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Chapter 6 Parasitic zoonoses.

TREMATODE ZOONOSES

Fascioliasis

Overview

Fascioliasis, mainly caused by the liver fluke Fasciola hepatica, has been known as a parasite of sheep and cattle for hundreds of years. The first record of F. hepatica was in 1379 by Jean de Brie who described large, flat worms as the cause of the disease liver rot in sheep. The fluke was first illustrated in 1668 by Francisco Redi, but it was not until 1737 when the microscopic cercariae were described by Jan Swammerdam. In 1758 Linnaeus gave the worm its scientific name (fasciola is Greek for band, which describes the characteristic shoulders of this fluke), but he considered the organism a leech. The first human infection with F. hepatica was described by Pallas in 1760. The role of mollusks in the life cycle of F. hepatica was not described until the mid-1800s when both A. P. Thomas and Rudolph Leuckart independently traced the development of the fluke to the same species of snail. In 1892, Brazilian scientist Adolph Lutz determined that ruminants (the definitive hosts) become infected by Fasciola flukes by ingesting juvenile flukes encysted on vegetation. Lutz was working with F. gigantica, a larger fluke of the genus Fasciola, which has the same biology as F. hepatica.

Fascioliasis in humans is characterized by fever, eosinophilia, and abdominal pain, although many people are asymptomatic. Humans are incidental hosts for F. hepatica, commonly known as the sheep liver fluke or common liver fluke, and F. gigantica, a rare and geographically isolated cause of fascioliasis. Fascioliasis in ruminants produces a variety of clinical signs ranging from devastating disease in sheep to asymptomatic disease in cattle.

Causative Agent

F. hepatica is one of the largest flukes (30 mm x 13 mm) and is leaf shaped with a pointed posterior end and a wide anterior end. Its oral sucker is small and located at the end of a cone-shaped projection at the anterior end. The body has a marked widening called an oral core that gives the fluke the appearance of shoulders. The testes are large and greatly branched, arranged in tandem behind the ovary. The smaller ovary is on the right side and the uterus is short. Vitelline follicles are extensive and fill most of the lateral body. F. hepatica cercariae have a simple, club-shaped tail about twice their body length. Operculated eggs are 90 x 150 [micro]m. The ova of F. hepatica are indistinguishable from that of Fasciolopsis buski.
F. hepatica is the most common disease-causing
liver fluke in temperate areas
of the world and the most important
trematode of domestic ruminants.


F. gigantica is a longer and more slender fluke than F. hepatica, but otherwise is very similar. Its operculated egg is 90 x 190 [micro]m.

Epizootiology and Public Health Significance

F. hepatica is found worldwide and is especially common in moist regions with high rainfall (favors the development of amphibious snails that serve as intermediate hosts). In the United States, F. hepatica is endemic along the Gulf Coast, the West Coast, the Rocky Mountain region, and other areas. It can also be found in eastern Canada, British Columbia, and South America, the British Isles, western and eastern Europe, Australia, and New Zealand.
There is not a second intermediate host
in the life cycle of Fasciola flukes.


F. gigantica is found in Africa, Asia, and Hawaii and is relatively common in herbivorous animals especially cattle. Its morphology, biology, and pathology are nearly identical to those of F. hepatica except different snail hosts are needed for each.

Fascioliasis causes considerable morbidity worldwide in temperate regions, except Oceania, with disease prevalence high in specific regions of Bolivia, Ecuador, Egypt, and Peru. Human fascioliasis is rare in the United States; however, an estimated 2 million cases exist worldwide. The incidence of human fascioliasis has increased since 1980.

Transmission

Infection occurs by ingestion of plants with metacercariae attached to them. The most common plants that have metacercariae attached are watercress, water lettuce, mint, parsley, khat, and other vegetables grown in water. Drinking surface water containing floating metacercariae can also lead to infection. Infection can also occur from eating raw liver containing immature liver flukes.

Pathogenesis

After oral ingestion of contaminated plants or water, the larvae excyst in the duodenum, migrate through the intestinal wall, and enter the liver from the peritoneal cavity. The young flukes penetrate the liver capsule and move in the parenchyma for several weeks, growing and destroying tissue. After 6 to 8 weeks of migration through the liver, the flukes penetrate the bile ducts (their final destination). In ruminants, sexual maturity is reached in approximately 10 weeks postinfection and in humans sexual maturity is reached in 3 to 4 months postinfection. Mature flukes consume hepatocytes and may reside for years in the hepatic and common bile ducts and occasionally in the gall bladder (causing the chronic adult biliary stage of infection). Adult fluke worms produce eggs about 4 months postinfection. Adult fluke eggs in the bile ducts are released into the common bile duct into the duodenum. Eggs are shed with feces into the environment. To develop further the egg must reach tepid (22[degrees]C to 26[degrees]C) surface water, where miracidia (larvae) develop and hatch within approximately 1 to 2 weeks. These miracidia then search for and invade many species of freshwater snails as intermediate hosts. After several developmental stages (sporozoites, redia, and cercariae) free-swimming cercariae leave the intermediate hosts, swim actively through water, and adhere to plants, where they encyst to form metacercariae (the infectious stage). Free-swimming cercariae may remain suspended in the water and encyst over a few hours (Figure 6-65).
Encysted cercariae (metacercariae) may
remain viable for many months unless
they become desiccated.


Clinical Signs in Animals

As immature F. hepatica flukes wander through the liver they destroy liver tissue and cause hemorrhage. In sheep and cattle, fascioliasis can be seen in three forms (determined by the number of metacercariae ingested over a period of time):

* Chronic. This form is often fatal in sheep and rarely fatal in cattle. Chronic fascioliasis is seen in all seasons; signs include anemia, unthriftiness, submandibular edema, and reduced milk secretion, but even heavily infected cattle may show no clinical signs. Heavy chronic infection is fatal in sheep. Chronic liver damage in sheep is cumulative over several years; fibrosis of liver tissues, cirrhosis, and calcification of bile ducts may be seen in cattle. Flukes may also be found in other sites such as the lungs.

* Acute. This form occurs primarily in sheep and is often fatal. In acute fascioliasis, there is extensive liver damage and the liver becomes enlarged and friable. In sheep, acute fascioliasis occurs seasonally and clinical signs in sheep include a distended, painful abdomen, anemia, and sudden death. Death can occur within 6 weeks of infection.

* Subacute. This form occurs in cattle and sheep. In subacute disease, survival is longer (typically 7 to 10 weeks) than the acute form. Clinical signs are associated with hepatic damage; death may occur as a result of hemorrhage and anemia.

[FIGURE 6-65 OMITTED]

Clinical Signs in Humans

In humans, fascioliasis may be asymptomatic (approximately 50%) or cause clinical disease with such clinical signs as intermittent fever, lethargy, weight loss, hepatomegaly (abdominal enlargement), abdominal pain, hives, cough, and gastrointestinal signs. In children, signs that resemble pancreatitis may be seen.
Fasciola hepatica infection in cattle
causes liver condemnation at slaugher
(termed liver rot).


Diagnosis in Animals

Fascioliasis is diagnosed by the identification of oval, operculated, golden eggs (130 x 75 [micro]m) via fecal sedimentation techniques (simple cup sedimentation using tap water, formaldehyde-ether concentration) (Figure 6-66). F. hepatica eggs cannot be observed in feces during acute fascioliasis and in subacute or chronic disease eggs are shed intermittently making repeated fecal examination necessary. ELISA tests are also available that can detect antibody levels about 2 to 3 weeks after infection. Necropsy samples show extensive liver damage and visualization of adult flukes in the bile ducts (immature stages may be squeezed or teased from the cut surface).

[FIGURE 6-66 OMITTED]

Diagnosis in Humans

Fascioliasis is diagnosed in humans using fecal stool samples (the small number of eggs shed intermittently in stool requires multiple specimens) and serologic methods (complement fixation, indirect hemagglutination, and ELISA).

Treatment in Animals

In ruminants several drugs are available to treat F. hepatica including clorsulon (cattle and sheep only) and albendazole in the United States and triclabendazole, netobimin, closantel, rafoxanide, and oxyclozanide in other countries (resistance to triclabendazole has developed in Australia and Europe). Most of these drugs have long withdrawal periods and need to be given at correct times to result in the optimal removal of flukes (each drug has varying efficacy against different ages of fluke). Treatments are determined by environmental factors (Gulf Coast states treat cattle before the fall rainy season and again in the late spring; northwestern states treat cattle at the end of the pasture season and in late January or February; European countries treat sheep in September or October, January or February, and again in April or May).

Treatment in Humans

People are treated with triclabendazole or bithionol.

Management and Control in Animals

Under ideal circumstances removing flukes in affected animals, reducing the snail population, and preventing livestock access to snail-infested pasture would be ways to control Fasciola hepatica in animals. Routine use of flukicides is practiced; however, molluscicide used to remove snails can be toxic to grazing animals and the ability to limit livestock access to snail-infested pasture may be cost prohibitive.

Management and Control in Humans

Avoiding ingestion of plants such as watercress found on moist ground in animal grazing areas and in areas with temporary flooding that may contain metacercariae will lower the cases of human fascioliasis. Avoiding consumption of unboiled or unfiltered surface water will also decrease the number of human fascioliasis cases.

Summary

Fascioliasis is caused by F. hepatica, one of the largest flukes that is found worldwide, especially in moist regions with high rainfall. In the United States, F. hepatica is endemic along the Gulf Coast, the West Coast, the Rocky Mountain region, and other areas. Infection occurs by ingestion of plants with metacercariae attached to them, drinking surface water containing floating metacercariae, or from eating raw liver containing immature liver flukes. In sheep and cattle, fascioliasis can be seen in three forms: chronic (anemia, unthriftiness, and potentially fatal in sheep), acute (distended abdomen and anemia in cattle and sheep, and sudden death in sheep), and subacute (hepatic disease, hemorrhage, and anemia in sheep and cattle). In humans, fascioliasis may be asymptomatic or cause clinical disease with such clinical signs as intermittent fever, lethargy, weight loss, hepatomegaly, abdominal pain, hives, cough, and gastrointestinal signs. Fascioliasis is diagnosed in animals by the identification of oval, operculated, golden eggs via fecal sedimentation techniques. ELISA tests are also available. Necropsy samples show extensive liver damage and visualization of adult flukes in the bile ducts. Fascioliasis is diagnosed in humans using fecal stool samples (the small number of eggs shed intermittently in stool requires multiple specimens) and serologic methods (complement fixation, indirect hemagglutination, and ELISA). In ruminants several drugs are available to treat F. hepatica infection including clorsulon (cattle and sheep only) and albendazole in the United States. People are treated with triclabendazole or bithionol. Fluke control includes removal of flukes in affected animals, reduction of the snail population, and prevention of livestock access to snail-infested pasture. Avoiding ingestion of plants found on moist ground in animal grazing areas and consumption of unboiled or unfiltered surface water will decrease the number of human fascioliasis cases.

Fasciolopsis

Overview

Fasciolopsis, caused by the trematode Fasciolopsis buski (commonly known as the giant intestinal fluke), is a clinically variable disease of the intestine. Fasciolopsis comes from the Latin fasciola meaning band (used to name Fasciola spp.) and the Greek opsis meaning appearance. The eggs of Fa. buski and F. hepatica are very similar in their appearance and their life cycles parallel each other. Busk first described Fa. buski in London after finding the flukes in the duodenum of a sailor in 1843. Barlow determined its life cycle in humans in 1925 and in 1947 Stoll estimated that worldwide there were 10 million human infections. Today fasciolopsis is found mainly in Asia and on the Indian subcontinent where humans raise pigs and consume freshwater plants.

Causative Agent

Fa. buski lives in the small intestine of its definitive host and is elongated, oval, and approximately 50 mm x 20 mm in size (it is one of the largest parasitic trematodes). The adult fluke has unbranched ceca, dendritic testes in its posterior half, branched ovaries that lie in the midline (anterior to the testes), extensive vitelline follicles, and a short uterus. The eggs are almost identical to the eggs of F. hepatica, being operculated and about 80 to 130 [micro]m in size (Figure 6-66).
There are more than 50 different species
of intestinal trematodes; however, only
a few species cause disease.


Epizootiology and Public Health Significance

Fa. buski is found in southeastern and eastern Asia. Endemic areas include China, Taiwan, India, Bangladesh, Indonesia, Thailand, and Vietnam where people and pigs eat fecal-contaminated water plants and their fruits.
Fa. buski flukes can produce about
25,000 eggs daily.


Infection with intestinal flukes in the United States is typically seen in immigrants from endemic areas. Intestinal flukes are endemic in the Far East and Southeast Asia. Death from infection is rare and is usually observed only in people with a heavy worm burden.

Transmission

Infection with Fa. buski occurs after oral ingestion of metacercariae adhered to raw water plants and their fruits such as water nuts and walnuts or ingestion of contaminated surface water. Water vegetation, particularly Trapa natans (water caltrop) or the water chestnut, thrives in ponds fertilized by night soil (human feces). Only the inner part of water caltrop is eaten, but human infection occurs if the outer covering of the infected plant is peeled using the person's teeth.
The definitive host of Fa. buski is the
pig.


Pathogenesis

Immature eggs are shed with feces into fresh water where miracidia hatch from eggs after 3 to 7 weeks. Miracidia invade the soft tissues of snails (intermediate hosts), asexually form sporocysts, then rediae, and finally cercariae. The cercariae are released from the snails and encyst (become metacercariae) on various water plants such as water caltrop, water chestnut, lotus (on the roots), and water bamboo. People and pigs become infected when they ingest water plants with metacercariae. After ingestion of metacercariae, Fa. buski attaches to the duodenal and jejunal mucosa (in severe infections, they may attach to the ileum or colon) and become adult flukes in approximately 3 months. At the point of attachment, intestinal flukes cause inflammation, ulceration, and mucus secretion. Severe infections may also cause intestinal obstruction or malabsorption. Some of the fluke's metabolites are toxic and when absorbed by the host can be fatal (Figure 6-67).
Disease conditions resulting from
Fa. buski infection are immunopathologic,
obstructive, and traumatic.


[FIGURE 6-67 OMITTED]

Clinical Signs in Animals

Pigs are the definitive hosts of Fa. buski and may be asymptomatic or may present with gastrointestinal disease such as weight loss, lethargy, abdominal pain, and diarrhea. Dogs may also be infected with a similar clinical picture.
Flukes do not multiply in humans, so
the intensity of infection is related to
the degree of exposure to the infective
larvae.


Clinical Signs in Humans

Many people infected with Fa. buski are asymptomatic. People with moderate infection may have occasional loose stools, weight loss, lethargy, and generalized abdominal pain. People with severe infection have diarrhea alternating with constipation, hunger pangs, facial edema, anemia, and ascites. Cachexia may follow these signs (possibly as a result of the toxic metabolic products) and death may occur.

Diagnosis in Animals

Diagnosis is made by fecal examination of operculated eggs.

Diagnosis in Humans

Diagnosis is made by fecal examination of operculated eggs filled with yolk material; however, Fa. buski eggs can easily be mistaken for F. hepatica eggs.

Treatment in Animals

Fasciolopsis is treated with praziquantel, diminazene aceturate, and imidocarb dipropionate. These drugs are not available in all endemic countries or their use may be restricted. Supportive treatment may include the use of anti-inflammatory drugs, antioxidants, and corticosteroids.

Treatment in Humans

Fasciolopsis in people is treated with praziquantel or triclabendazole (not available for humans in United States). Supportive care may include the use of anti-inflammatory drugs, antioxidants, corticosteroids, or blood transfusions.

Management and Control in Animals

Animal disease can be prevented by adequate waste removal of human and pig feces. Snail control may also be attempted.

Management and Control in Humans

Human disease can be prevented by immersing vegetables in boiling water for a few seconds to kill the metacercariae. Hands may become contaminated while handling the contaminated plants and should be thoroughly washed in clean water. People should avoid cracking water nuts and chestnuts with their teeth. Adequate waste removal of human and pig feces as well as not using human or pig feces for fertilizer would also control disease. Snail control may be attempted.

Summary

Fasciolopsis is caused by Fa. buski, a fluke that lives in the small intestine of its definitive host. Fa. buski is elongated, oval, and approximately 50 mm x 20 mm in size. Fa. buski is found in southeastern and eastern Asia. Infection with Fa. buski occurs after oral ingestion of metacercariae adhered to raw water plants and their fruits such as water nuts and walnuts or ingestion of contaminated surface water. Pigs are the definitive hosts of Fa. buski and may be asymptomatic or may present with gastrointestinal disease such as weight loss, lethargy, abdominal pain, and diarrhea. Dogs may also be infected with a similar clinical picture. Many people infected with Fa. buski are asymptomatic. People with moderate infection may have occasional loose stools, weight loss, lethargy, and generalized abdominal pain. People with severe infection have diarrhea alternating with constipation, hunger pangs, facial edema, anemia, and ascites. Diagnosis is made by fecal examination of operculated eggs. Fasciolopsis in animals is treated with praziquantel, diminazene aceturate, and imidocarb dipropionate. Fasciolopsis in people is treated with praziquantel or triclabendazole (not available in United States). Human and animal disease can be prevented by adequate waste removal of human and pig feces as well as not using human or pig feces for fertilizer. Snail control may be attempted. Human disease can be prevented by immersing vegetables in boiling water for a few seconds to kill the metacercariae. Hands may become contaminated while handling the contaminated plants and should be thoroughly washed in clean water. People should avoid cracking water nuts and chestnuts with their teeth.

Schistosomiasis

Overview

Schistosomiasis is the second most prevalent tropical disease in the world (first is malaria) with 200 million people infected (120 million are symptomatic) and 600 million people in 74 countries at risk of contracting the disease. Schistosomiasis, mainly caused by three species of Schistosoma blood trematodes, is also known as bilharzia in honor of Theodore Bilharz who first identified the etiological agent in Egypt in 1851. Schistosoma is derived from the Greek skhizein meaning to split or cleft and soma meaning body describing the male fluke, which has a deep cleft extending the length of his body where the female is held during copulation.

Schistosomiasis has been known since antiquity dating from about 1200 B.C. in Egyptian mummies to the curse Joshua placed on Jericho in the Bible. The first Europeans to contract schistosomiasis were surgeons in Napoleon's army in Egypt in 1799. These surgeons observed many cases of hematuria but the cause was not identified until Bilharz identified and named the parasite Distomum haematobium in 1851. In 1858, Weinland changed the name to Schistosoma and 3 months later it was named Bilharzia by Cobbold. Although many people still identify the parasite as Bilharzia, its official name is Schistosoma.

The three main species that cause disease are Sc. haematobium, Sc. mansoni, and Sc. japonicum. Sc. haematobium was the single species for some time; however, some infected people had eggs with terminal spines while other people had eggs with lateral spines. In 1905, Sir Patrick Manson proposed that intestinal and urinary bladder schistosomiasis were distinct diseases caused by distinct flukes. Another scientist named Sambon agreed with the two species concept and named the fluke that produced laterally spined eggs Sc. mansoni. In 1915 Leiper was working in Egypt and discovered that the laterally spined eggs came from a different snail than the eggs with terminal spines solidifying the argument for two different species of fluke. Meanwhile Japanese scientists were discovering another species on their own and in 1904 Katsurada recognized adult flukes in dogs and cats that resembled those found in humans and named the flukes Sc. japonicum. Additional species that are rarer causes of human disease are Sc. intercalatum, Sc. mekongi, Sc. mattheei, and Sc. bovis. Schistosomes are also called blood flukes because they live in the vascular system of humans and other vertebrates.

Schistosome cercarial dermatitis is another disease manifestation caused by species of Schistosoma. This disease has been known for a long time and has been referred to by a variety of names including swimmer's itch, rice paddy itch, clam diggers itch, sawah (Malaysia), kubure or kobanyo (Japan), hoi con (Thailand), and duckworms (New Jersey). Schistosome cercarial dermatitis has been around as long as mankind and has been recorded as early as the 19th century, particularly in people involved in the logging industry. In 1855 a description of a disease by LaValette producing erythemous maculopapular eruptions is presumed to have been cercarial dermatitis. The disease was first identified in the United States (Douglas Lake, MI) in 1928 by Cort who determined that the cause of swimmer's itch was the larva stage of trematodes whose adult form lives in the blood vessels of birds and mammals (prior to this discovery it was believed that the disease was only caused by human schistosomes). Originally the disease was called schistosome dermatitis by Cort in 1928; however, in 1930, Vogel used the term cercarial dermatitis. Since then cases of cercarial dermatitis have been increasing and occur commonly in people who swim in the Great Lakes and other recreational waters.

Causative Agent

Schistosoma spp. are flukes that have considerable sexual dimorphism with males being shorter and stouter than females. Males have a ventral, longitudinal groove called a gynecophoral canal where the female normally resides. The mouth has a strong oral sucker with the suckers of females being smaller and not so muscular. Paired intestinal ceca meet and fuse at the worm's midpoint and continue as a single gut to the posterior end. Variation in the number of testes and length of uterus varies among the species as described in Table 6-12.
The pathophysiology of schistosomiasis
is a result of the immune response
against the schistosome eggs.


The eggs of schistosomes are nonoperculate, possess a spine, and contain a miracidium. The microscopic appearance of the egg allows diagnostic differentiation of the species as described in Table 6-12.

Adult worms live in veins that drain abdominal organs of the host: Sc. haematobium lives mainly in the veins of the urinary bladder, Sc. mansoni lives in the veins draining the large intestine, and Sc. japonicum lives in the veins of the small intestine. Sc. intercalatum and Sc. malayensis are found in Asia and are rarer causes of schistosomal disease.

Cercarial dermatitis is caused by the cercariae of certain species of schistosomes whose normal hosts are birds and mammals other than humans. These cercariae seem to have a chemotrophic reaction to secretions from the skin and are not as host-specific as other types of schistosomes. Species of schistosomes that infect animals are described in the clinical signs in animals section. One species of schistosome often implicated in cases of cercarial dermatitis is Austrobilharzia variglandis, whose normal hosts are ducks.

Epizootiology and Public Health Significance

Humans are the main host for the three main species of schistosomes; however, some animal reservoirs exist for each species. Sc. haematobium is prevalent in North Africa and is also present throughout Africa and in the Middle East. Urinary schistosomiasis caused by Sc. haematobium affects 54 countries in Africa and the eastern Mediterranean. Animal reservoir hosts of Sc. haematobium are baboons, monkeys, pigs, and rodents. Sc. mansoni is found in Egypt, the Middle East, northern and western South America, the Caribbean islands, and East, Central, and West Africa. Animal reservoir hosts of Sc. mansoni are monkeys and rodents. Sc. japonicum is found in Southeast and East Asia. Sc. japonicum is endemic in China, the Philippines, Indonesia, and Thailand. Animal reservoir hosts of Sc. japonicum include domestic ruminants, dogs, and rodents.
Female Schistosoma worms live in the
gynecophoral canal (groove) of males
whose muscles help the paired worms
work their way up into smaller veins
where the female deposits eggs.


The number of people with schistosomiasis in the United States is estimated at more than 400,000 infected persons. The appropriate snail intermediate host is not endemic to the United States and most U.S. cases are found in immigrants or in someone who acquired the infection outside the United States. Intestinal schistosomiasis caused by Sc. mansoni occurs in 52 countries, including Caribbean countries, eastern Mediterranean countries, South American countries, and most countries in Africa.

Schistosomal cercarial dermatitis occurs worldwide with cases reported from every continent except Antarctica. In the United States, cases are most commonly reported from the Great Lakes region.

Transmission

The infectious forms of schistosomes are the cercariae swimming in water. Invasion of cercariae into the final host is percutaneous.

Pathogenesis

The life cycle of schistosomes involves a sexual stage in the human and an asexual stage in the fresh water snail host (intermediate host). Adult worms mate and lay eggs, which are excreted with feces or urine (depending on the species). When the eggs reach the fresh water, the miracidia (ciliated larva) hatch from the egg and the miracidia penetrate the snail. In the snail the fork-tailed cercariae asexually develop following two sporocyst stages (within 3 to 5 weeks) and leave the snail. The cercariae swim close to the water surface until they meet a suitable host (human or animal), where they adhere to the skin, shed their tail (become schistosomulae), and penetrate the skin within minutes with the help of secreted enzymes. Once inside the host, cercariae travel to the heart, the lungs, and through the systemic circulation to reach the portal veins where they develop into adult worms. They then travel to their final destination of intestinal veins (Sc. mansoni and Sc. japonicum) or urinary bladder veins (Sc. haematobium). The time between cercariae penetration and the first egg production is 4 to 6 weeks. The spined eggs reach the capillaries where miracidia develop in the eggs. The eggs get stuck in capillaries where they induce granulomatous reactions. If these reactions occur close to the mucosal surface, eggs break through into the lumen of the organ. Otherwise miracidia die within 3 to 4 weeks and the granulomas get organized by connective tissue. Humans excrete approximately 50% of the eggs, whereas the rest are trapped in various parts of the body (Figure 6-68).

[FIGURE 6-68 OMITTED]

The life cycle of avian schistosomes varies from other schistosomes in that the cycle is typically maintained in birds and humans are accidental, dead-end hosts. Adult worms (Trichobilharzia, Gigantobilharzia, Austrobilharzia, Ornithobilharzia, Microbilharzia, and Heterobilharzia) are found in the gastrointestinal tract of the bird host. Adult worms release eggs that are shed in the bird's feces and upon immersion in water, a miracidium emerges. Miracidia infect snails and develop into sporocysts, which in turn undergo asexual reproduction producing cercariae. Cercariae swim to the surface of the water to infect another bird. After infecting a bird, the cercaria develops into a schistosomulum and migrates to the gastrointestinal tract where it matures and mates producing eggs starting a new cycle (Figure 6-69).

Clinical Signs in Animals

Most species of pathogenic schistosomes of animals are found in the hepatic portal system. The principal clinical signs seen in animals are associated with passage of the spined eggs through the tissues to the lumen of the gastrointestinal tract. Species of Schistosoma in animals include:

* Sc. bovis is the predominant species infecting ruminants in northern and eastern Africa, southern Europe, and the Middle East.

* Sc. mattheei is the predominant species infecting ruminants in southern and central Africa.

* Sc. nasale found in the veins of the nasal mucosa of ruminants and horses in India, Sri Lanka, and Burma, where it may cause upper respiratory disease.

* Sc. curassoni, Sc. margrebowiei, and Sc. leiperi are other species infecting ruminants in Africa.

* Sc. spindale, Sc. nasale, Sc. indicum, Sc. incognitum, and Sc. japonicum are widespread in livestock in Asia.

* Sc. mansoni can infect rodents, baboon, cattle, and dogs and is widespread in Africa, the eastern Mediterranean, the Caribbean, and South America.

* Sc. mekongi can infect dogs and monkeys and is found in Southeast Asia.

* Sc. intercalatum is found in cattle, sheep, antelope, and goats in Central Asia.

* Species of avian schistosomes include Trichobilharzia, Gigantobilharzia, Austrobilharzia, Ornithobilharzia, Microbilharzia, and Heterobilharzia and are found worldwide.

[FIGURE 6-69 OMITTED]

Clinical signs associated with the intestinal and hepatic forms of schistosomiasis in ruminants include bloody diarrhea, anemia, and emaciation, which develop after the onset of egg excretion. Severely affected animals may die within a few months of infection, whereas those less heavily infected develop chronic disease that may present as slowed growth. Nasal schistosomiasis is a chronic disease of cattle, horses, and occasionally buffalo producing large amounts of mucopurulent discharge and dyspnea with milder cases frequently asymptomatic.

Clinical Signs in Humans

Clinical signs in people correlate with the life cycle of the parasite. Clinical signs associated with the parasitic form include:

* Cercariae (Figure 6-70). The syndrome caused by cercarial penetration of skin is called schistosome cercarial dermatitis (also called swimmer's itch or paddy field dermatitis). Skin penetration of cercariae produces an allergic dermatitis at the site of entry (Figure 6-71). Within minutes to days after swimming in cercariae contaminated water, tingling, burning, or itching of the skin occurs. Vesicles appear within 12 hours after the first clinical signs. Scratching the areas may result in secondary bacterial infections. Itching may last up to a week or more, but is self-limiting. Upon second exposure, a pruritic papular rash occurs. Schistosome cercarial dermatitis is caused by avian schistosomes (Trichobilharzia, Gigantobilharzia, Austrobilharzia, Ornithobilharzia, Microbilharzia, and Heterobilharzia in North America) found in ducks, geese, gulls, and swans, and by schistosomes of certain aquatic mammals (such as muskrats and beavers). This larval form searches for a suitable host (bird, muskrat), but will burrow into human skin if a suitable host is not found. These larvae cannot develop inside a human and typically die rapidly. Cercarial dermatitis typically occurs on the exposed skin outside of closefitting garments. Other organisms that can cause cercarial dermatitis are Sc. spindale and Sc. bovis.

* Schistosomula. These are tailless cercariae that are transported through blood or lymphatic vessels to the right side of the heart and lungs producing symptoms such as cough and fever.

* Adult worm. Adult worms do not multiply inside the body, however, the adult male and female worms mate in the veins producing eggs 4 to 6 weeks after cercarial penetration. Adult worms are rarely pathogenic.

* Eggs. Eggs cause Katayama fever (named after an endemic region of Japan) and schistosomiasis. Katayama fever is a condition caused by the high worm and egg antigen stimulus that results from immune complex formation and leads to a serum sickness-like illness, Schistosomiasis, a condition caused by immunologic reactions to Schistosoma eggs trapped in tissues stimulating a granulomatous reaction resulting in inflammation, collagen deposition and fibrosis, and organ damage.

[FIGURE 6-70 OMITTED]

[FIGURE 6-71 OMITTED]

Chronic disease and its manifestations depend on the species of schistosome causing infection, the duration and severity of infestation, and the immune response to the eggs.

[FIGURE 6-72 OMITTED]

Sc. mansoni and Sc. japonicum cause intestinal tract (fatigue, abdominal pain, diarrhea, and dysentery) and liver disease (abdominal pain, flatulence, anemia, weakness, edema, and melena); Sc. haematobium only rarely causes intestinal or liver disease but characteristically causes urinary tract disease (dysuria, urinary frequency, and hematuria) (Figure 6-72). Spread to other organs may also be seen including the respiratory system (cough, wheezing, and low-grade fever), cardiovascular system (palpitations, dyspnea, and hemoptysis), and CNS (seizures, headache, and paresthesia).

Diagnosis in Animals

Schistosomiasis in animals is diagnosed via identification of eggs in feces, rectal scrapings, or nasal mucus of infected animals. Other species of Schistosoma that may be found in animals include Sc. bovis (eggs are 200 x 60 [micro]m and spindle shaped), Sc. mattheei (eggs are 177 x 55 [micro]m and spindle shaped), Sc. spindale (eggs are 380 x 70 [micro]m and are elongated and flattened on one side), and Sc. nasale (eggs are 455 x 65 [micro]m and are boomerang shaped). At necropsy adult flukes can be found in blood vessels.
The female adult worm lives for
approximately 3 to 8 years and lays eggs
throughout her life span.


Diagnosis in Humans

Diagnosis of schistosomiasis is by fecal or urine detection of eggs (Figure 6-73). The urine is most likely to yield positive results for Sc. haematobium from 10 A.M until 2 P.M. Quantification of egg excretion determines the severity of infection and is calculated by collecting 24-hour urine or fecal samples, homogenizing the sample, and counting the eggs in a measured sample (less than 100 eggs per gram is a light infection, 100 to 400 eggs per gram is a moderate infection, and greater than 400 eggs per gram is a heavy infection). The egg viability test is used to assess the effectiveness of treatment and involves mixing feces or urine with room temperature distilled water and observing for hatching miracidia.

[FIGURE 6-73 OMITTED]

Active infection produces viable eggs, whereas treated or past infection results in nonviable eggs and an absence of miracidia. Serologic tests can be used in nonendemic areas using antibody detection methods specific to the worm species (mansoni adult worm microsomal antigen [MAMA], haematobium adult worm microsomal antigen [HAMA], and japonicum adult worm microsomal antigen [JAMA]) have been used in the falcon assay screening test (FAST), enzyme-linked immunoassay (ELISA), and immunoblot assays.

Treatment in Animals

Praziquantel is highly effective treatment in animals and requires two treatments 3 to 5 weeks apart.

Treatment in Humans

Praziquantel is the treatment of choice for all species of schistosomiasis causing systemic disease.

Schistosome cercarial dermatitis can be treated symptomatically with corticosteroid cream or calamine lotion, applying cool compresses to the affected areas, and bathing in epson salts, baking soda, or oatmeal.

Management and Control in Animals

Control measures are mainly used in China, where infected livestock are important reservoirs of human infection. Transmission can be reduced by controlling snails using molluscicides such as niclosamide, by modifying snail habitats, or by fencing off contaminated bodies of water and providing clean drinking water.
Infected snails produce cercariae their
entire life.


Management and Control in Humans

People should avoid contact with surface water in endemic areas or areas with snails, towel dry or shower immediately after leaving the water, avoid attracting birds to areas where people swim, and encourage health officials to post signs on shorelines where swimmer's itch is a current problem.

Summary

Schistosomiasis is the second most prevalent tropical disease in the world (first is malaria) with 200 million people infected (120 million are symptomatic) and 600 million people in 74 countries are at risk of contracting the disease. Schistosomiasis is mainly is caused by three species of Schistosoma blood trematodes: Sc. haematobium, Sc. mansoni, and Sc. japonicum. Adult worms live in veins that drain abdominal organs of the host: Sc. haematobium lives mainly in the veins of the urinary bladder, Sc. mansoni lives in the veins draining the large intestine, and Sc. japonicum lives in the veins of the small intestine. Humans are the main host for the three main species of schistosomes with Sc. haematobium prevalent in North Africa (its reservoir hosts are baboons and other monkeys, pigs, and rodents; some believe it is only found in humans); Sc. mansoni prevalent in Egypt, the Middle East, northern and western South America, the Caribbean islands, and East, Central, and West Africa (its reservoir hosts are monkeys and rodents); and Sc. japonicum prevalent in Southeast and East Asia (its reservoir hosts include domestic ruminants, dogs, horses, cats, swine, deer, and rodents). The infectious forms of schistosomes are the cercariae swimming in water. Invasion of cercariae into the final host is percutaneous. The life cycle of schistosomes involves a sexual stage in the human and an asexual stage in the fresh water snail host. The cercariae swim close to the water surface until they meet a suitable host (human or animal), where they adhere to the skin, shed their tail, and penetrate the skin. Once inside the host, cercariae travel through veins to a variety of organs where they develop into adult worms. Species of Schistosoma in animals include Sc. bovis, Sc. mattheei, Sc. nasale, Sc. curassoni, Sc. margrebowiei, Sc. leiperi, Sc. spindale, Sc. nasale, Sc. indicum, Sc. incognitum, and Sc. japonicum. Cercarial dermatitis (swimmer's itch) is caused by several genera of avian schistosomes including Trichobilharzia, Gigantobilharzia, Austrobilharzia, Ornithobilharzia, Microbilharzia, and Heterobilharzia. Clinical signs associated with the intestinal and hepatic forms of schistosomiasis in ruminants include bloody diarrhea, anemia, and emaciation. Clinical signs in people correlate with the life cycle of the parasite and are associated with the parasitic form: cercariae (cercarial dermatitis), schistosomula (producing symptoms such as cough and fever), adult worm (rarely pathogenic), and eggs (causing Katayama fever and schistosomiasis). Schistosomiasis in animals is diagnosed via identification of eggs in feces, rectal scrapings, or nasal mucus of infected animals. Diagnosis of schistosomiasis in people is by fecal or urine detection of eggs, quantification of egg excretion, and egg viability tests. Serologic tests such as the falcon assay screening test (FAST), enzyme-linked immunoassay (ELISA), and immunoblot assays are also available. Praziquantel is the treatment of choice for all species of schistosomiasis. Control measures include controlling snails using molluscicides such as niclosamide, by modifying snail habitats, or by fencing off contaminated bodies of water and providing clean drinking water. People should avoid contact with surface water in endemic areas or areas with snails, towel dry or shower immediately after leaving the water, avoid attracting birds to areas where people swim, and encourage health officials to post signs on shorelines where swimmer's itch is a current problem. (For less common zoonotic trematodes, see Table 6-13.)

ARTHROPOD DISEASE

Overview

Arthropods have been known since ancient history with fossils present in the geological record as far back as the Paleozoic age. The phylum Arthropoda contains over 80% of all animal species making it the largest group of living organisms. Arthropod is derived from the Latin arthro meaning joint and podos meaning foot, which describes one of this groups key evolutionary features that allow these organisms to move quickly from one place to another. Arthropods are characterized by jointed chitinous exoskeletons (protects the organism and provides an area for muscle attachment), segmented bodies (compartmentalization and specialization of function), and jointed appendages (rapid movement). Arthropods have a true coelom (filled with fluid that supplies nutrients), a small brain, an extensive nervous system, and separate sexes. Any of numerous invertebrate animals of the phylum Arthropoda is divided into three classes: insects (95% of all arthropods), arachnids, and crustaceans. Insects have three body regions (head, thorax, and abdomen), three pairs of legs (6 total), and highly specialized mouthparts. Examples of insects are mosquitoes, flies, true bugs, lice, and fleas. Arachnids have two body regions (cephalothorax and abdomen), four pairs of legs (8 total), and mouthparts that are used for capturing and tearing. Examples of arachnids are ticks, mites, spiders, and scorpions. Crustaceans are typically aquatic arthropods that have a pair of appendages associated with each segment (such as crabs and crayfish). Crustaceans such as Cyclops spp. are intermediate hosts of parasites such as Di. latum and Paragonimus westermani. These diseases were covered in previous chapters and will not be covered here.

Arthropods are involved in nearly every kind of parasitic relationship. Arthropods may affect human and animal health directly (by bites, stings, or infestation of tissues) or indirectly as vectors, definitive hosts, and intermediate hosts (disease transmission). The direct effects of arthropod bites are typically irritation, pruritus, and secondary infections. The most significant diseases produced by arthropods are through vector-borne transmission. Arthropods can serve as means of biological transmission (blood-feeding arthropods such as mosquitoes and ticks acquire the pathogen, it multiplies within the arthropod vector, and is transmitted when the arthropod takes a blood meal) or mechanical transmission (arthropods physically carry pathogens from one host or place to another).

Causative Agents

Adult Diptera

Arthropods in the order Diptera are commonly known as true flies and include insects such as mosquitoes, blackflies or gnats, midges, sand flies, tsetse flies, and house flies. The name Diptera was coined by Aristotle in the 4th century B.C. and is derived from the Greek words di meaning two and ptera meaning wings in reference to the fact that true flies have only a single pair of wings (two total). The Diptera are divided into three suborders: Nematocera (small, delicate insects with long, multisegmented antennae), Brachycera (compact, robust flies with short, stylate (pointed) antennae), and Cyclorrhapha (compact, robust flies with short, aristate (flagellum-like) antennae). Flies can be found worldwide except Antarctica. Many species are particularly important as vectors of disease. The earliest fossil flies are from the Mesozoic geological period (225 million years ago) and over time they have become one of the largest most diverse groups of organisms.

Flies have a life cycle that involves a major change from a soft-bodied, wingless larval stage to a hardened, winged adult. The larvae typically have a variety of common names such as wriggler and maggot. Fly larvae of different species have different feeding habits with some species having very precise requirements. Many fly larvae consume decaying organic matter, whereas others are parasitic on other insects and organisms. Adult flies are typically free-living, fly during the day or night depending on the species, and consume liquid food (nectar) or decomposing organic matter.

The major morphological feature, which distinguishes flies from other insects is their reduced hind wings (halteres). The halteres are small, club-like structures that help the fly balance during flight. Adult flies have only one pair of functional wings (the forewings). The mesothorax of Diptera species has become enlarged to contain the enormous flight muscles as a result of their reliance on the forewings for flight. The mouthparts of flies may be modified for stabbing and piercing other insects. Table 6-14 summarizes some diseases caused by adult Diptera arthropods.

Larval Diptera or Myiasis

Myiasis, also known as fly-strike or fly-blown, is the infestation of live vertebrate animals with dipteran larvae (first-stage that develop into third-stage) that feed on the host's dead or living tissue. Flies causing myiasis can be obligate parasites that can develop only on live hosts or facultative parasites that can develop on either live hosts or organic debris. The adult flies are not parasitic, but when they lay their eggs in open wounds the eggs hatch into their larval stage (maggots or grubs), the larvae feed on live and/or necrotic tissue, causing myiasis to develop (Figure 6-78). Accidental infestations with fly larvae can occur if eggs or larvae are inhaled or swallowed inadvertently with food and are referred to as pseudomyiasis. Myiases are often classified according to the area of the animal that the larvae infest or the appearance of the lesion they cause. When larvae are deposited into open wounds the myiasis is known as traumatic, when the appearance is boillike the myiasis is termed furuncular, when the larvae migrate beneath the skin and its path can be traced the myiasis is called creeping, and when the larvae are bloodsucking the myiasis is called sanguinivorous.

[FIGURE 6-74 OMITTED]

[FIGURE 6-75 OMITTED]

[FIGURE 6-76 OMITTED]

[FIGURE 6-77 OMITTED]

[FIGURE 6-78 OMITTED]

Most myiasis-causing flies belong to one of three major families: Oestridae (botflies), Sarcophagidae (fleshflies), or Calliphoridae (blowflies) (Figure 6-79). The family Oestridae has approximately 150 species of flies. All botflies (Gastrophilus spp.) cause myiasis and are obligate parasites typically with a high degree of host specificity. The family Sarcophagidae has only two genera of medical importance: Sarcophaga and Wohlfahrtia. Members of Sarcophagidae are larviparous (deposit first-stage larvae instead of laying eggs). The family Calliphoridae can be divided into two groups: the nonmetallic flies such as the Congo floor maggot and tumbu fly and the metallic flies such as the blowflies (bluebottles and greenbottles). The zoonoses caused by Diptera myiasis are summarized in Table 6-15.

[FIGURE 6-79 OMITTED]

Fleas

Fleas belong to the order Siphonaptera, one of the major groups of bloodsucking insects with more than 2,500 species and subspecies of fleas. The name Siphonaptera is derived from the Greek words siphon meaning a tube or pipe and aptera meaning wingless. Adult fleas are obligatory hematophages (blood feeders) that parasitize warm-blooded animals (94% parasitize mammals and 6% parasitize birds). Fleas are wingless insects that have a flattened body. Flea bodies have three major parts: head (triangular in shape with club-shaped antennae and downward pointed mouthparts), thorax (contains three pairs of legs and has three distinct segments called the prothorax, mesothorax, and metathorax), and abdomen (contains 10 segments in adults with the caudal segment containing a dorsal sensilium that detects vibrations and temperature changes for host detection). The flea body may be light yellow, yellowish black, brown black, or jet-black in color.

Fleas have four phases of development: the egg, the free-living larva, the pupa, and the adult. Eggs are deposited in debris such as nests, burrows, cracks, or crevices (sometimes they are deposited on the host where the eggs typically fall to the ground) (Figure 6-80). Eggs hatch in about 5 days depending on species, temperature, and humidity. A legless, wormlike larva with a small blackish eyeless head containing small antennae emerges from the egg. Larvae are very active, avoid light, cannot tolerate humidity extremes, and seek protection in cracks, crevices, burrows, and nest debris. Larvae feed on organic debris with the larval stage lasting approximately 2 to 3 weeks. There are typically three larval instars (stages). At the end of the larval period the larva empties the alimentary canal and spins a white silken cocoon from silk produced by its salivary glands. After 2 to 3 days in the cocoon the larva pupates. Adults emerge from the pupa after approximately 5 to 14 days depending on temperature. The adult flea requires a stimulus (vibrations or chemicals) to cause it to escape from the cocoon. Adult fleas avoid light and are therefore found among animal hair or feathers or on people's clothing (Figure 6-81).

[FIGURE 6-80 OMITTED]

[FIGURE 6-81 OMITTED]

Fleas are attracted by vibrations, sound, or chemical signals. Most species of fleas have preferred hosts, but they are not entirely host-specific. Typically people are not attacked by fleas if there is a more suitable host available. Fleas move by jumping (fleas can jump up to 150 times their body length). Ideally fleas feed daily for 2 to 15 minutes, but may survive for months between feedings. Fleas are capillary feeders that produce bites frequently occurring in groups or on a line. The bites are pruritic within a few minutes. Hypersensitivity reactions may occur and severe itching may produce large papules. Secondary infections may also occur as a result of intense scratching. Flea bites can be treated with anti-itch creams or systemic antipruritic drugs such as antihistamines or corticosteroids. Antibiotics may be used to treat secondary infections. Fleas and flea bites can be prevented by avoiding dark places where fleas prefer to live, by controlling fleas on animals, and environmental hygiene. Zoonoses caused by fleas are summarized in Table 6-16.
Both sexes of fleas take blood meals
and are equally important a vectors of
disease.


Heteroptera

The suborder Heteroptera is a diverse insect group with approximately 30,000 species. The name Heteroptera is derived from the Greek hetero meaning different and ptera meaning wings referring to the difference in texture of the front wings (leathery) from those at the apex (membranous). At rest, these wings cross over one another to lie flat along the insect's back. Heteroptera insects also have elongate, piercing mouthparts, which arise from either the ventral (hypognathous) or anterior (prognathous) part of the head. Antennae are slender with 4 to 5 segments. The mandibles and maxillae interlock with one another and are long and thread-like to form a flexible feeding tube (proboscis). The immature forms look similar to the adults and always are wingless. Heteroptera insects live in a broad range of habitats including land, water, and semi-aquatic environments. Bed bugs, members of the ectoparasites family Cimicidae, live as ectoparasites on birds and mammals (including humans) (Figure 6-82). Triatoma bugs, members of the family Reduviidae, colonize habitats built by people or nests of animals (Figure 6-83).

[FIGURE 6-82 OMITTED]

[FIGURE 6-83 OMITTED]

Blood-sucking Heteroptera are attracted by warm temperature and CO2. Blood is required by all stages for molting and egg deposition. Eggs are laid in bunches in cracks and crevices and hatch after about 8 to 15 days. Nymphs emerge from the eggs depending on environmental temperature. Nymphs are similar in appearance to the adults and feed on blood for approximately 10 minutes. There are five nymph stages and the nymphal period lasts 2 to 7 weeks. Adults can survive for up to 4 years and more than 1 year without a bloodmeal (bedbugs) or 4 to 6 months without a bloodmeal (Triatomine bugs). The zoonoses caused by Heteroptera (bugs) are summarized in Table 6-17.
Both sexes of bedbug and kissing
bug take blood meals and are equally
important as pests.


Ticks

Ticks are covered in chapter 4.

Mites

Acariasis is disease caused by a variety of mites. Mites are in the Acari subclass in the Arachnidea class of arthropods that contain approximately 30,000 species worldwide (Figure 6-84). Acari is derived from the Greek word akari meaning mite. Mites are not insects and are small with six legs as larvae and eight legs as nymphs and adults. Mites are wingless, lack antennae, and usually have flat or round bodies. There are numerous free-living, ectoparasitic and endoparasitic species.

[FIGURE 6-84 OMITTED]

Female mites deposit eggs either on the host or in the environment. Eggs hatch into larvae and then typically pass through two nymphal stages before becoming adults. All mites causing acariasis are transmitted by direct contact. Indirect transmission via fomites is important in some, but not all, species of mites (Figure 6-85).

[FIGURE 6-85 OMITTED]

[FIGURE 6-86 OMITTED]

All acariases are highly contagious for susceptible species. Close contact may be needed to spread mites if their numbers are low. Scabies is the classic mite disease whose name was derived from the Latin word for itch (scabies) and the verb scabere meaning to scratch (Figure 6-86). Zoonotic mites do not reproduce on people; therefore, the clinical signs they produce are self-limiting. Disease in people tends to be temporary (pruritus and discomfort) and are not fatal. The zoonoses caused by mites are summarized in Table 6-18.

Lice

Louse infestation is prevalent throughout the animal kingdom. Lice are divided into two orders: Mallophaga (chewing lice, which are common in birds and domestic animals with humans being accidental hosts) and Anoplura (sucking lice, which only parasitize mammals with the three types of human lice belonging to this group). Examples of Mallophaga lice include Menopon gallinae (fowl louse), Menacanthus stramineus (chicken and turkey louse), Bovicola spp. (cattle, horses, sheep, and goat species), Trichodectes canis (dog louse), and Felicola subrostratus (cat louse). Mallophaga lice are significant pests of animals and birds and may rarely cause short-term irritation to humans (animal lice may live for only a few hours on humans). The three types of human lice are in the Anoplura order producing disease called pediculosis because of the genera of lice involved (Pediculus humanus capitis (head louse), Pediculus humanus corporis (body louse), and Pthirus pubis (crab louse). These lice are not zoonotic, but may rarely be transmitted to animals (particularly swine). Lice can transmit disease such as trench fever (Ba. qunitana), relapsing fever (Bo. recurrentis), and epidemic typhus (Rickettsia prowazekii) (Figure 6-87).
Demodex mites cause significant disease
in puppies (spread from bitch to nursing
pups during the first days of life), but
are not zoonotic.


[FIGURE 6-87 OMITTED]

Lice are permanent ectoparasites (spend their entire life on the host) and are highly host specific. Lice eggs are called nits and are glued to the host hair (Figure 6-1A). The egg stage lasts approximately 5 to 10 days (longer if the eggs are on clothing or are in a cool environment). The louse hatches from the egg as a nymph and passes through three nymphal instars (stages) before becoming an adult. They nymph stage lasts from 7 to 12 days and is also temperature dependant. The entire louse life cycle is approximately 2 to 3 weeks.
Lice are highly host specific and
lice found on animals do not cause
pediculosis in people.


Summary

The phylum Arthropoda contains over 80% of all animal species making it the largest group of living organisms. Arthropods are characterized by jointed chitinous exoskeletons, segmented bodies, and jointed appendages. Arthropods have a true coelom, a small brain, an extensive nervous system, and separate sexes. The phylum Arthropoda contains three classes: insects, arachnids, and crustaceans. Arthropods may affect human and animal health directly (by bites, stings, or infestation of tissues) or indirectly as vectors, definitive hosts, and intermediate hosts (disease transmission). Arthropods in the order Diptera are commonly known as true flies and include insects such as mosquitoes, blackflies or gnats, midges, sand flies, tsetse flies and house flies. Diptera adults cause zoonotic disease by serving as vectors of a variety of diseases. Myiasis is the infestation of live vertebrate animals with dipteran larvae (first-stage that develop into third-stage) that feed on the host's dead or living tissue. The adult flies are not parasitic, but when they lay their eggs in open wounds the eggs hatch into their larval stage (maggots or grubs), the larvae feed on live and/or necrotic tissue, causing myiasis to develop. Most myiasis-causing flies belong to one of three major families: Oestridae (botflies), Sarcophagidae (fleshflies), or Calliphoridae (blowflies). Fleas belong to the order Siphonaptera, one of the major groups of blood-sucking insects with more than 2,500 species and subspecies of fleas. Adult fleas are obligatory hematophages that parasitize warm-blooded animals (94% parasitize mammals and 6% parasitize birds). Most species of fleas have preferred hosts, but they are not entirely host-specific. Typically people are not attacked by fleas if there is a more suitable host available. The suborder Heteroptera is a diverse insect group that includes bed bugs (that live as ectoparasites on birds and mammals) and Triatoma bugs (that colonize habitats built by people or nests of animals). Acariasis is disease caused by a variety of mites. Mites are in the Acari subclass in the Arachnidea class of arthropods that contain approximately 30,000 species worldwide. All acariases are highly contagious for susceptible species. Close contact may be needed to spread mites if their numbers are low. Scabies is the classic mite disease. Zoonotic mites do not reproduce on people; therefore, the clinical signs they produce are self-limiting. Louse infestation is prevalent in animals. Lice are divided into two orders: Mallophaga (chewing lice, which are common in birds and domestic animals with humans being accidental hosts) and Anoplura (sucking lice, which only parasitize mammals with the three types of human lice belonging to this group). Lice do not produce zoonotic disease. Lice rarely cause skin irritation in people (lice can only live a few hours on a person's skin and lice may rarely be transmitted to animals). Lice are permanent ectoparasites (spend their entire life on the host) and are highly host specific.

Review Questions

Multiple Choice

1. Parasites are organisms that

a. provide nourishment and protection to other living organisms.

b. are found on the exterior of the host.

c. live independently until they are exposed to an optimal host.

d. live in association with, and at the expense of, other organisms.

2. Protozoa are types of parasites that

a. are unicellular, eukaryotic organisms.

b. cannot make their own food.

c. are found typically in moist environments.

d. all of the above.

3. Helminths are

a. unicellular worms that cannot make their own food.

b. divided into groups called Platyhelminthes (flatworms) and Nematoda (roundworms).

c. parasites typically found in moist environments outside of an animal's body.

d. all of the above.

4. Protozoa are classified based upon their

a. movement.

b. digestive system.

c. segmented body.

d. type of nutritional status.

5. Trematodes are commonly referred to as -- and cestodes are commonly referred to as --.

a. flukes, roundworms

b. roundworms, tapeworms

c. flukes, tapeworms

d. tapeworms, roundworms

6. What two key evolutionary adaptations do arthropods possess?

a. jointed appendages and an exoskeleton

b. jointed appendages and wings

c. wings and an exoskeleton

d. antennae and wings

7. Instar is a term used in arthropods to describe a/an

a. egg.

b. nymph.

c. larval stage.

d. adult insect.

8. What type of parasitic life cycle involves a definitive host and one or more intermediate hosts?

a. direct

b. indirect

c. vertical

d. horizontal

9. What best defines a reservoir host?

a. A reservoir host is the host in which asexual reproduction of the parasite occurs.

b. All hosts in the parasitic life cycle are reservoir hosts.

c. A reservoir host is an organism that harbors the same stage of the parasite that is found in humans.

d. A reservoir host is an organism that accidentally becomes infected with a parasite that normally infects another species.

10. A stool sample is collected from a patient suspected of having an En. histolytic infection. The stool sample is liquid. What stage of the life cycle of Entamoeba is most likely to be observed in the specimen?

a. cyst

b. trophozoite

c. sporozoite

d. tryptomastigote

11. A protozoan cyst measuring 15 [micro]m is observed on examination of a concentrated fecal specimen. It has four nuclei, with even peripheral chromatin, a central small endosome, and rounded chromatin bars and ingested erythrocytes in the cytoplasm. The organism is most likely

a. Endolimax nana.

b. Entamoeba coli.

c. Iodamoeba butschlii.

d. Entamoeba histolytica.

12. This organism causes severe, chronic, and potentially fatal diarrhea in immunocompromised people. It also produces mild to moderate diarrhea in calves. The disease in humans usually results from drinking contaminated water. This organism is identified in fecal samples by using the acid-fast staining method. The organism is

a. Pneumocystis carinii.

b. Cryptosporidium parvum.

c. Giardia duodenalis.

d. Toxoplasma gondii.

13. People involved in what occupation are most likely to become infected with Balantidium coli?

a. sewage workers

b. beef ranchers

c. nurses

d. pork ranchers

14. The tsetse fly (Glossina) serves as the vector for

a. leishmaniasis.

b. African sleeping sickness.

c. Chagas' disease.

d. filariasis.

15. The definitive host for Toxoplasma gondii is the

a. dog.

b. cat.

c. pig.

d. flea.

16. What parasite has the greatest risk to unborn human fetuses?

a. En. coli

b. G. lamblia

c. Cry. parvum

d. T. gondii

17. An infection with this sporozoan is characterized by the gradual onset of headache, chills, sweating, and fatigue following a tick bite. The disease is usually self-limiting and is caused by

a. Babesia microti.

b. Borellia burgdorferi.

c. Colorado tick fever parasite.

d. Trypanosoma brucei.

18. Heavy infections with -- nematode) can cause rectal prolapse (especially in children).

a. Taenia saginata

b. Trichuris trichiura

c. Angiostrongylus cantonensis

d. Paracapillaria philippinensis

19. The infective stage of Strongyloides stercoralis is the

a. embryonated egg.

b. rhabditiform larva.

c. filariform larva.

d. adult worm.

20. The most common agent of cysticercosis is

a. Diphyllobothrium latum.

b. Ascaris lumbricoides.

c. Trichuris trichiura.

d. Taenia solium.

21. Human infection with Di. latum results from consuming infected

a. freshwater fish.

b. saltwater fish.

c. pork.

d. snails.

22. Cystic hydatid disease is caused by

a. Echinococcus granulosus.

b. Taenia solium.

c. Taenia saginata.

d. Schistosoma mansoni.

23. A 7-year-old girl is small for her age and anemic. She has experienced alternating bouts of diarrhea and dysentery for at least 6 months. A fecal examination reveals football-shaped eggs with polar plugs at both ends. How was the child most likely infected?

a. By eating raw fish

b. By eating contaminated snails

c. Oral-fecal route

d. By playing in contaminated soil

24. You are an epidemiologist working for the WHO in charge of investigating cystic hydatid disease in Spain. Your most effective advice to the local authorities to help prevent the spread of this disease to humans would include which of the following? (1) To increase the chlorine content of local drinking water; (2) To thoroughly educate the public on personal hygiene measures; (3) To stop the feeding of animal organs to other domestic animals; (4) To trap and sample wild carnivores for disease, then to recommend extensive hunting of carnivores if they carry the disease.

a. 1, 2

b. 3, 4

c. 1, 4

d. 2, 3

25. Which parasites are commonly associated with domestic dogs? (1) Toxocara canis; (2) Dirofilaria spp.; (3) Ancylostoma caninum; (4) Echinococcus granulosus.

a. 1, 2, 3

b. 2, 3, 4

c. 1, 3, 4

d. 1, 2, 3, 4

26. The parasites that cause cutaneous larva migrans

a. cause diarrhea in animals and skin lesions in people.

b. include Ancylostoma and Strongyloides.

c. can be transmitted transplacentally and through colostrum/milk in animals.

d. all of the above.

27. Visceral larva migrans

a. tend to cause organ and ocular disease.

b. is as a result of organisms such as Toxocara, Baylisascaris, and Ascaris.

c. can cause gastrointestinal disease in animals or animals may be asymptomatic.

d. all of the above.

28. Infection with Diptera larvae is called

a. mange.

b. myiasis.

c. taeniasis.

d. infestation.

29. What is very host specific?

a. fleas

b. lice

c. mosquitoes

d. flies

30. Arthropods cause disease by what method?

a. biological transmission

b. mechanical transmission

c. irritation from bites and stings

d. all of the above
Matching

31. -- Echinococcus granulosus       A. hookworm causing CLM
32. -- Dracunculus medinensis        B. protozoan that parasitizes
                                        erythrocytes causing anemia

33. -- Taenia solium                 C. hemoflagellate transmitted by
                                        sandflies

34. -- Ancyclostoma caninum          D. cystic hydatid cyst

35. -- Entamoeba histolytica         E. alveolar hydatid cyst

36. -- Babesia microti               F. cestode acquired from eating
                                        fish

37. -- Cryptosporidium parvum        G. Guinea worm larvae released
                                        from skin blisters

38. -- Giardia duodenalis            H. fluke that may cause liver
                                        disease in sheep and cattle
                                        and abdominal pain and fever
                                        in humans

39. -- Leishmania spp.               I. burrowing mites that tunnel
                                        in skin

40. -- Toxoplasma gondii             J. avian schistosomes that may
                                        cause cercarial dermatitis

41. -- Trypanosoma cruzi             K. cysticercus

42. -- Trypanosoma brucei            L. protozoan causing amebic
                                        colitis

43. -- Diphyllobothrium latum        M. common protozoan in calves
                                        that is typically transmitted
                                        to people via contaminated
                                        water

44. -- Echinococcus multilocularis   N. flagellated protozoan that may
                                        cause chronic diarrhea in
                                        animals and humans

45. -- Trichinella spiralis          O. mite that causes walking
                                        dandruff in animals and a
                                        mild pruritic dermatitis
                                        in people

46. -- Trichuris trichiura           P. coccidian protozoan that can
                                        cause congenital defects in
                                        humans if the mother contracts
                                        the disease while pregnant

47. -- Fasciola hepatica             Q. flagellate that may cause
                                        myocarditis in puppies and
                                        children

48. -- Trichobilharzia and           R. flagellate that may cause
       Microbilharzia                   neurological disease in
                                        animals and sleeping sickness
                                        in humans

49. -- Sarcoptes scabiei             S. nematode acquired by ingestion
                                        of encysted larvae in muscle
                                        tissue (especially pork)

50. -- Cheyletiella spp.             T. whipworm that may cause
                                        diarrhea and anemia in
                                        dogs and rectal prolapse
                                        in children


Case Studies

51. A 4-year-old boy was taken to his physician for evaluation of diarrhea. One month prior to the examination, he had been having chronic abdominal pain and gas to frank diarrhea for the preceding 1 to 2 weeks. On the morning of the examination, the boy's diarrhea was yellow, foul-smelling, and frothy. Stool was collected for culture and ova and parasite examination. After 48 hours, the stool culture was reported as negative for enteric pathogens and the ova and parasite examination was positive for pear-shaped trophozoites that were 12 to 15 [micro]m long with two bilateral nuclei.

a. What parasite was found in the stool specimen?

b. How is this parasite transmitted?

c. How can infection with this organism be prevented?

52. A 10-year-old boy was admitted to the hospital with fever of unknown orgin and nasal discharge of 1-week duration. On numerous occasions he had been observed eating dirt in the yard. The family has no house pets; however, the family raised pigs on their land for their own use. On admission the boy was pale with a temperature of 101[degrees]F, and had a slightly enlarged spleen. His hemoglobin was 7.2 g/dl, WBC count was 32,000/[micro]l, with 64% eosinophils. Radiographs of the chest showed abnormal shadows in the right cranial and caudal lobes. A tentative diagnosis was made of iron deficiency anemia and possible visceral larva migrans. He was treated and sent home. Three weeks later the boy was readmitted in a semicomatose state with a 10-day history of shaking spells and ataxia. His WBC count was 9,100/[micro]l with 4% eosinophils. The boy developed convulsions that were difficult to control and he died 3 weeks later (7 weeks following his first admission). On autopsy granulomatous lesions were found in the liver, heart, and brain. The liver and heart lesions were considered to be chronic, but the brain lesions appeared to be more recent in origin.

a. What do you think is the most likely causative agent?

b. Explain why this boy had such severe clinical symptoms.

53. A 20-year-old man developed a rash on his forearm that progressed to vesicles and crusts. The skin lesions were pruritic and he was developing breaks in the skin as a result of his itching of the lesions. This man had recently purchased two hunting dogs that also have pruritis. A skin scraping was taken and an oval mite that had eight legs was found.

a. What parasite do you think this person has?

b. How did this person acquire the infection?

c. How is this disease treated in both animals and people?

d. What should be done with the animals until they clear the infection?

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Table 6-12 Morphology of the Three Most Common Schistosomes in People

Schistosoma
Species           Papillae         Size

Sc. Haematobium   Small            Male: 10 to 15 mm x 0.8 to 1.0 mm
                  tubercles        Female: 20 mm x 0.25 mm

Sc. mansoni       Large papillae   Male: 10 to 15 mm x 0.8 to 1.0 mm
                  with spines      Female: 20 mm x 0.25 mm

Sc. japonicum     Smooth           Male: 12 to 20 mm x 0.5 mm
                                   Female: 26 mm x 0.3 mm

Schistosoma       Number
Species           of Testes   Ovary Position   Uterus

Sc. Haematobium   4 to 5      Midbody          Averages 50 eggs

Sc. mansoni       6 to 9      Anterior half    Short; few eggs at
                                               a time

Sc. japonicum     7           Posterior to     Long; may contain
                              midbody          up to 300 eggs

Schistosoma
Species           Egg

Sc. Haematobium   Elliptical with sharp
                  terminal spine

Sc. mansoni       Elliptical with sharp
                  lateral spine

Sc. japonicum     Oval to spherical;
                  rudimentary lateral spine

Table 6-13 Less Common Zoonotic Trematodes

                                    Type of          Predominant
Trematode         Disease           Trematode        Signs in People

Clonorchis        Clonorchiasis     Liver fluke      Minor infection is
senensis          First discov-     Transparent,     common and may
(named for        ered in the       lancet-          be asymptomatic.
its branched      bile ducts        shaped fluke     Inflammation
testicles;        of a Chinese      that is 1 to     of the bile ducts
klon is Greek     carpenter in      5 mm by 8        with the severity
for branch,       Calcutta in       to 25 mm. It     depending on
orchis is Greek   1875              has paired       the number of
for testicle,                       testes in the    flukes and the
and sinos for                       posterior        duration of their
oriental); also                     part of its      persistence.
known as the                        body.            Symptoms
Chinese liver                       Eggs             include fatigue,
fluke                               contain a        anorexia,
                                    well-            jaundice, and
                                    developed        gastrointestinal
                                    miracidium.      problems.
                                                     Persistent
                                                     infections
                                                     produce liver
                                                     cirrhosis,
                                                     hepatomegaly,
                                                     edema, and
                                                     ascites.

Heterophyes       Heterophyiasis    Intestinal       People are
heterophyes       Also known as     fluke            typically
                  metagonimiasis    Small flukes     asymptomatic;
                  or intestinal     that are less    only rarely do
                  dwarf fluke       than 1 mm in     people show
                  infection         size             gastrointesti-
                                                     nal symptoms
                                                     (diarrhea and
                                                     abdominal
                                                     pain) and even
                                                     rarer do they
                                                     show extrain-
                                                     testinal signs
                                                     (infarcts to
                                                     the heart and
                                                     CNS)

Opisthorchis      Opisthorchiasis   Liver fluke      Liver disease
felineus;                           Eggs are small   similar to that
Opisthorchis                        (15 to 30        seen with
viverrini                           [micro]m),       clonorhchiasis:
                                    operculated,     cholangitis
                                    and yellow-      and in later
                                    brown in         stages liver
                                    color.           cirrhosis and
                                                     pancreatitis

Paragonimus       Paragon-          Lung fluke       Migrating
westermani        imiasis,          Eggs are         parasites cause
and eight         pulmonary         60 to 90         inflammation of
other known       distomatosis      [micro]m         the peritoneum,
species                             and are          pleura, and lungs
                                    operculated      producing cough-
                                                     ing, fever, and
                                                     chest pain

                  Clinical Signs
Trematode         in Animals          Transmission

Clonorchis        * Dogs and          People are
senensis          cats are res-       infected by
(named for        ervoir hosts        ingesting raw
its branched      * Clinical signs    or undercooked
testicles;        in dogs and         freshwater fish
klon is Greek     cats include        (mainly carp).
for branch,       biliary             Feces of infect-
orchis is Greek   disease and         ed vertebrates
for testicle,     pancreatitis        containing eggs
and sinos for     because of          can contami-
oriental); also   pancreatitic        nate surface
known as the      duct obstruc-       water.
Chinese liver     tion; carci-
fluke             nomas of
                  the bile and
                  pancreatic
                  ducts have
                  been seen
                  in chronic
                  cases.

Heterophyes       * Main              Ingestions of
heterophyes         hosts are         raw or under-
                    piscivorous       cooked fish
                    birds and         (second inter-
                    mammals           mediate hosts)
                    such are          that contain
                    dogs and          metacercariae
                    cats that
                    are usually
                    asymptom-
                    atic

Opisthorchis      * Mild to           Ingestion
felineus;           severe            of raw or
Opisthorchis        fibrosis of       undercooked
viverrini           the bile          freshwater
                    ducts and         fish containing
                    gallbladder       metacercariae

Paragonimus       * Chronic,          Ingestion of
westermani          deep,             undercooked
and eight           intermittent      meat of fresh-
other known         cough, and        water crabs and
species             lethargy in       other crusta-
                    dogs, cats,       ceans
                    and other
                    animals

                  Animal              Geographic
Trematode         Source              Distribution

Clonorchis        Snails are the      * Endemic
senensis          first intermedi-    and widely
(named for        ate host.           distributed
its branched      Freshwater          in Japan,
testicles;        fish such as        Korea,
klon is Greek     carp are the        China,
for branch,       second inter-       Taiwan, and
orchis is Greek   mediate host.       Vietnam
for testicle,     Dogs, cats,         * Prevalence
and sinos for     and wild            rates in
oriental); also   fish-eating         humans in
known as the      mammals             these areas
Chinese liver     are reservoir       is up to
fluke             hosts.              50%

Heterophyes       Spores are          * Worldwide,
heterophyes       common in the         but found
                  environment           predomi-
                                        nantly in
                                        tropical
                                        climates of
                                        Southeast
                                        and East
                                        Asia,
                                        Northern
                                        Africa, and
                                        the Middle
                                        East

Opisthorchis      Snails are the      * Worldwide
felineus;         first intermedi-      with
Opisthorchis      ate host.             prevalence
viverrini         Fish are the          in Central
                  second inter-         Europe
                  mediate host.
                  Dogs, cats,
                  and wild
                  fish-eating
                  mammals
                  are reservoir
                  hosts.

Paragonimus       Snails are the      * Central,
westermani        first intermedi-      Southeast,
and eight         ate host.             and East
other known       Crustaceans           Asia; China;
species           are the second        Japan; Far
                  intermediate          East
                  host.               * Paragonimus
                  Dogs, cats,           kellicotti
                  and wild              occurs main-
                  fish-eating           ly in North
                  mammals               America
                  are reservoir
                  hosts.

Trematode         Pathology

Clonorchis        Infection occurs when an animal
senensis          or person eats raw or partially
(named for        cooked freshwater fish or dried,
its branched      salted, or pickled fish infected
testicles;        with the metacercariae (the devel-
klon is Greek     opmental form in encapsulated
for branch,       cercariae). In the duodenum, the
orchis is Greek   cyst is digested and an immature
for testicle,     larva is released. The larva enters
and sinos for     the biliary duct, where it devel-
oriental); also   ops and matures into an adult
known as the      worm. The adult worm feeds on
Chinese liver     the mucosal secretions and lays
fluke             fully embryonated operculated
                  eggs (which are excreted in the
                  feces). Upon reaching freshwater
                  and upon ingestion by a suitable
                  species of snail (first intermediate
                  host), the eggs hatch to produce
                  a miracidium. Inside the snail,
                  the miracidia multiply asexually
                  through a single generation of
                  sporocysts and two generations of
                  rediae to fork-tailed cercariae.
                  The cercariae escape from the snail
                  to the water and penetrate under
                  scales of freshwater fish (second
                  intermediate host). In the fish, the
                  cercariae lose their tails and encyst
                  in the scale or muscle of the fish
                  to the metacercariae (the form,
                  which is infectious to humans
                  and animals). When ingested, the
                  infected fish cause infection in
                  humans and animals.

Heterophyes       Eggs are shed with feces and are
heterophyes       taken up by freshwater snails.
                  Cercariae emerge from the snails,
                  invade freshwater fish, and encyst
                  to metacercariae. After ingestion
                  by the final host they are released
                  in the intestinal tract, enter the
                  mucosa, and develop into adult
                  flukes in 1 to 2 weeks.

Opisthorchis      Eggs are released with the mira-
felineus;         cidia (larvae) infected feces of
Opisthorchis      infected hosts. Miracidia must
viverrini         reach water for further develop-
                  ment; in water they are ingested
                  by snails (first intermediate host).
                  In the snails several asexual repli-
                  cations take place producing cer-
                  cariae. Cercariae leave the snails
                  and enter fish (second intermedi-
                  ate host). In the fish the parasites
                  encyst to become metacercariae.
                  Metacercariae are ingested by
                  suitable hosts where young flukes
                  are released into the duodenum.
                  From the duodenum they migrate
                  to the bile ducts, where they
                  mature to adults.

Paragonimus       Adult parasites live in pairs in cysts
westermani        in the connective tissue of lung.
and eight         Eggs are coughed up, swallowed,
other known       and excreted in feces. Eggs reach
species           water for further development
                  to miracidia. Miracidia hatch and
                  enter snails (first intermediate
                  host). After asexual replication,
                  cercariae develop and leave the
                  snails to enter crustaceans (second
                  intermediate host). In crustaceans
                  the parasite encysts in muscles
                  and internal organs. The final
                  hosts (humans and other mam-
                  mals) ingest the contaminated
                  meat, young flukes are liberated
                  in the gastrointestinal tract, pen-
                  etrate the intestinal mucosa, and
                  migrate to the peritoneal cavity.
                  In the lung the parasites become
                  encapsulated by connective tissue
                  and begin egg laying.

                  Diagnosis           Treatment
Trematode
                  Identification      Praziquantel or
Clonorchis        of eggs in          albendazole.
senensis          feces and           Control of the
(named for        duodenal            disease can be
its branched      secretions.         achieved by
testicles;        Concentration       cooking fish
klon is Greek     techniques          or the use of
for branch,       (such as            gamma irradia-
orchis is Greek   formol-ether        tion of fish to
for testicle,     sedimenta-          kill the meta-
and sinos for     tion) may be        cercariae. Fecal
oriental); also   needed due to       contamination
known as the      the small size      of water should
Chinese liver     of the eggs.        also be avoided.
fluke             Serologic tests
                  are available
                  in people.

                  Microscopic         Praziquantel or
Heterophyes       examination         nicolsamide
heterophyes       of operculated,     Disease can be
                  brownish eggs       prevented by
                  of 15 to 30         avoiding con-
                  [micro]m in stool   sumption of raw
                                      or undercooked
                                      fish.

                  Direct detec-       Praziquantel
Opisthorchis      tion in feces       Disease can be
felineus;         using zinc sul-     controlled by
Opisthorchis      fate floatation.    eating properly
viverrini         Serologic tests     cooked fish or
                  are available.      fish that have
                                      been irradiated.

                  Direct detec-       Praziquantel or
Paragonimus       tion in sputum      triclabendazole
westermani        and occasion-       Surgery may be
and eight         ally feces          needed in some
other known       using zinc sul-     cases.
species           fate floatation.    Disease can be
                  Serologic tests     controlled by
                  are available.      eating properly
                                      cooked
                                      crustaceans.

Table 6-14 Zoonotic Diseases Caused by Diptera Adult Arthropods

Diptera
Suborder       Scientific Name            Features

Nematocera     * Culicidae mosquitoes     * Mosquitoes need stagnant
                 (Anopheles, Culex,         or slowly moving water
                  Aedes)                    for breeding
                 (Figure 6-76)            * Eggs may be deposited on
                                            damp soil or vegetation,
                                            tree holes, containers,
                                            pools, swamps, or directly
                                            in water.

               * Phlebotomidae sand       * Sand flies are hairy
                 flies (Phlebotomus)        insects with long legs
                 (Figure 6-74)              and are inactive during
                                            the day.

               * Simuliidae blackflies    * Blackflies need running
                 (Simulium, Odagmia)        water for breeding sites
                 (Figure 6-75)              (breeding occurs in
                                            temperate zones in later
                                            spring and early summer).
                                          * Blackflies fly long
                                            distances and attack
                                            during the day in the
                                            open, but will not enter
                                            dwellings.
               * Ceratopogonidae          * Midges are small and lay
                 midges (Culicoides)        eggs in damp places
                 (Figure 6-77)              (edge of ponds and
                                            swamps).

Brachycera     * Tabanidae (horseflies)   * Blood-sucking insects
                 arthropods (Tabanus,     * Females bite and feed on
                 Haematopota, Chrysops)     warm-blooded animals
                                          * Most live near waters
                                          * Strong fliers
                                          * Breed in moist earth or
                                            leaf mold

Cyclorrhapha   * Glossinidae arthropods   * Glossina are found only
                 (Glossina)                 in Africa south of the
                                            Sahara (other Glossinidae
                                            are found worldwide).
                                          * Many species of Glossina
                                            are bloodsucking, but
                                            only a few attack
                                            humans.

Diptera        Disease
Suborder       Transmitted         Bite Reaction      Prevention

Nematocera     * Equine            * Bites cause      * Insect
                 encephalitis,       local signs        repellent
                 dengue fever,       such             * Long pants and
                 yellow fever,       as swelling,       sleeves
                 and a variety       pruritus,        * Bed netting
                 of viral            pain, and        * Remove water
                 diseases            erythema.          source for
               * Leishmaniasis                          breeding
               * Onchocerciasis                       * Insecticides
               * Loiasis                                for buildings

Brachycera     * Tularemia         * Bites cause      * As above
               * Trypanosomiasis     relatively       * Avoid infested
                                     large, painful     areas during
                                     bite wounds        the daytime
                                     and secondary
                                     hemorrhages

Cyclorrhapha   * Trypanosomiasis   * Bites are not    * As above
                                     immediately      * Avoid infested
                                     recognized but     areas during
                                     in time become     the daytime
                                     tender, hard
                                     swellings;
                                     some may
                                     produce
                                     anaphylactic
                                     reaction

Note: Mosquitoes and flies find their hosts by chemical signals such
as C[O.sub.2] or valerianic acid in sweat.

Table 6-15 Zoonotic Diseases Caused by Diptera Larvae

Type of Myiasis              Scientific Name

Dermal, cutaneous, or        * Cordylobia anthropophaga (Mango fly)
subdermal myiasis            * Cordylobia rodhaini (New World
                               screwworm)
                             * Cochliomyia hominivorax (New World
                               screwworm)
                             * Chrysomya bezziana (Old World
                               screwworm)
                             * Dermatobia hominis (human botfly)

Wound myiasis                * Lucilia sericata and Lucilia cuprina
                             * Sarcophaga spp. (fleshflies)
                             * Calliphora spp. (blowflies)

Ophthalmomyiasis             * Oestrus ovis (botflies)
(ocular myiasis)             * Rhinoestrus purpurea
                             * Wohlfahrita magnifica (Old World
                               fleshflies)
                             * Wohlfahrita nuba

Creeping myiasis             * Gasterophilus spp. (horse botflies)
                             * Hypoderma spp. (cattle botflies)

Sanguinivorous myiasis       * Auchmeromia luteola (Congo floor maggot)

Auricular myiasis            * Wohlfahrtia spp.
                             * Cochliomyia hominivorax
                             * Musca domestica

Urogenital and rectal        * Calliphora spp. (bluebottle flies)
myiasis                      * Fannia spp.
                             * Musca domestica

Gastrointestinal myia-       * Calliphora spp.
sis, enteric myiasis, or     * Sarcophaga spp.
pseudomyiasis                * Fannia spp.
                             * Musca domestica

Type of Myiasis              Animal Host

Dermal, cutaneous, or        * Dogs (Africa)
subdermal myiasis            * Rodents (Africa)
                             * Cattle (Central and South
                               America)
                             * Cattle (Africa, Asia, and Middle
                               East)
                             * Cattle mainly; other mammals
                               (Central and South America)

Wound myiasis                * Sheep (worldwide)
                             * Facultative myiasis organism
                               (worldwide)
                             * Facultative myiasis organism
                               (worldwide)

Ophthalmomyiasis             * Sheep (worldwide)
(ocular myiasis)             * Equine (Europe, Africa, and Asia)
                             * Rats and rabbits (Mediterranean)
                             * Camels (Africa to Pakistan)

Creeping myiasis             * Equine (worldwide)
                             * Cattle (worldwide)

Sanguinivorous myiasis       * Pigs (Africa)

Auricular myiasis            * Rodents (worldwide)
                             * Cattle (Central and South America)
                             * Facultative myiasis organism
                               (worldwide)

Urogenital and rectal        * Sheep; many animal species;
myiasis                        facultative myiasis organism
                               (worldwide)
                             * Facultative myiasis organism
                               (worldwide)
                             * Facultative myiasis organism
                               (worldwide)

Gastrointestinal myia-       * Many animals (worldwide)
sis, enteric myiasis, or     * Facultative myiasis organism
pseudomyiasis                  (worldwide)
                             * Facultative myiasis organism
                               (worldwide)
                             * Facultative myiasis organism
                               (worldwide)

Type of Myiasis              Features

Dermal, cutaneous, or        * Larvae burrow several centimeters deep
subdermal myiasis              causing subdermal lesions on various
                               parts of the body. Larvae develop from
                               first to third stage in 5 to 8 days.

                             * Larvae cause painful lesions in skin
                               typically on head, back, abdomen, arm,
                               thigh, axilla, or orbit of eye.

                             * Larvae feed for 4 to 12 weeks on host.

Wound myiasis                * Adults deposit eggs (Sarcophaga
                               spp. are larviparous and deposit first
                               stage larvae) into wounds and under
                               bandages where larvae stage for
                               approximately 5 days during which time
                               they feed on necrotic tissue and secrete
                               bacteriostatic chemicals.
                             * May be used therapeutically.

Ophthalmomyiasis             * Adult flies deposit eggs or larvae on
(ocular myiasis)               eyelids, in the conjunctival sac, or on
                               adjacent skin.
                             * Clinical signs appear quickly producing
                               pain and irritation.
                             * Clinical signs range from conjunctivitis
                               to anterior uveitis.
                             * I* natural hosts maggots invade the
                               nasopharynx.

Creeping myiasis             * Hatched larvae penetrate the subcuta-
                               neous tissue and migrate.
                             * In definitive hosts larvae migrate
                               extensively, develop in subcutaneous
                               boils (cattle botflies) or in the
                               gastrointestinal tract (equine
                               botflies) after being licked
                               off fur by host.
                             * In humans, larvae penetrate skin, cause
                               swelling, and migrate in the epidermis
                               where they persist for long periods of
                               time.

Sanguinivorous myiasis       * Larvae cause skin lesions and feed on
                               blood (mainly at night)
                             * Larvae repeatedly attack requiring 5 to
                               20 blood meals for development during
                               a 10-week period

Auricular myiasis            * Larvae burrow several centimeters deep
                               causing subdermal disfiguring lesions
                               and discharge in the auditory canal and
                               nasal cavity (may extend to bone, carti-
                               lage, and brain)

Urogenital and rectal        * Larvae invade the vagina, ureter, or
myiasis                        rectum

Gastrointestinal myia-       * Eggs or larvae are ingested with con-
sis, enteric myiasis, or       taminated food
pseudomyiasis                * Larvae survive transport through the
                               gastrointestinal tract causing
                               irritation, intestinal pain, vomiting,
                               and diarrhea
                             * Larvae may cause ulceration
                             * Larvae may be found in vomit or feces

Type of Myiasis              Treatment and Prevention

Dermal, cutaneous, or        * Remove maggots
subdermal myiasis            * Treat any secondary bacterial infection
                             * Personal hygiene
                             * Male flies may be irradiated so that
                               they are sterile and cannot fertilize
                               females

Wound myiasis                * Remove maggots
                             * Treat any secondary bacterial infection
                             * Personal hygiene

Ophthalmomyiasis             * Remove maggots (may need to be
(ocular myiasis)               surgically removed)
                             * Treat any secondary bacterial infection
                             * Personal hygiene

Creeping myiasis             * Remove maggots
                             * Treat any secondary bacterial infection
                             * Personal hygiene

Sanguinivorous myiasis       * Remove maggots
                             * Treat any secondary bacterial infection
                             * Personal hygiene

Auricular myiasis            * Remove maggots
                             * Treat any secondary bacterial infection
                             * Personal hygiene

Urogenital and rectal        * Remove maggots
myiasis                      * Treat any secondary bacterial infection
                             * Personal hygiene

Gastrointestinal myia-       * Remove maggots
sis, enteric myiasis, or     * Treat any secondary bacterial infection
pseudomyiasis                * Personal hygiene

Note: Facultative myiasis organisms can feed on organic debris.

Table 6-16 Zoonotic Diseases Caused by Fleas

Scientific Name              Host

* Ctenocephalides canis      * Dogs, humans, foxes, mammals
* Ctenocephalides felis      * Cats, dogs, humans
* Pulex irritans             * Humans, pigs, dogs, cats
* Ceratophyllus gallinae     * Poultry, humans, cats
* Echidnophaga gallinacean   * Chickens, dogs, humans
* Spilopsyllus cuniculi      * Rabbits, humans
* Nosopsyllus fasciatus      * Rats, rodents, humans
* Archaeopsylla erinacei     * Hedgehogs, humans, mammals
* Xenopsylla cheopis         * Rats, mice, humans, domestic
                               animals
* Tunga penetrans            * Humans, large mammals

Scientific Name              Disease Transmitted

* Ctenocephalides canis      * Diphylidium larvae
* Ctenocephalides felis      * Cestode larvae
* Pulex irritans             * Yersinia pestits, cestode larvae
* Ceratophyllus gallinae     * Mechanical transmission of pathogens
* Echidnophaga gallinacean   * Mechanical transmission of pathogens
* Spilopsyllus cuniculi      * Francisella tularensis
* Nosopsyllus fasciatus      * Yersinia pestis, Rickettsia typhi,
                               bacteria
* Archaeopsylla erinacei     * Mechanical transmission of pathogens
* Xenopsylla cheopis         * Cestode larvae, Yersinia pestis,
                               rickettsiae
* Tunga penetrans            * Bacteria

Note: Fleas are temporary ectoparasites of humans and animals and
transmitters of disease.

Table 6-17 Zoonotic Diseases Caused by Heteroptera (Bugs)

Common Name       Scientific Name                    Host

Bedbugs           * Cimex lectularius                * Mammals, bats,
                    (worldwide)                        birds, humans
                  * Cimex hemipterus (tropics)
                  * Leptocimex boueti
                    (West Africa)

Kissing bug,      * Triatoma spp. (tropics)          * Marsupials,
assassin bug,     * Rhodnius spp. (tropics)            rodents,
cone-nose bug     * Panstrongylus spp. (tropics)       carnivores,
                  * Dipetalogaster spp. (tropics)      bats, birds,
                                                       humans

Common Name       Disease

Bedbugs           * Bites on unprotected body parts
                    (face, neck, forearm, leg) develop into
                    severely pruritic, hemorrhagic urticaria
                    within hours
                  * Lesions may occur in linear pattern

Kissing bug,      * Bites on unprotected body parts
assassin bug,       (face, neck, forearm, leg) are usually
cone-nose bug       painless and go recognized
                  * Triatomine bugs are intermediate
                    hosts and transmit Trypanosoma cruzi
                    (Chagas' disease)

Common Name       Treatment & Prevention

Bedbugs           * Anti-inflammatory creams
                    to reduce pruritus
                  * Insecticidal sprays or foggers
                    with pyrethroids can be used
                    in buildings

Kissing bug,      * Anti-inflammatory creams
assassin bug,       to reduce pruritus
cone-nose bug     * Insecticidal sprays or foggers
                    with pyrethroids can be used
                    in buildings

Table 6-18 Zoonotic Diseases Caused by Mites

Family                     Scientific Name              Animal Host

Sarcoptidae                * Sarcoptes scabiei          * Dogs,
These are burrowing          Sarcoptic mange is           horses,
mites that live in           caused by several            cattle, pigs,
tunnels in the skin.         subtypes of Sarcoptes        humans
They complete their life     that may have host
cycle on the host (do        preference. Subtypes
not survive for long         include Sarcoptes
periods in the               scabiei var. hominis
environment).                (people), Sarcoptes
                             scabiei var.canis
                             (dogs), Sarcoptes
                             scabiei var.suis
                             (pigs), Sarcoptes
                             scabiei var.equi
                             (horses), Sarcoptes
                             scabiei var.bovis
                             (cattle), and
                             Sarcoptes scabiei
                             var.ovis (sheep).
                             Most are found
                             worldwide with some
                             species rarely found
                             in some countries.

Sarcoptidae                * Notoedres cati             * Felines
These mites are              (worldwide)
burrowing mites with a
life cycle similar to
Sarcoptes scabiei.

Cheyletiellidae            * Cheyletiella yasguri       * Dogs, cats,
These are nonburrowing       (causes clinical disease   rabbits,
mites that remain            in puppies, but not        wild mam-
of the surface of skin       adults)                    mals
and feed on scales or      * Cheyletiella blakei
tissue fluids/blood.       * Cheyletiella parasi
They can survive for         tovorax (all are
short periods in the         found worldwide)
environment.

Macronyssidae              * Ornithonyssus bacoti       * Rodents
These are nonburrowing       (worldwide)                  and small
mites that                 * Ornithonyssus bursa          marsupials
can survive for longer       (tropical areas) and       * Birds
periods of time in the       Ornithonyssus sylviarum
environment (hence           (Australia, New Zealand,
they can be transmitted      and temperate zones of
by fomites).                 northern hemisphere)

Dermanyssidae              * Dermanyssus gallinae       * Birds
These are                    (also known as the         * Small
nonburrowing mites.          chicken mite and poultry    rodents
Zoonotic species can         mite; found worldwide)
spend most of their        * Liponyssoides sanguineus
life in the environment      (Asia, Europe, United
(drop off the host after     States, and northern
feeding).                    Africa)

Trombiculidae              * Eutrombicula spp.          * Mammals
Also known as chiggers,    * Neotrombicula spp.          and birds
these mites are            * Schoengastia spp.
only parasitic in their    * Euschoengastia spp.
larval stage. These        * Acomatacarus spp.
mites deposit their        * Siseca spp.
eggs on the ground         * Blankaartia spp.
or on low bushes.            (all are found
Hatched larvae feed          worldwide)
on tissue juices then
drop to the ground to
develop into nymphs.

Family                     Animal Disease

Sarcoptidae                * Dogs: lesions are first found on ventral
These are burrowing          chest and abdomen. Other affected areas
mites that live in           include ears, periorbital area, elbows,
tunnels in the skin.         and legs. Lesions are pruritic with thick
They complete their life     yellowish crusts.
cycle on the host (do      * Pigs: lesions appear first on the head
not survive for long         (periorbital area, snout, and ears)
periods in the               then spread to hind legs. Affected skin
environment).                is pruritic, red, and inflamed with
                             scabs, erosions, ulcers, or cracks. Skin
                             eventually becomes wrinkles with clinical
                             signs resolving in 12 to 18 weeks. Some
                             pigs may become unthrifty.
                           * Cattle: lesions are first found on the
                             head and neck or above the scrotum or
                             udder. Lesions are pruritic and may
                             appear as papules, crusts, and alopecia.
                             Skin may thicken and develop large folds.
                           * Sheep: lesions are similar to those in
                             cattle and are typically found on nonwooly
                             skin areas.
                           * Horses: lesions first appear on head,
                             neck, and shoulders. Initial lesions are
                             pruritic, papules, and vesicles which
                             later become crusts. Alopecia, crusting,
                             and skin thickening may be seen as the
                             disease progresses.

Sarcoptidae                * Notoedric mange is intensely pruritic
These mites are              in most felines. Lesions start on
burrowing mites with a       pinna, face, eyelids, dorsum of neck,
life cycle similar to        and paws. Initial papular rash may
Sarcoptes scabiei.           become red and have areas of partial
                             or complete alopecia. Crusts,
                             thickened skin, and enlarged lymph
                             nodes may also be seen.

Cheyletiellidae            * Dogs and cats: lesions appear on
These are nonburrowing       dorsum as dry, scaly dermatitis with
mites that remain            dandruff. Pruritus may be mild to
of the surface of skin       moderate. Occasionally there may be
and feed on scales or        red, excoriated lesions with
tissue fluids/blood.         alopecia. Cats may develop miliary
They can survive for         dermatitis. Adults may also be
short periods in the         subclinical carriers.
environment.               * Rabbits: clinical signs vary from loose
                             body fur; thick reddish-brown crusts
                             in ears; scaly, red, oily patches on
                             dorsum and head; dandruff; to absence
                             of clinical signs.

Macronyssidae              * Laboratory and pet rodents: anemia,
These are nonburrowing       debililty, weakness, pruritus, and
mites that                   death.
can survive for longer       Ornithonyssus bacoti is a vector for
periods of time in the       murine typhus, Q fever, and plague.
environment (hence         * Birds: blackened feathers, cracks and
they can be transmitted      scabs around the cloaca, decreased
by fomites).                 productivity

Dermanyssidae              * Poultry: anemia (feeds on blood at
These are                    night) and lowered productivity
nonburrowing mites.          (decreased weight gain or egg
Zoonotic species can         production)
spend most of their        * Pet birds: restlessness, anemia,
life in the environment      excessive preening, pruritus, and death
(drop off the host after   * Rodents: may develop papules in area
feeding).                    of bite. Mice and gerbils tend to be
                             asymptomatic.

Trombiculidae              * Dogs and cats: lesions are variable
Also known as chiggers,      with bites usually on area that has
these mites are              contact with the ground. Bites
only parasitic in their      result in pruritic papules,
larval stage. These          alopecia, crusts, and scabs.
mites deposit their        * Horses: lesions are in areas in
eggs on the ground           contact with the ground. Bites
or on low bushes.            lesions are similar to dogs and
Hatched larvae feed          cats, but wheals and larvae in the
on tissue juices then        lesions may also be seen.
drop to the ground to      * Birds: large numbers of mites may
develop into nymphs.         produce anorexia, lethargy, and
                             death in birds.

Family                       Human Disease

Sarcoptidae                * Severe pruritus, papular
These are burrowing          rash (particularly on
mites that live in           shoulders, spaces between
tunnels in the skin.         fingers and toes, and
They complete their life     skin folds), burrows on
cycle on the host (do        skin are pathognomonic.
not survive for long         Secondary bacterial
periods in the               infections may be seen.
environment).              * Zoonotic scabies is almost
                             always self-limiting with
                             mites disappearing in a
                             few days and clinical signs
                             resolving in 1 to 3 weeks.
                           * Host specific species may
                             cause temporary derma-
                             titis in other hosts, but do
                             not reproduce except on
                             their preferred host.

Sarcoptidae                * Presents with signs of
These mites are              scabies but is self-limiting
burrowing mites with a       in several weeks.
life cycle similar to
Sarcoptes scabiei.

Cheyletiellidae            * Humans are aberrant
These are nonburrowing       hosts that can get a
mites that remain            self-limiting, mildly
of the surface of skin       pruritic dermatitis on
and feed on scales or        abdomen, chest, arms,
tissue fluids/blood.         and legs.
They can survive for
short periods in the
environment.

Macronyssidae              * Painful or pruritic lesions
These are nonburrowing       may cause irritation and
mites that                   localized dermatitis. Most
can survive for longer       of these mites cause papules,
periods of time in the       but may also produce
environment (hence           vesicles, urticaria, and
they can be transmitted      hemorrhagic necrosis.
by fomites).               * Ornithonyssus bursa can
                             carry Western equine
                             encephalitis virus.

Dermanyssidae              * Painful or pruritic lesions
These are                    may cause irritation and
nonburrowing mites.          local dermatitis. Most of
Zoonotic species can         these mites cause papules,
spend most of their          but may also produce
life in the environment      vesicles, urticaria, and
(drop off the host after     hemorrhagic necrosis.
feeding).                  * Liponyssoides sanguineus
                             can transmit Rickettsia
                             kauri (human rickettsialpox)

Trombiculidae              * Dermatitis and allergic
Also known as chiggers,      reactions are possible.
these mites are              The first lesion is typically
only parasitic in their      a small red papule with
larval stage. These          intense pruritus; a wheal
mites deposit their          develops that may bleed.
eggs on the ground         * Some bites remain painful
or on low bushes.            for weeks.
Hatched larvae feed        * Some species of chiggers
on tissue juices then        can transmits scrub typhus
drop to the ground to        (Orientia tsutsugamushi).
develop into nymphs.

Family                       Treatment and Prevention

Sarcoptidae                * Animals: acaricides include lyme
These are burrowing          sulfur, amitrax, pyrethrins,
mites that live in           coumaphos, malathion, rotenone, or
tunnels in the skin.         carbaryl. Ivermectin and doramectin
They complete their life     are effective for treating mites.
cycle on the host (do      * People: topical lotions containing
not survive for long         permethrin or lindane may be used,
periods in the               but most human cases are
environment).                self-limiting. Ivermectin is also
                             effective.
                           * Sarcoptic mange in cattle is a reportable
                             disease in the United States.
                           * Isolation of affected animals and proper
                             disinfection is important in preventing
                             disease spread.
                           * Zoonotic disease can be prevented by
                             treating affected animals and wearing
                             protective clothing and gloves when
                             handling animals.

Sarcoptidae                * Same as for Sarcoptes
These mites are
burrowing mites with a
life cycle similar to
Sarcoptes scabiei.

Cheyletiellidae            * Same as for Sarcoptes
These are nonburrowing
mites that remain
of the surface of skin
and feed on scales or
tissue fluids/blood.
They can survive for
short periods in the
environment.

Macronyssidae              * Same as for Sarcoptes
These are nonburrowing
mites that
can survive for longer
periods of time in the
environment (hence
they can be transmitted
by fomites).

Dermanyssidae              * Same as for Sarcoptes
These are
nonburrowing mites.
Zoonotic species can
spend most of their
life in the environment
(drop off the host after
feeding).

Trombiculidae              * Same as for Sarcoptes
Also known as chiggers,
these mites are
only parasitic in their
larval stage. These
mites deposit their
eggs on the ground
or on low bushes.
Hatched larvae feed
on tissue juices then
drop to the ground to
develop into nymphs.

Notes: Diagnosis is skin scraping or collection of mite for
examination.

Psoroptic mange (Psoroptes ovis in the family Psoroptidae) only
affects domestic animals such as sheep and cattle and is not
zoonotic. Psoroptic mange is a reportable disease in the United States.
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Article Details
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Title Annotation:Part 5: TREMATODE ZOONOSES-References
Author:Romich, Janet Amundson
Publication:Understanding Zoonotic Diseases
Article Type:Disease/Disorder overview
Geographic Code:1USA
Date:Jan 1, 2008
Words:17578
Previous Article:Chapter 6 Parasitic zoonoses.
Next Article:Chapter 7 Viral zoonoses.
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