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Chapter 6 Parasitic zoonoses.

Objectives

After completing this chapter, the learner should be able to

* Describe properties unique to the different types of parasites

* Describe the different types of medically important parasites

* Identify the visual appearance of parasites either microscopically or to the naked eye

* Describe the roles arthropods play in disease

* Differentiate direct versus indirect parasitic life cycles based upon the types of hosts involved

* Briefly describe the history of specific parasitic zoonoses

* Describe the causative agent of specific parasitic zoonoses

* Identify the geographic distribution of specific parasitic zoonoses

* Describe the transmission, clinical signs, and diagnostic procedures of specific parasitic zoonoses

* Describe methods of controlling parasitic zoonoses

* Describe protective measures professionals can take to prevent transmission of parasitic zoonoses

Key Terms

amastigote

arthropod

autoinfection

cestode

coenurosis

cyst

cysticercosis

definitive host

dioecious

direct life cycle

ectoparasite

endoparasite

epimastigote

helminth

hemoflagellete

hermaphrodite

host

indirect life cycle

intermediate host

metamorphosis

nematode

oocyst

ovoviviparous

parasite

parthenogenic

platyhelminthes

promastigote

protozoa

reservoir host

rostellum

sporozoite

trematode

trophozoite

trypomastigote

OVERVIEW

Parasitology is the study of unicellular and multicellular organisms that parasitize people and animals. Parasites are organisms that derive nourishment and protection from other living organisms. Parasites are organisms that live in association with, and at the expense of, other organisms. The organism that provides the nutrition is called the host, and the organism obtaining the nutrition is the parasite. Parasites found on the exterior of the host are called ectoparasites (such as lice or fleas), whereas those found inside the host are called endoparasites (such as worms, amebae, and malaria protozoa) (Figure 6-1A and B). The effect a parasite has on a host ranges from causing minor harm (allowing the host to live and reproduce normally and complete its normal life cycle) to completely interfering with reproduction or causing the premature death of the host.

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Parasites range in size from tiny, single-celled organisms to worms visible to the naked eye. Several parasites have emerged as significant causes of illness especially foodborne and waterborne disease. These organisms live and reproduce within the tissues and organs of infected human and animal hosts, and are often excreted in feces.

Types of Parasitic Relationships

A host is a source of nourishment and housing for parasites. There are many different types of hosts involved in parasitic life cycles. Examples of different types of hosts include:

* The host in which a parasite reaches sexual maturity and reproduces sexually is referred to as the definitive host. In parasites that only reproduce asexually the host in which the parasite spends the majority of its life cycle is also referred to as the definitive host. The definitive host of Toxoplasma gondii is the cat because sexual reproduction occurs in the epithelial tissue of the feline intestine.

* Intermediate hosts harbor an immature stage of a parasite and are required for parasite development. A parasitic life cycle may include one or more intermediate hosts. Intermediate hosts are sometimes called alternate hosts. Arthropods such as mosquitoes, ticks, and flies are common intermediate hosts for parasitic disease. Intermediate hosts for Toxoplasma gondii are humans, wild animals, and domestic animals other than cats.

* Reservoir hosts serve as a source of the parasite that can be transmitted to humans from animals, even if the animal is the normal host of the parasite. For example, beavers can be reservoir hosts of Giardia duodenalis.

Types of Parasites

Parasites can be described as obligate (meaning the parasite cannot live apart from its host) or facultative (meaning the organism may be parasitic on another organism but is also capable of living independently). Most parasites are obligate parasites even if they have free-living stages outside the host. Facultative parasites are not normally parasitic, but can become parasitic when they are accidentally eaten or enter a wound or other body orifice. An example of a facultative parasite is Naegleria fowleri, a free-living ameba that causes primary amoebic meningoencephalitis.

Parasites are classified into different groups based upon their cell type, size, locomotion, and a variety of other factors. General categories of medically significant parasites include protozoa, helminths, and arthropods.

* Protozoa are unicellular, eukaryotic organisms that cannot make their own food and are usually found in moist, if not fully aquatic, environments. Protozoa are classified by the way they move: some move by tube-like structures called pseudopodia that project and withdraw to cause movement (such as Entamoeba histolytica), some move by flagella which causes whip-like movement (such as G. duodenalis), some move by cilia that cause waves of movement (such as Balantidium coli), whereas others have no organs for motion in the adult form (such as T. gondii).

* Another type of parasite is helminths commonly known as worms. Helminths are multicellular, eukaryotic organisms that cannot make their own food and are usually found in animals. Platyhelminthes, commonly called "flatworms," and nematodes, commonly called "roundworms," are the two medically important groups of helminths. Adult flatworms have flattened bodies and consist of two groups: cestodes ("tapeworms") and trematodes ("flukes"). Roundworms have cylindrically shaped bodies usually tapered at both ends.

* A third type of parasite is arthropods. Arthropods are multicellular organisms with jointed appendages and an exoskeleton. They may function as vectors of disease (such as the Anopheles mosquito that helps transmits Plasmodium organisms that cause malaria) or may cause disease directly (such as lice which are blood-sucking insects that live on skin).

Protozoa

The Kingdom Protista contains organisms called protozoa whose single unifying trait is that they exist as a single cell. There are over 50,000 species of protozoa; however, only about one-fifth are parasitic, whereas the majority are free-living. Parasitic protozoa are small, eukaryotic, and have high rates of reproduction.
Structurally, a protozoan is equivalent to
single eukaryotic cell.


One way protozoa are classified into phyla is according to their method of movement. There are seven phyla of protozoa; however, only four are parasitic in humans and animals. The four phyla of medical concern are Sarcomastigophora (motile by means of pseudopods such as ameba or by flagella such as Trichomonas vaginalis), Apicomplexa (nonmotile when mature such as Cryptosporidium parvum), Cilophora (motile by means of cilia such as Bal. coli), and Microspora (lack locomotion extensions on their body such as Enterocytozoon spp.).

* Sarcomastigophora consist of the amebae and flagellates. Amebae are motile by cytoplasmic extensions called pseudopods and most amebae have cyst and trophozoite stages in their life cycle. The cyst stage is the infective stage, whereas the active stage that feeds and moves is called the trophozoite (commonly called a troph). In most species of ameba the trophozoite is not infectious because it is unable to survive the harsh conditions of the stomach. An example of a pathogenic ameba is En. histolytica; examples of ameba that are nonpathogenic in healthy people and animals include En. coli, Endolimax nana, and Iodamoeba butschlii (Figure 6-2).

Flagellates are also members of the phylum Sarcomastigophora and are motile by one or more long, thin cytoplasmic extensions called flagella. Flagellates are surrounded by a flexible cell covering called a pellicle that gives them more definite shape than ameba. Flagellates also have a cyst stage that is infective and an active trophozoite stage. Some flagellates have a deoxyribonucleic acid (DNA) containing structure in the mitochondrion called a kinetoplast. Two examples of pathogenic flagellates are G. duodenalis and Tr. vaginalis. A subcategory of the flagellate group is the hemoflagellates which infect blood. Hemoflagellates are transmitted by the bite of a blood-sucking arthropod and are found in the blood smears of infected individuals. Examples of hemoflagellates are Trypanosoma and Leishmania spp. (Figure 6-3).

* Apicomplexa is the largest taxon of parasitic protozoa with about 4,000 known species. Protozoa in the group Apicomplexa have an apical complex, which is a unique arrangement of microtubules, vacuoles, and fibrils at one end of the cell. Apicomplexans infect both invertebrates and vertebrates; they may be relatively benign or may cause serious illnesses. Examples of Apicomplexa parasites include species of Plasmodium (Figure 6-4), the protozoa that cause malaria in humans and other animals, Eimeria spp., which cause coccidiosis, Cry. parvum, and T. gondii.

Apicomplexans have varied and complex life cycles involving two different hosts (usually a mammal and a mosquito). Their life cycles have both asexual and sexual phases that involve the formation of a thick-walled oocyst after fertilization, followed by meiosis to produce infective spores. Apicomplexans are transmitted to new hosts in various ways; some, like the malaria parasite, are transmitted by infected mosquitoes, whereas others may be transmitted in the feces of an infected host (Cry. parvum) or when a predator eats infected prey (T. gondii).

* Ciliophora contain the ciliates, which are free-living protozoal forms. Relatively few are parasitic, and only one species, Bal. coli, is known to cause human disease (Figure 6-5). Some ciliates cause diseases in fish; others are parasites or commensals on various invertebrates; others live in great numbers in the digestive tracts of many hoofed mammals, where they help breakdown cellulose.

Ciliates move by means of cilia allowing them to rotate and move forward and backward. Many ciliates have one or more macronuclei and micronuclei. The micronucleus functions in mitosis and meiosis, whereas the macronucleus maintains routine cell function. Their life cycle consists of a trophozoite and a cyst. Ciliates reproduce asexually by division: the micronucleus undergoes mitosis, whereas in most ciliates the macronucleus simply pinches apart into two.

* Members of Microspora are tiny, obligate intracellular parasites that infect many vertebrates and invertebrates. Transmission is by direct contact or by an intermediate host. As a result of their small size (1 to 4 [micro]m), electron microscopy is needed to recognize and identify these organisms. Five genera of microsporidians have been reported in immunocompromised humans: Encephalitozoon, Pleistophora, Nosema, Microsporidium, and Enterocytozoon.
Protozoa enter the cyst stage when
moisture and food supplies dwindle; the
cyst stage is the tougher, dormant form,
whereas the trophozoite stage is the
motile, active form.


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[FIGURE 6-4 OMITTED]

[FIGURE 6-5 OMITTED]

Helminths

Helminths are macrocytic worms in the Kingdom Animalia. The typical helminth life cycle includes three stages:

* Egg. Adults produce eggs either by production of eggs and sperm in the same worm (hermaphroditic) or in separate male and female worms (dioecious). Fertilized eggs are usually released in the environment with a protective shell and food source to aid their development into larvae. Most eggs are vulnerable to environmental extremes such as heat, cold, and drying; therefore, certain helminths lay hundreds of thousands of eggs in an attempt to successfully complete its life cycle.

* Larva. Larvae emerge from eggs and are sometimes called the juvenile form. The larva stage is found on the intermediate host. There may be more than one intermediate host as is the case with Diphyllobothrium latum, the fish tapeworm, which has a freshwater crustacean and a freshwater fish as intermediate hosts.

* Adult. As the larvae mature they become adults. The adult stage is found in the definitive host. There may be more than one definitive host as is the case with Dipylidium caninum, the dog tapeworm, which can have a definitive host of a dog, cat, or human.

Helminth infections are typically acquired by ingestion of the larval stage (some exceptions are larval injection into the body via an insect bite or entry into the body by skin penetration). Helminths that are parasitic in animals belong to two phyla: Platyhelminthes and Nematoda.

[FIGURE 6-6A OMITTED]

[FIGURE 6-6B OMITTED]

Platyhelminthes

Platyhelminthes are the flatworms and have no body cavity other than the gastrointestinal tract, which lacks an anus. The same pharyngeal opening in flatworms takes in food and expels waste. Because of the lack of any other body cavity, in larger flatworms the digestive system is often highly branched in order to transport food to all parts of the body. Flatworms consist of two medically significant groups. The Trematoda, or flukes, are all parasitic, and have complex life cycles specialized for parasitism in animal tissues. The Cestoda, or tapeworms, are intestinal parasites in vertebrates. Trematodes, named from the Greek trema meaning hole, which refers to the muscular sucker containing a mouth (hole) at the anterior end of the fluke, are unsegmented, are flattened dorsoventrally, have leaf-shaped or elongated bodies, and have suckers or hooks, which attach the organism to the host allowing them to draw fluids from host tissue. Trematodes are hermaphroditic (contain both types of sex organs) and can cross-fertilize. Trematodes have indirect life cycles and infection occurs by ingestion or penetration of infective larvae (Figure 6-6A). As seen in Table 6-1, trematodes are named according to the tissue they inhabit in the definitive host (for example, Fasciola hepatica is the liver fluke).

Cestodes, named from the Greek kestos meaning embroidered belt as a result of their appearance, have long, ribbon-like, segmented bodies (Figure 6-6B). Adult cestodes inhabit the intestines of animals and they do not have their own digestive system. Cestodes obtain nutrients by absorption through their external covering. A specialized attachment organ called the scolex is located at the cranial end and has hooks, suckers, or both. The segments of the tapeworm are called proglottids and contain male and female sex organs. Tapeworms cross-fertilize or self-fertilize and eggs are produced in the proglottids. As the tapeworm grows, proglottids are pushed to the caudal end where they eventually break free to be passed in the host's feces or the proglottids disintegrate releasing ova. Most species of cestodes have two hosts. The adult worm lives in the definitive host and the larva stage lives in the intermediate host. The intermediate host must ingest the egg for the life cycle to be completed. For humans to get infected the larva stage must be ingested.
Trematodes and cestodes are
hermaphroditic (monoecious), whereas
nematodes are dioecious (have separate
sexes).


Nematoda

Nematoda is a diverse animal phylum containing more commonly known parasites such as hookworms (Necator americanus and Ancylostoma duodenale in people, An. caninum in dogs, and An. braziliense in cats), pinworms (Enterobius vermicularis), Guinea worms (genus Dracunculus), and intestinal roundworms (genus Ascaris). Nematodes are long, cylindrical worms whose bodies are covered by a tough external covering and are tapered at both ends (Figure 6-7). Nematodes have a complete digestive system with both a mouth and an anus. Longitudinal muscle fibers allow the roundworms to move with a side-to-side motion. Nematodes are dioecious (the sexes are separate), undergo sexual reproduction, and males are typically smaller than females. The females are extremely prolific, producing hundreds or thousands of eggs that must embryonate (incubate) outside of the host. After this embryonation period, they are infectious to another individual. Some nematodes require an intermediate host. Nematodes are transmitted by ingestion of eggs or infective larvae or by penetration of the skin by larvae. The pathogenicity of the worms is often determined by the worm burden (the number of adult worms living in the intestine). The adults will mate in the intestine and the females will lay eggs that will pass out with the feces. Most ova are not infective immediately after passing out of the body in the feces (if most ova were infective immediately humans would have great numbers of worms in their bodies).
Pinworm infections in people are often
blamed on the family's pet dog or cat;
however, Enterobius vermicularis (the
pinworm) does not parasitize dogs or
cats. Pinworms parasitize such animals as
mice, rats, monkeys, rabbits, and horses.


[FIGURE 6-7 OMITTED]

Arthropods

Arthropods are found in the Kingdom Animalia and make up over 80% of all species on Earth. There are over 750,000 species of insects and the vast majority of arthropods are either beneficial or harmless to us.

Arthropods are involved in virtually every type of parasitic relationship serving as both definitive and intermediate hosts for protozoa, trematodes, nematodes, and other arthropods, functioning as vectors to transmit infective stages of parasites to vertebrates, and serving as parasites in their own right (such as fleas, ticks, and some crustaceans). Arthropod life cycles are as varied as their structures, which can provide a series of challenges in attempting to control parasitic infestations.

[FIGURE 6-8 OMITTED]

The Arthropoda phylum can be divided into three subphyla, each of which contains medically significant organisms (Figures 6-8 and 6-9). The subphylum Crustacea are arthropods with two pairs of antennae, two-branched appendages, and are usually aquatic. Examples in Crustacea are crayfish and copepods. The subphylum Uniramia are arthropods that have one pair of antennae, non-branched appendages, a distinct head, thorax, and abdomen, and in the class Insecta never have more than six legs. Examples in Uniramia include organisms in the class Insecta (insects) and the class Myriapoda (centipedes and millipedes). The subphylum Chelicerata are arthropods that have no antennae, no wings, a typically fused head and thorax, no more than eight legs, and possess specialized feeding appendages. Examples in Chelicerata are in the order Acarina (ticks and mites), the order Araneae (spiders), and the order Scorpiones (scorpions).

[FIGURE 6-9 OMITTED]

Two key evolutionary adaptations are important in understanding arthropods. One of these adaptations is the development of jointed appendages. Jointed appendages allow arthropods to move quickly. The other important adaptation of arthropods is the rigid carbohydrate exoskeleton. The exoskeleton not only provides protection but also serves as an anchor for muscles that attach to the inner surface of this exoskeleton. The arthropod body can only grow so much before it expands into its exoskeleton, thus limiting the size arthropods can attain. The vast majority of arthropods are less than 1 centimeter in length. One problem that arthropods encounter with this exoskeleton is that the animal is enclosed in a rigid, nonexpandable covering. The solution to this problem is a series of molts (changes) that all arthropods go through during their development. The larval stages between each molt are referred to as instars. The length of time between molts depends on the species involved. The number of instars also varies with species, the season or annual cycle, and sometimes with the species' nutritional state. The process of molting is controlled by hormones.

[FIGURE 6-10 OMITTED]

Arthropods differ from one another in their postembryonic development. In most species embryos develop into larvae. A larva is a life cycle state that is structurally distinct from the adult, normally occupies an ecological niche separate from the adult, is sexually immature, and must undergo a structural reorganization (metamorphosis) before becoming an adult. Metamorphosis is the change in size, form, and function that takes place as an immature arthropod reaches adulthood. In insects, two types of metamorphoses exist: complete and incomplete (also known as gradual) (Figure 6-10).

* In complete metamorphosis, the immature stages bear little resemblance to the adult, they are often found in different environments, and there is a pre-adult "resting" stage called the pupa. The stages in this type of metamorphosis are egg, larva, pupa, and adult (ELPA). If there is more than one larva stage, they are referred to as [L.sub.1], [L.sub.2], and so on and each larval stage is referred to as an "instar." An example of an insect that undergoes complete metamorphosis is the flea (insect order Siphonaptera).

* In incomplete or gradual metamorphosis, the immature stages look very similar to the adults (just miniature versions of them) and are usually found in the same environment. The stages in this type of metamorphosis are egg, nymph, and adult (ENA) and there are several nymphal instars. An example of an insect that undergoes incomplete metamorphosis is the bed bug (insect order Hemiptera).

In some arthropods, the terms complete and incomplete metamorphosis are not used to describe development of these organisms. The terms egg, larva, nymph, and adult (ELNA) are used to describe the developmental stages found in arachnids (spiders). Larva of arachnids have six legs (three pairs), whereas nymph and adult stages have eight legs (four pairs). In other cases (such as ticks), there's only one instar per stage (for example, one larva stage, which then molts into a nymph, which then molts into an adult). In still other cases (such as mites), there may be two or more nymph instars.

How Are Parasites Transmitted?

Three elements are necessary for the transmission of parasitic diseases: a source of infection; a mode of transmission; and a susceptible host. The source of infection may be the cyst or less often the trophozoite of a protozoan. Eggs or infective larvae are the source of infection for helminths.

Parasites may be transmitted from host to host through consumption of contaminated food and water, by injection of the parasite into the host by an arthropod vector, by transplacental transmission, or by putting anything into a mouth that has touched the stool (feces) of an infected person or animal. Insects and arachnids can transmit infectious agents to humans and animals either mechanically or biologically. Mechanical transmission involves the physical transfer of an infectious agent from an infected host to a noninfected host by the insect or arachnid. For example a horse with equine infectious anemia (EIA), a viral infection of horses, is fed upon by a blood-feeding horse fly. The blood on the fly's mouthparts is transferred to another horse when the fly feeds again. This is the same outcome as if blood from one horse has been transferred to another by a needle.

Biological transmission uses the insect or arachnid as a necessary part of the transmission cycle. Depending upon the organism, the infectious agent multiplies and/or matures within the insect/arachnid, and when the insect/arachnid feeds on an uninfected host, the agent is transferred. This may seem similar to the arthropod being an intermediate host; however, the term intermediate host is only used when the arthropod transmits parasitic organisms (not viruses, rickettsiae, or bacteria). An example of biological transmission is Ixodes scapularis, the vector for Lyme disease. Uninfected larval ticks acquire the infection by feeding on rodents (usually white-footed mice), which have Borrelia burgdorferi bacteria circulating in their blood. The organism multiples in the larval tick and persists as the tick molts to the nymph stage, and when the nymph tick feeds on an uninfected human or animal, the bacteria are transmitted to that host.

How Do Parasites Reproduce?

Parasites reproduce sexually or asexually or a combination of both. Asexual reproduction in parasites can be accomplished by several methods including mitotic fission and schizogony. Simple mitotic fission is common in protozoan parasites and occurs by production of two daughter cells. En. histolytica and Tr. vaginalis reproduce by mitotic fission. Many protozoa have increased their reproductive potential with a method known as schizogony. Schizogony (multiple fission) occurs when the nucleus undergoes several mitotic divisions after which the cytoplasm is divided among daughter cells. Plasmodium is a protozoan parasite that reproduces by schizogony.

Sexual reproduction in parasitic organisms is accomplished in several ways. Protozoa reproduce by conjugation and the production of gametocytes. Helminths reproduce by cross-fertilization and self-fertilization. Many cestodes and trematodes are hermaphroditic and can self-fertilize.

Some organisms undergo both sexual and asexual reproduction. Bal. coli, a ciliate, reproduces asexually by fission and sexually by conjugation. Sexes are separate in most arthropods and reproduction is generally sexual.

What Are Parasitic Life Cycles?

Parasitic life cycles are complex and have exact requirements. There are two basic types of parasitic life cycles: direct (simple) and indirect (complex). Direct life cycles involve only one definitive host, whereas indirect life cycles involve a definitive host and one or more intermediate hosts. Both types of life cycles are seen in protozoa and helminths.

Direct life cycles involve a single host where the parasite often spends most of its life (usually as an adult) and where it reproduces. In direct life cycles the parasites are transmitted from one host to another by direct contact or by ingestion of a form of the parasite (usually a cyst or egg) (Figure 6-11).

Indirect life cycles are more complicated than direct life cycles as a result of the need for one or more intermediate hosts and typically further development of the parasite that will only occur in a specific environment (Figure 6-12).

[FIGURE 6-11 OMITTED]

How Do Parasites Cause Disease?

Most human parasites cause disease by going through three general stages. The first stage is transmission of the parasite to the animal/human host from a source such as water, soil, food, or other animal. The microbe then invades and multiplies in the host, producing more parasites that can infect other suitable hosts. The final stage is when the parasite leaves the host in large numbers by a specific means and, in order to survive, must find and enter a new host. There are variations on this scheme that can include the parasite invading more than one host species (using an intermediate host), the parasite being spread by means of vectors, and the parasite being spread through body fluids.

[FIGURE 6-12 OMITTED]

Arthropods are unique in that they may cause disease by three different categories: direct effect, miscellaneous effects, and transmission of infectious diseases.

* Direct effect is the physical irritation caused by bites or stings of the arthropod. Examples of arthropods that cause disease by direct effect are mites, ticks, spiders, scorpions, lice, bedbuds, kissing bugs, fleas, bees, flies, mosquitoes, and beetles.

* Miscellaneous effects include tissue invasion and psychological effects caused by the arthropod. Examples of arthropods that cause disease by miscellaneous effects are scabies (invasion of the skin), demodetic mange (invasion of hair follicles), screwworm larvae (invasion of the brain), and spinal paralysis (continued biting by Ixodidae ticks). Some of these arthropods also cause direct effects.

* Transmission of infectious diseases is the arthropod serving as a means of moving infectious organisms from one individual to another. Examples of arthropods serving as means of infectious disease transmission include mosquitoes (malaria, encephalitis, dengue fever, yellow fever), tsetse flies (trypanosomiasis), black flies (onchocerciasis), fleas (plague), ticks (Lyme disease, ehrlichiosis), and kissing bugs (Chagas' disease).

How Are Parasitic Diseases Diagnosed?

A variety of techniques are used to detect and identify parasitic infections.

* Stool specimens. The examination of fecal samples for parasitic organisms begins with examination of unpreserved fecal samples. Protozoan trophozoites and oocysts are found in diarrhea, whereas cyst forms are found in formed stool. A direct wet film (commonly called a fecal smear or direct smear) may be made from a small portion of the fecal sample and a drop of saline on a clean slide. Specimens that cannot be promptly examined need to be preserved in a fixative such as polyvinyl alcohol (PVA).

* Blood specimens. The examination of blood samples usually involves the preparation of both thick and thin blood smears. Thin preps must be thin enough to avoid obscuring the parasites with blood cells (ideally one cell layer thick). Thick preps will contain more organisms per field; however, this technique can cause some distortion. Buffy coat preps (using the layer between the red blood cells and serum after centrifugation) concentrate the white blood cells and are especially useful for detecting Leishmania and trypanosomes.

* Sputum specimens. Pulmonary infection with parasites can be detected by examination of sputum (lower respiratory tract secretion). The best sample for examination is induced, free of saliva, and collected early in the morning. Fresh sputum is examined by wet mount or is fixed with PVA.

* Urine and vaginal specimens. Wet mounts are used for examination of urine, vaginal secretions, or prostate exudate.

* Cerebrospinal fluid (CSF) specimens. CSF is best examined promptly (within 20 minutes of collection) and is often centrifuged to concentrate parasitic organisms.

* Serologic tests. Serologic tests are used when conventional methods of parasite identification fail to yield results. Many test kits are available; however, cross-reaction with other antigens/antibodies is possible, which limits their usefulness. Some examples of serologic tests include complement fixation (Chagas' disease, Leishmania, and Paragonimus), agglutination tests (Chagas' disease), and Western blot tests (schistosomiasis).
Table 6-1 Classification of Trematodes According
to Their Habitat

Blood flukes        * Schistosoma haematobium
                    * Schistosoma mansoni
                    * Schistosoma japonicum
                    * Schistosoma mekongi

Liver flukes        * Fasciola hepatica
                    * Clonorchis sinensis
                    * Opisthorchis felineus
                    * Opisthorchis viverrini

Lung flukes         * Paragonimus westermani

Intestinal flukes   * Fasciolopsis buski
                    * Heterophyes heterophyes
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Title Annotation:Part 1: OVERVIEW
Author:Romich, Janet Amundson
Publication:Understanding Zoonotic Diseases
Article Type:Disease/Disorder overview
Geographic Code:1USA
Date:Jan 1, 2008
Words:4639
Previous Article:Chapter 5 fungal zoonoses.
Next Article:Chapter 6 Parasitic zoonoses.
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