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Chapter 15 Parasite control.

Internal parasites are those that are found on the inside of the horse. They include single-celled animals (protozoa), roundworms (nematodes), and flatworms (flukes and tapeworms). These parasites usually are found in the gastrointestinal tract but may be located in other internal organs. External parasites--lice, ticks, and mites--are found on the skin, in the ears, and at other places on the body. Signs associated with both internal and external parasites depend on the type and number of parasites present and can range from no apparent effects to general unthriftiness, weakness, debilitation, and ultimately death of the host.

Every horse is infected by one or more of these parasites. So, horses should be on a parasite prevention and control program. A general knowledge and understanding of the nature of these parasites and their development is essential before necessary prevention and control measures can be effectively applied.


After completing this chapter, you should be able to:

* Describe the life cycle of a typical internal parasite with an intermediate host

* Describe the symptoms of a parasite-infected horse

* List management techniques that help prevent parasite infections

* Give the scientific names for five common parasites

* Name the general categories of chemicals used to treat horses with parasites

* Describe the life cycle and damage caused by strongyles and ascarids

* Identify flying insects that are carriers of disease

* List the parts of the digestive system that internal parasites may affect

* Distinguish the different effects of mites, ticks, chiggers, and lice

* Discuss why young horses are more severely affected by parasites than older horses are

* Discuss the six sanitation and management practices used for reducing or controlling parasites

* Explain how a horse is checked for parasites

* List outward appearances of parasites on horses


abdominal worm










eye worm



intermediate host












stomach hairworm

stomach worms


summer sores






More than 150 types of internal parasites are known to infect horses. The most important ones are strongyles, ascarids, tapeworms, and bots. The digestive tract, or stomach and intestines, is the most commonly affected area, although larvae migrate through all tissues of the horse's body. Larvae, the first stage of the parasite, must go through several stages to become adults; each stage appears somewhat different (Figure 15-1).

Life cycles of strongyles and ascarids are similar. They are classified into a large group of parasites known as roundworms. Bots are the larvae of an insect, the botfly.

Internal parasites are widespread. Unless control measures are practiced, they are likely to increase and cause severe injury or death of the horse. Injury or harm inflicted on the horse is related to:

* Kind of parasites

* Number involved

* Time over which the parasites are acquired

Strongyles are the most injurious. Ascarids, bots, and tapeworms are generally less harmful. The horse may tolerate a few parasites without apparent signs of ill effect, but larger numbers are quite likely to be harmful. Acquiring a large number within a few days may overwhelm and kill a horse. Getting the same number over a period of weeks or months is generally much less harmful.

Horses affected the most by parasites are young sucklings or weanlings and yearlings. In general, ascarid problems are restricted to young horses. This is because, in most cases, resistance or immunity is built up by the time a horse is 2 or 3 years old. Strongyles and bots affect horses of all ages. Even so, the young are much more severely affected than older horses. Table 15-1 gives a brief outline of some common internal parasites.

Life Cycles and General Characteristics

To survive and propagate themselves, well-adapted parasites live in harmony with their hosts, the animals from which the parasite obtains food. If a parasite were always to kill its host, it would be responsible for its own death, because by definition parasites are organisms that live in or on another organism of a different species for the purpose of obtaining food.

Parasites include protozoa, nematodes (roundworms), cestodes (tapeworms), trematodes (flukes), and acanthocephalans (spiny-headed worms). Most of the parasites affecting horses are nematodes.


Protozoans. Protozoans are single-celled animals that occur in the bloodstream and intestinal tract of horses. These organisms multiply by dividing and may be transmitted from horse to horse by an arthropod vector (carrier) or simply by being ingested in food or water as a result of fecal contamination.

Nematodes. Roundworms are by far the most serious and economically important of the worms that occur in horses. These, as their name implies, are elongated, cylindrical worms ranging in size from 2 millimeters to 35 centimeters in length. Although the large worms cause significant problems, the small worms are far more important from both an economic and health point of view.

Some roundworm parasites damage the host by sucking blood. Others cause damage by migrating through body tissues such as the lungs, and still others can cause severe colic in horses by forming a mass of worms in the intestine that interferes with intestinal motility and to some extent, absorption of nutrients.

Most equine nematode parasites have a direct life cycle. This type of parasite requires no other organism except the definitive, or final, host to complete its life cycle.

Typically, females that live in the digestive tract lay eggs, which are passed to the outside with the horse's feces. The eggs hatch in 2 to 3 days, depending on temperature and humidity, into small wormlike organisms called first-stage larvae ([L.sub.1]). First-stage larvae develop and molt to second-stage larvae ([L.sub.2]), which molt to third-stage larvae ([L.sub.3]). The [L.sub.3] are infective to the final host. They migrate up blades of grass, and the horse ingests them when grazing. These preparasitic stages are much the same for most of the strongyle parasites of the horse.

When the horse ingests the third-stage larvae, they develop into fourth-stage larvae, which may wander extensively through the body of the horse before becoming adults in the intestinal tract--large strongyles--or they may develop into adults in the gut with no migration through other organs--small strongyles.

Some nematode parasites require a second host in order to complete their life cycles. This second host is an invertebrate and is called the intermediate host. Typically, the intermediate host eats the eggs or first-stage larvae, which then develop in the intermediate host instead of on the ground. The definitive host becomes infected when the intermediate host (fly, tick, etc.) injects the infective stage of the parasite while it is taking a blood meal. Sometimes the definitive host gets the infective stage by eating the infected intermediate host.

Nematodes have a complete digestive system. They have a mouth through which they suck blood or intestinal juices, and they excrete their waste through an anus (Figure 15-2).

Cestodes. All cestodes--tapeworms--that occur in horses use pasture mites as intermediate hosts. The final host becomes infested by ingesting the mite containing the infective cysticercoid while grazing. Mites become infected by ingesting tapeworm eggs deposited on the pasture with the host's feces.


The tapeworm that occurs in horses is a large worm consisting of a head, which attaches to the intestine of the horse, and a long ribbonlike body with similar segments called proglottids. Unlike nematodes, in which the sexes are separate (males and females mate to produce the next generation), tapeworms contain both sexes within the same worm.

Tapeworms absorb nutrients through their skin, having no mouth or anus (Figure 15-3).

Trematodes. Flukes also require an intermediate host, most often a snail. Although flukes do occur in horses, they are of minor significance and will not be discussed here.

Large Strongyles

The group of nematodes called the large strongyles are the most damaging of all the parasites that occur in horses. Adult worms range in size from approximately 12.5 millimeters up to about 31 millimeters in length. They live in the large intestine and cecum, where they feed by eating plugs of the mucosal lining.

Far more damaging than the adult are the larvae that migrate through internal organs of the host. Some prefer to live in one of the large arteries supplying the small intestine of the horse. These larval strongyles damage the artery's lining, causing it to react and become very thickened, producing an aneurysm.


Often, blood clots form and are carried by the bloodstream to smaller vessels, where they can block the blood supply to a part of the intestine. Where other vessels supply this part of the intestine, no real damage is done. But if no other blood supply exists, this part of the intestine dies. Unless corrected surgically, the condition can be life threatening. Sometimes these blood clots find their way back to the arteries that supply the hind legs and can cause rear limb lameness.

The parasite causing these problems is called Strongylus vulgaris. Other large strongyles (S. edentatus and S. equinus) migrate through different organs, notably the liver and pancreas, and inflict damage in their own particular way.

Small Strongyles

As their name implies, small strongyles are much smaller than the large strongyles--usually about 13 millimeters in length, although some are smaller. These nematodes are present in much larger numbers than the large strongyles. There may be hundreds of large strongyles in a horse, but usually there are thousands of small ones. While the small strongyles do not cause the damage or present the danger that large strongyles do, they can cause colic due to decreased intestinal motility, in addition to producing unthriftiness, diarrhea, rough hair coat, and other signs associated with heavy parasitic infections. These species of parasites usually are clumped into one or two genera (Triodontophorus or Trichonema).

As with the large strongyles, horses become infested by these parasites through ingesting the infective third-stage larvae while grazing. But unlike the large strongyles, these parasites require little time to reach maturity, start producing eggs, and further contaminate pastures. Consequently, they quickly build up large numbers of larvae to reinfest horses and assure their propagation. Like the large strongyles, small strongyles inhabit the large intestine and the cecum.

Hairlike Worms

Smallest of the nematode parasites occurring in horses is the stomach hairworm, Trichostrongylus axei. It is about 4 or 5 millimeters in length and very thin and hairlike. As with the large and small strongyles, the life cycle is direct. Eggs are passed in the feces, hatch, and develop into third-stage infective larvae in 4 or 5 days. Horses become infested by eating the larvae on the grass.

This parasite occurs in the stomach and the small intestine and damages the lining of these organs, sometimes causing bleeding into the gut. This is associated with dark, fetid diarrhea and, with heavy infections, can cause a rapid loss of condition.

The equine intestinal threadworm, Strongyloides westeri, is somewhat unique. These small, hairlike worms are 8 to 9 millimeters long and only the adult female is parasitic.

To add to the uniqueness of this parasite, the adult males and females can exist outside the host in a free-living state. When conditions become unfavorable for existence on the outside, the females produce eggs that hatch into third-stage infective larvae and either are eaten by the horse or penetrate the skin. If they penetrate the skin, the larvae migrate to the lungs, penetrate the alveoli, and after reaching the trachea, are coughed up and swallowed. They continue to develop to adulthood in the gut.

Some of these migrating larvae do not develop, but remain dormant in muscle tissue of mares until they foal. Then they migrate into the mammary gland and infest nursing foals via the colostrum. S. westeri has been thought to contribute to foal heat diarrhea, which occurs 12 to 13 days after birth.


The most common tapeworm in horses is in the genus Anoplocephala. This cestode, or flatworm, Anoplocephala magna, is called the "large horse tapeworm" and occurs most often in the small intestine. It also is found in the stomach and sometimes in the cecum.

This is a fairly robust tapeworm about 25 centimeters long, with very short segments. It retains its position in the host by attaching to the small intestine lining by four suckers located on the head (scolex). Like all tapeworms, both sexes are contained in each individual segment (proglottid).

This worm has an indirect life cycle: the eggs are passed in the host's feces and are eaten by pasture mites, which are the intermediate hosts. After the horse eats the oribatid mite containing the infective cysticercoid (larva), the larva develops into an adult in the horse's small intestine in 6 to 10 weeks. Typically, these worms don't live very long in their host.

With light infestations, horses show no signs. Heavy infestations cause horses to suffer colic and diarrhea and possibly go off feed. They often are depressed, may become dehydrated, and spend a lot of time lying down. When many worms are present, sometimes few feces are passed because the worms cause an intestinal obstruction. Heavy infestations of this parasite can produce complications that result in death.

Horse tapeworm infection is diagnosed by finding the eggs in the feces. But many horses do not pass eggs, especially with heavy infestations, so tapeworms may not be diagnosed when present.

Anoplocephaliasis in the horse is usually a disease of yearlings at pasture.


Although this parasite is most common in donkeys, horses also harbor worms (Dictyocaulus arnfieldi) that live in their lungs. The lungworm is another nematode, or roundworm, parasite. Females are about 60 millimeters long, and males can be a little over half that. Adults live in the lungs of horses, where they mate; the eggs produced by the females are coughed up by the horse, swallowed, and passed with the feces.

Dictyocaulus has a direct life cycle. Horses become infected by ingesting the third-stage larva. In horses, the lungworm adults may never produce eggs; in donkeys, they may start producing eggs in 3 to 4 months. Although lungworms cause few clinical problems in donkeys, in horses they may cause coughing, an increased respiratory rate, and some nasal discharge. Because eggs often are not produced in horses, diagnosis becomes difficult; veterinarians cannot find eggs in the feces, so they must rely on history--including whether donkeys are grazing with horses--and clinical signs to diagnose the disease.

Stomach Worms

The habronemas, which consist of Habronema muscae, H. majus, and H. megastoma (Draschia megastoma), are the equine stomach worms that cause two rather distinct diseases in horses: gastric and cutaneous habronemiasis. Habronemas have indirect life cycles, with house and stable flies serving as intermediate hosts.

Habronema eggs, which pass with the horse's feces, are eaten by fly maggots and mature with the fly as it becomes an adult. Infective larvae are deposited around the horse's lips and nostrils where the flies feed, thereby gaining entrance into the horse's mouth. Horses may also become infected by ingesting infected adult flies that have become entrapped in food or water. The larvae are freed in the horse's stomach and develop into adults in about 2 months. In the stomach of horses, these parasites produce fibrous tumors, or numerous nodules that, if close together, form a tumor.

Another type of disease caused by this parasite is cutaneous habronemiasis or summer sores. This also is caused by species of Habronema, but is due to the larvae that the intermediate host deposits in existing wounds in the skin. (Some parasitologists think that the larvae can penetrate healthy skin.) Cutaneous habronemiasis occurs during the summer and is most common on areas where horses cannot switch flies, such as the inside of the legs, over the withers, the penile sheath, and fetlocks.

These lesions are brownish-red, angry-looking sores that may ooze serum tinged with blood. They seem to itch intensely and often disappear when cold weather sets in--only to reappear when the weather warms up again. The appearance of cutaneous skin lesions in the summer time when flies are numerous would suggest summer sores.

The gastric form of habronemiasis is more difficult to diagnose since few eggs are passed and, because the larvae don't float very well, they are sometimes missed during routine fecal floatation examination. Adult females are about 25 millimeters long. Males are somewhat smaller--usually about two-thirds to three-quarters the size of the female.


Parascaris equorum is the horse ascarid. This is a very large, robust roundworm. Females grow up to 35 centimeters long, though the males are somewhat smaller. The life cycle is direct; but instead of ingesting the larvae on the pasture, foals ingest the infective eggs that contain larvae. Because it takes about 2 weeks for the eggs to become infective, a foal could ingest freshly passed feces (a common habit) and not become infected.

After infective eggs are ingested, they hatch and penetrate the wall of the intestine, migrate in the bloodstream to the lungs where they may cause some respiratory problems, and are then coughed up, swallowed, and mature in the small intestine.

The adults start producing eggs about 12 weeks after the foal becomes infected. Because of a developing immunity, foals often shed the infection at about 7 months of age.

Clinical signs of ascarid infection in foals include a dry hair coat, potbelly, and abdominal discomfort (sometimes these foals kick their flanks). They will be undersized for their age and breed, and very often they have dry stools covered with mucus, although diarrhea sometimes is present. It should be noted that ascarid eggs are very resistant and can survive for years in the soil.

Because some anthelmintics render these parasites unable to move, impaction due to a large mass of immobile worms sometimes can occur following deworming. A veterinarian can suggest an appropriate anthelmintic to use with a heavy ascarid infection.


Horses, like people and unlike dogs, can have pinworms. Two kinds of pinworms occur in horses. A rather large one, the females of which can be up to 63 millimeters long, is Oxyuris equi. The minute horse pinworm, Probstmayria sp., is only about 2 millimeters long and is of little consequence.

The life cycle of Oxyuris species is direct, and like horse ascarids, the egg is the infective stage. It is infectious for 3 to 5 days after being laid and is ingested by the horse with food or water. The parasite matures in the mucosa of the cecum, colon, and rectum and starts producing eggs in 120 to 150 days.

Because the females migrate out of the anus to lay eggs and then return to the colon, this disease causes an intense itching around the anus of horses. Owners will see horses rubbing their hindquarters, often resulting in all the hair being rubbed off over the tailhead.

Often these horses become restless, go off feed, and lose condition. Sometimes young mares may appear to be in heat. Although adult female pinworms occasionally can be seen around the horse's anus, diagnosis is by finding the eggs, usually with a transparent tape swab.


Equine piroplasmosis, or equine babesiasis, is a protozoan disease occurring in horses, mules, and donkeys in the southeastern part of the United States, particularly Florida and Georgia.

Two species of Babesia--B. caballi and B. equi--are known. These small, protozoan parasites occur in red blood cells. The life cycle is indirect. The tropical horse tick (Dermacentor nitens) serves as intermediate host. The brown dog tick (Rhipicephulus sanguineus) may be able to serve as an intermediate host for B. equi.

Equine babesiasis causes horses to have a fever and to become listless and depressed. They may go off feed, and may develop central nervous system disturbances causing rear leg weakness or even paralysis. The limbs may become swollen--stocked-up.

This disease usually lasts 8 to 10 days and can cause death, although most horses recover and return to normal.
American Association of Equine Practitioners

An equine veterinarian is a veterinarian
who treats horses. Horses are different
from other domestic species in anatomy,
physiology, pathology, pharmacology, and
husbandry. Most veterinary schools produce
graduates able to practice on a wide variety of
species. Specialization in equine veterinary
medicine is normally developed after becoming
a DVM. Equine veterinarians in the United States
are certified by the American Association of
Equine Practitioners (AAEP).

The AAEP was founded in Louisville, Kentucky,
in 1954 by 11 veterinarians. Its mission was to
demand excellence among its practitioners and
ensure methodical concern for the health and
welfare of the horse. Since 1954, the AAEP
membership has grown to over 8,000
veterinarians and veterinary students from 57
countries. Focuses of the AAEP include:

* Finding cures and treatments for
particular problems

* Participating in equine research and
development programs

* Educating the public through programs on
preventing and treating injury and disease,
providing nutrition, and dealing with
parasite control

* Maintaining a presence with schools of
veterinary medicine, equine research
institutions, and organizations throughout
the world

* Addressing animal welfare, medication,
and injury issues

* Promoting the public image of the equine
veterinary profession

* Improving ethics and standards, practice
management, and owner education

* Providing an authoritative voice within the
equine industry

Ethics and guidelines promoted by the AAEP
can be downloaded from the AAEP Web site

Equine Protozoal Myeloencephalitis (EPM)

EPM probably occurs throughout North America, affecting male and female horses equally. Thoroughbreds and Standardbreds are affected more frequently. Signs can appear at any age, but most cases occur in horses under 4 years old.

Symptoms vary depending on where in the central nervous system the organism is located. The most frequent signs include stumbling and incoordination. When cranial nerves are involved, the head tilts. Signs are frequently progressive, affecting several different areas of the horse and usually affecting one side of the horse more than the other. Because EPM also affects the fibers that run to the muscles, muscle wasting is seen in some cases.

Most likely the protozoan Sarcocystis neurona causes EPM. This protozoan requires two secondary hosts, one of which is the opossum (Figure 15-4). Opossums pass infective organisms in their feces, so the most likely source of infection to the horse is feed and water contaminated with opossum feces. The second host has been harder to find. Infective feces of opossums cannot infect opossums directly. Their feces must first pass through the intermediate host. Horses are "dead end" hosts: they become infected but do not pass the infective organism to others. Birds have been suspected to be the secondary or intermediate host. Recently cats have been shown to be a successful secondary host. If cats or birds ingest the infective oocysts (eggs) passed in the opossum feces, the protozoan can be found in the tissues of the cat or bird. Once the cat or bird dies and is ingested by an opossum, the life cycle ends and the protozoan is passed out the opossum's feces. Other intermediate hosts could include minks, skunks, and raccoons.

Other diseases cause symptoms similar to those of EPM; they include bacterial and viral diseases of the brain and spinal cord, trauma, malformations of the spinal cord, poisons, hind-limb lameness, and vitamin E deficiency. Diagnosing EPM is a matter of matching up the history and symptoms and then ruling out other possible causes to arrive at a tentative diagnosis. Several blood tests are available, but none is definitively diagnostic. Many horses test positive on the blood tests but exhibit no symptoms. Recently, tests on cerebrospinal fluid have greatly improved the ability to accurately diagnose the disease. One of these tests is called a Western blot test. It detects the horse's antibodies to the organism.


Since the life cycle of the protozoan is not completely understood, no proven methods of preventing EPM now exist. However, a vaccine is being tested.

Treatment regimens for EPM are costly; often of limited effectiveness; and in a small number of treated animals, symptoms recur when therapy is discontinued. Two drugs used to combat EPM are trimethoprim sulfa and pyrimethamine. The cost of treatment ranges from $150 to $300 per month, depending on the size of the horse and the drug formulations used.


Onchocerca species are nematodes that occur as adults in connective tissue of horses, mules, and donkeys. They are fairly common parasites. About 75 percent of horses surveyed in the Midwest were infected with Onchocerca cervicalis. Adult females are quite long--up to 30 centimeters--but the males are small, 6 to 7 millimeters long. Females of Onchocerca cervicalis occur in the ligamentum nuchae of horses and mules.

Onchocerca reticulata occurs in the flexor tendons and suspensory ligaments. This nematode also requires an intermediate host and uses midges (Culicoides sp.) as an arthropod vector. Biting midges pick up the microfilaria in the skin of horses; these develop to an infective stage in the midge in about 3 weeks. When the midge takes a blood meal from a horse, the infective stage is injected--thus completing the cycle.

In addition to the dermatitis this organism can cause, it sometimes causes eye problems. O. reticulata causes occasional lameness. Species of Onchocerca do not cause fistulous withers or poll evil, as formerly believed. Because the new parasiticide, ivermectin, quickly kills Onchocerca microfilaria, horses sometimes mount an immune response to these dead microfilaria--resulting in tissue edema. This condition resolves itself spontaneously in about 7 days.

Eye Worm

The equine eye worm, Thelazia lacrymalis, is about 19 millimeters long and lives in the tear duct and conjunctival sac of the horse's eye. The female worms produce living larvae; they don't lay eggs. These first-stage larvae wander into the eye secretions and are picked up by face flies that serve as the intermediate host. In the fly, the larvae develop into the infective stage and can be transferred to another host when the face fly feeds on eye secretions.

Although most eye-worm infections go undetected, heavy infections cause mild eye irritation. On rare occasions, they result in blindness, probably due to secondary bacterial infection.

Diagnosis is made by observing adult worms in the eye. Treatment is best achieved by removing the adults from the conjunctival sac under ophthalmic anesthesia and tranquilization. Decreasing the prevalence of eye worms is best achieved by controlling face flies.

Sometimes the abdominal worm, Setaria equina, develops in the eye and causes damage. Normally, these nematode parasites, which use mosquitoes as intermediate hosts, occur in the abdominal cavity and are of little or no consequence.


External parasites of horses include ticks, mites, chiggers, and lice. These cause irritation and may carry disease.


Three kinds of ticks occur commonly on horses. Each has a preferred location on the horse. Some are more common in specific parts of the country.

The winter tick, Dermacentor albipictus, has become widely distributed because horses now are commonly transported from one part of the country to another. Although this tick occurs primarily on the horse, it is found on other farm animals, such as cattle, sheep, and goats, so these animals can also be involved in its spread.

Ticks differ from one another in that some use only one host as they develop from larvae to nymphs to adults, while other ticks use more than one host. The winter tick is one of the ticks that uses only one host. The entire life cycle takes place on the horse (Figure 15-5).

Tick infestations, like those of lice, are more common in the winter than in the warm seasons. Large numbers of winter ticks can cause horses to become weak; lose their appetite and become thin; and, because of the blood loss, sometimes develop an anemia that makes them more susceptible to other diseases. Ticks can cause death, especially in foals.

The Pacific coast tick, Dermacentor occidentalis, is found chiefly in coastal areas of the West. Unlike the winter tick, this tick drops off the host to lay its eggs, and the larvae and nymphs feed on small mammals before becoming adults and parasitizing horses.

The Pacific coast tick can transmit Rocky Mountain spotted fever, Colorado tick fever, African horse fever (piroplasmosis), and other diseases. It also can produce a condition called tick paralysis that can affect humans, dogs, and calves. Consequently, horses should be inspected for ticks after trail rides or cross-country pleasure rides in areas where these ticks occur.


The ear tick, Otobius megnini, like Dermacentor albipictus, is a one-host tick. It is common on horses but is also found in the ears of cattle, sheep, dogs, cats, and, occasionally, people. These ticks, however, do not occur on the horse in their adult stage: only the larvae and nymphs are found in the horse's ears. Adults have nonfunctional mouth parts but may survive for 2 years on the ground.

Ear ticks on horses cause irritation evidenced by excessive head tossing or rubbing of the ears. Horses that have drooping ears and shake their heads a lot may have ear ticks. Ear ticks also predispose the animal to secondary bacterial infection of the middle and inner ear and can, consequently, cause serious problems. This tick, unlike Dermacentor, does not itself transmit any diseases. Several topical preparations are available for treating ear ticks.

Mange Mites

Mites are ectoparasites that are closely related to ticks and cause a skin condition called mange. The entire life cycle of mange mites occurs on the horse. Mating occurs on the skin or in burrows the mites make in the skin. The eggs hatch on the host after about 4 days and are mature, egg-laying adults 12 to 15 days later.

Sarcoptic mange (Sarcoptes) causes lesions usually found on the neck, shoulders, head, chest, and flanks of horses. These mites burrow under the skin and cause severe irritation and itching. In trying to relieve the source of itching, horses will bite and rub the affected area until the hair is lost and large, scabby areas often result (Figure 15-6).


Chorioptic mange (Chorioptes) produces lesions like sarcoptic mange, but since the mites occur more commonly on the lower extremities, it often is called foot mange. Horses affected with these mites will paw, lick, and bite at their lower legs in an attempt to relieve the itching.

Psoroptic mange occurs primarily on the poll or the tail. This mange mite (Psoroptes) also causes intense itching, with hair loss and scabs if the horse traumatizes itself extensively.

Mange mites can live off the host for a short time and can be transferred from one host to another on combs, blankets, and so forth. In the past, mange has been extremely difficult to control; but with the new ivermectins, mange should become a less-serious problem.

Chiggers and Lice

Chiggers affect horses in much the same way they affect people. Chiggers are the larval stage of harvest mites (Trombicula) that affect horses' feet and muzzles as they walk and feed on infested pastures.

Lice can be a very serious problem in horses. There are two kinds of lice: biting lice (Damalinia), which feed on skin and hair, and sucking lice (Haematopinus), which pierce the skin and suck blood and tissue fluids (Figure 15-7).

Mites and lice are very host specific. They will not pass from horses to cows, sheep, goats, dogs, or other animals.

Infestation with both sucking and biting lice can be debilitating to horses. Biting lice cause skin irritation and itching, and horses will rub, bite, and kick at themselves to relieve the source of irritation. This results in a rough coat with loss of hair; if serious enough, secondary bacterial infection can cause major skin lesions. In addition, heavy louse infections can produce serious unthriftiness and weight loss.

Sucking lice and a heavy infestation of biting lice can remove enough blood to cause a horse to become seriously anemic, in addition to producing irritation and debilitation because of itching.

Louse infestations usually are more severe in late winter and early spring. Frequent grooming and applications of topical pesticides are helpful in louse control.


Although not permanently associated with their host as worms and mites are, flies, mosquitoes, gnats, and other flying insects are important not only because of the worry and loss of condition they cause but also because some are carriers of disease.

Fly control depends to a great extent on sanitation, good grooming, and common sense. Flies breed in manure and sometimes spilled grain, especially if it is wet. Removing spilled grain and manure from stalls on a regular basis, and changing bedding as it becomes soiled with feces and urine, will aid in fly control. Some flies serve as carriers of parasitic worms and viral diseases.


Mosquitoes transmit equine encephalomyelitis. Black flies and "no-see-ums" very often cause intense itching and attendant lesions in horses' ears, although they will bite other thinskinned areas of the horse as well.


Bots are fly larvae that are parasites in the stomach of horses. Gastrophilus intestinalis, the common horse bot, and G. nasalis, the throat botfly, are the two common botflies found in this country.

Adult flies look somewhat like bees and are not seen often. These adult flies lay eggs on the hair of the legs or around the chin and throat of horses.

G. intestinalis lays its eggs on the forelegs and shoulders of horses. The eggs hatch when the horse licks itself, so the larvae quickly gain entrance to the horse's mouth. G. nasalis lays its eggs around the chin and throat, where they hatch spontaneously (the horse doesn't need to lick the eggs for them to hatch). G. nasalis eggs hatch and burrow under the horse's skin into the mouth.

Both species remain for about a month in the lining of the tongue and cheeks, where they may cause severe ulcers around the teeth and cause horses to go off feed. After about a month, the larvae are found in the stomach, where they produce a condition called gastric myiasis. Although in small numbers, bots cause virtually no clinical signs, in heavy infections there may be almost no part of the horse's stomach wall that does not have a bot attached (Figure 15-8).

These botfly larvae are fairly large, about 2 centimeters long, and have large oral hooklets that they attach to the stomach wall. Sometimes they completely penetrate the stomach wall, causing peritonitis and subsequent severe problems.


Adult flies seriously annoy horses when depositing their eggs on the legs and chin. Washing the legs and chins of horses with warm water containing an organophosphate insecticide every week during botfly season aids in control.

Warble flies, which cause cattle grub, can affect horses. But they are seldom a problem except in cow ponies used to work range cattle.


Sanitation and good management practices should be used to control parasite infections. Foals are born free of internal parasites; their buildup of internal parasite infections is related to the degree of contact, either direct or indirect, with older animals carrying the infections. All of the worm parasites discussed here use feces or manure to spread infections by contamination of feed and water supplies or the environment.

Transfer stages of these worm parasites do not actively seek the host to complete the infection process. Instead, they rely on chance to be picked up and swallowed. Thus, only a very small percentage actually complete this step in the life cycle. To compensate for this, female worms produce large numbers of eggs to start the transfer process.

Sanitation and sound management practices aid in controlling or minimizing the spread of infections. These practices assist the natural destructive forces such as sunlight and drying during transfer stages. Susceptible animals also should be allowed only limited contact with contaminated pastures, paddocks, or stables. A checklist of sanitation and management practices effective in reducing numbers of parasites and flying insects includes the following:

1. Proper manure disposal

* Stable manure: Compost before spreading on pasture, or spread on cropland and other ungrazed areas

* Small corrals or paddocks: Pick up all manure and compost or dispose of as above

2. Pasture management

* Practice frequent mowing and chain harrowing

* Avoid overstocking

* Rotate grazing as much as practical

* Graze young animals separate from older horses

* Follow horses with cattle or sheep before returning pastures to horses

3. Feed

* Provide mangers, racks, or bunks for hay and grain

* Do not feed off the ground

4. Water

* Provide a clean water supply

* Avoid water sources contaminated with feces

5. Removal of bot eggs

* Clip egg-bearing hairs or sponge affected areas with warm water

6. Regular deworming of horses should be practiced under the supervision of a veterinarian who is familiar with local land and weather changes.

Anthelmintic Drugs

For effective parasite control, horses often need to be treated with specific drugs, commonly referred to as anthelmintics (Table 15-2). These drugs remove the parasites from the intestinal tract. The treated animal is relieved of the immediate damage or injury caused by parasites, but probably more important, removal of parasites breaks the cycle. This serves to reduce contamination of the environment with transfer stages, limiting the spread of infections and protecting animals from reinfection.

The best method of strongyle control currently recommended is to administer a small quantity of a deworming compound to the individual horse's ration daily. This compound is very effective in those management systems in which horses are fed individually.

The best routine deworming program for any particular management system should be designed around individual client/horse requirements and may differ depending on the number of horses on a pasture, amount of time spent at pasture compared with time spent in a stable, availability of alternative pastures (for rotational grazing), age of the horses, and other conditions. The local veterinarian should guide the horse owner regarding the optimal deworming strategy.

If daily deworming cannot be undertaken, regular use of ivermectin is recommended (every 6 to 10 weeks depending on the local circumstances). Ivermectin has profoundly affected parasite burdens in horses. It is so effective that the old-fashioned deworming strategies that used stomach tubing or promoted the "rotation" of a number of weaker deworming compounds are no longer recommended. Traditionally, dangerous and toxic compounds were used to eliminate bots. Fortunately, ivermectin is as effective against bots as it is against strongyles and ascarids. Although tapeworm problems are uncommon, once-yearly deworming against tapeworms is recommended for horses that are routinely dewormed with ivermectin. This is especially true if the horse has access to permanent pasture grazing. The best method of deworming against tapeworms is to use a high dose of pyrantel pamoate.

The veterinarian's services can include a microscopic examination of fecal samples for an indication of the kinds and relative numbers of worm parasites in the animals. This, along with other information such as numbers and ages of animals and type and amount of pasture, provide the veterinarian with a rational basis for the selection of drugs and frequency of treatments for the particular situation.

With primary emphasis on strongyle control, some operations may require only one or two treatments per year, whereas others with factors or circumstances favoring heavy infections may take as many as six treatments per year to maintain effective control. All horses on a farm should be included in the control program. New stock or temporary boarders should be treated and quarantined for a week or so before they are placed on pasture or otherwise allowed to mingle with other horses.

Drug Resistance

Many anthelmintics introduced over the last several years are benzimidazole analogs to which nematode parasites are starting to develop a resistance. Resistance develops when a wormer does not kill all of the target worm population; some survive to contaminate pastures. Over time, pastures become contaminated with a high proportion of larvae that, when eaten, will develop into adults able to tolerate the doses of anthelmintic normally administered.

Before concluding that lack of response is due to benzimidazole resistance, other reasons should be considered to explain why horses have eggs in their feces after worming. Among them might be a low plane of nutrition, rapid reinfection, wrong choices of anthelmintic, an inappropriate dose, or faulty administration.

If anthelmintic resistance is a problem, two processes can be used. One is to use a given class of anthelmintic--for instance, a benzimidazole--for a year and then use a different compound such as pyrantel, an organic phosphate, or ivermectin for a year. Simply changing from one benzimidazole to another does not constitute changing anthelmintics. The second option is to change the classes of anthelmintics each time horses are wormed.

A prime objective of any strongyle control program should be to keep pasture contamination of larvae to a minimum. The worming protocol needed to accomplish this objective will depend on worm burden, stocking rate, and climatic conditions and so will vary from farm to farm. The horse owner with 50 acres to support two horses will have far fewer problems than will an owner who is trying to keep two horses on two acres.


Parasites are grouped as internal and external. Many parasites can affect horses, but internal parasites create the greatest health problems. The person working with horses must learn to recognize the signs of parasitic infections early. Also, many good management practices will prevent or lessen the chance of severe infections. Based on local geography and weather conditions, a veterinarian can help develop a control program that also combines the use of chemicals to kill internal parasites.

Parasite infections are difficult to identify and hard to eliminate. Success in controlling them must be a determined and sustained effort. A continuing battle must be waged against internal parasites, the most common danger to the health and well-being of horses.


Success in any career requires knowledge. Test your knowledge of this chapter by answering these questions or solving these problems.

True or False

1. Keeping pasture contamination to a minimum will help in parasite control.

2. Botflies transmit equine encephalomyelitis.

3. All parasites require an intermediate host.

4. Lungworms cause more problems in horses than they do in donkeys.

5. Horses and people can have pinworms.

Short Answer

6. Name six sanitation and management practices for reducing parasites.

7. List three hosts of parasites.

8. Name five symptoms of any parasite infestation.

9. List five internal and five external parasites.

10. Name three ways the veterinarian can help in a deworming program.

11. List the major internal parts of the digestive system that any type of worm infestation may affect in the horse.

Critical Thinking/Discussion

12. What is the purpose of an anthelmintic?

13. Explain the life cycle or stages of a parasite.

14. Explain the difference in how ticks, lice, chiggers, and mites affect horses.

15. Why are young horses affected more by parasites than older horses are?

16. How are horses checked for parasites?


1. Based on the information in this chapter, develop a checklist of things to observe when examining horses for signs of parasites. Make the checklist complete enough that it could be given to a new employee. Using a word processor, put this checklist in a table format.

2. Visit or contact a veterinary clinic. Find out how to submit a fecal sample to be checked for parasitic infection.

3. Using the generic names for the antiparasitic drugs in Table 15-2, identify the brand name or trade name used to sell these drugs. This can be done by visiting a livestock supply store, veterinary clinic, or reading a livestock or equine supply catalog.

4. Invite a veterinarian or sales representative to discuss the types of anthelmintics sold. Also, find out what types of restrictions are placed on the sale of these medicines.

5. Research and report the role of bacteria in human health and commerce. Some bacteria cause disease, while other bacteria have a beneficial role in human welfare.

6. Develop a program for regular deworming based on your location and weather conditions. Using a word processor, put this deworming program into a table format.

7. Learn how to check a horse for ticks. Write a paper describing this process.

8. Compare the types of parasites found in cattle, pigs, and sheep to those found in horses. Describe any similarities or differences. Put this information into a table format.



Cheek, P. R. (2004). Applied animal nutrition: Feeds and feeding (3rd ed.). Upper Saddle River, NJ: Prentice Hall.

Griffin, J. M., & Gore, T. (1989). Horse owner's veterinary handbook (2nd ed.). New York: Howell Book House.

Hawcroft, T. (1997). First aid for horses: The essential quick-reference guide. New York: Howell Book House.

Hill, C. (1997). Horse health care. North Adams, MA: Storey Communications.

Kahn, C. M. (Ed.). (2005). The Merck veterinary manual (9th ed.). Whitehouse Station, NJ: Merck & Co.

Kellon, E. M. (1995). Equine drugs and vaccines: A guide for owners and trainers. Ossining, NY: Breakthrough Publications.

Siegal, M. (Ed.), & School of Veterinary Medicine. (1996). Book of horses: A complete medical reference guide for horses and foals. New York: Harper Collins.

University of Missouri-Columbia Extension Division. (n.d.) Missouri horse care and guide book. Columbia: Cooperative Extension Service, University of Missouri and Lincoln University.

U.S. Department of Agriculture. (1923). Special report on diseases of the horse. Washington, D.C.: U.S. Government Printing Office.

Worth, M. (2004). Storey's guide to feeding horses: Lifelong nurition, feed storage, feeding tips, pasture management. North Adams, MA: Storey Publishing.


Internet sites represent a vast resource of information, but remember that the URLs (uniform resource locator) for World Wide Web sites can change without notice. Using one of the search engines on the Internet such as Yahoo!, Google, or, find more information by searching for these words or phrases:


equine encephalomyelitis

parasite control

parasites in horses

(internal and external)

A search for any of the specific parasites will produce additional information. Table A-18 in the appendix also provides a listing of some useful Internet sites that can serve as a starting point for further exploration.
TABLE 15-1 Common Internal Parasites of the Horse

Parasite                     Where Found

Habronema adult              Stomach, injured skin
Habronema larvae
(stomach worm)

Gasterophilus (bots)         Stomach, gums

Parascaris                   Small intestine
(large white worm)

Strongyloides (threadworm)   Small intestine

Anoplocephala (tapeworm)     Small intestine

Strongylus (bloodworm)       Large intestine and colon

Triodontophorus              Large intestine and colon
Poteriostomum Trichonema
and others (small

Oxyuris (pinworm)            Large intestine

Parasite                     Damage

Habronema adult              Causes tumors of wall,
Habronema larvae             granulomatous ulcers
(stomach worm)

Gasterophilus (bots)         Inflammation, perforation
                             of stomach wall, gums

Parascaris                   Irritate intestinal wall,
(large white worm)           possible obstruction

Strongyloides (threadworm)   Erosion of intestinal
                             mucosa, enteritis

Anoplocephala (tapeworm)     Ulceration of ileocecal
                             valve, enteritis

Strongylus (bloodworm)       Adults suck blood, cause
                             ulcers on mucosa. Larvae
                             cause enlargement and
                             aneurysms of anterior
                             mesenteric artery

Triodontophorus              Irritate intestinal wall,
Poteriostomum Trichonema     causing thickening and
and others (small            nodules with larvae in them
strongyles)                  feeding on blood

Oxyuris (pinworm)            Adults feed on gut contents;
                             larvae feed on mucosa

Parasite                     Signs

Habronema adult              Gastritis, digestive
Habronema larvae             disorders; summer sores,
(stomach worm)               often healing
                             spontaneously after first

Gasterophilus (bots)         Digestive upsets and
                             bowel irritation

Parascaris                   Flatulence, diarrhea,
(large white worm)           rough hair coat; "hay
                             belly" more common in
                             young horses

Strongyloides (threadworm)   Anorexia, loss of weight,
                             diarrhea, anemia; common
                             cause of trouble in suckling

Anoplocephala (tapeworm)     Unthriftiness

Strongylus (bloodworm)       Anemia, unthriftiness,
                             colic, anorexia, malaise,
                             soft feces with a foul odor;
                             in large infections, legs
                             and abdomen swell

Triodontophorus              Anemia, anorexia, dark or
Poteriostomum Trichonema     black manure, soft feces
and others (small            with a foul odor; in large
strongyles)                  infections, legs and
                             abdomen swell

Oxyuris (pinworm)            Restlessness, irregular
                             feeding with consequent
                             loss of condition, dull hair
                             coat, tail rubbing

TABLE 15-2 Antiparasitic Compounds for Internal Parasites

Class                Generic Name            Methods (1)

Avermectins          Ivermectin              P, T

Benzimidazoles       Fenbendadzone (FBZ)     T, F, P

                     Mebendazole (MBZ)       T, F, P

                     Oxfendazole (OFZ)       T, F

                     Oxibendazole (BZ)       T, F, P

                     Thiabendazole (TBZ)     T, F

                     MBZ + TCF               T, F, P

                     OFZ + TCF               P

                     TBZ + TCF               T, F

Oranophospates       Trichlorfon (TCF)       T, P

Phenylguanidines     Febantel (FBT)          T, F, P

                     FBT + TCF               P

Pyrimidines          Pyrantel-pamoate        T, F, P

                     Pyrantel-               F
                     tartrate (2)

                                             Percent Effectiveness
Class             Generic Name
                                               Bots     Strongyles

Avermectins       Ivermectin                  95-100      95-100

Benzimidazoles    Fenbendadzone (FBZ)              0      95-100

                  Mebendazole (MBZ)                0       65-95

                  Oxfendazole (OFZ)                0      95-100

                  Oxibendazole (BZ)                0      95-100

                  Thiabendazole (TBZ)              0      90-100

                  MBZ + TCF                   95-100       65-95

                  OFZ + TCF                   95-100      95-100

                  TBZ + TCF                   95-100      90-100

Oranophospates    Trichlorfon (TCF)           95-100           0

Phenylguanidines  Febantel (FBT)                   0      95-100

                  FBT + TCF                   95-100      95-100

Pyrimidines       Pyrantel-pamoate                 0      65-100

                  tartrate (2)

                                            Percent Effectiveness
Class             Generic Name
                                             Pinworms    Ascarids

Avermectins       Ivermectin                  95-100      90-100

Benzimidazoles    Fenbendadzone (FBZ)         95-100      90-100

                  Mebendazole (MBZ)           95-100      95-100

                  Oxfendazole (OFZ)           95-100      90-100

                  Oxibendazole (BZ)           95-100      90-100

                  Thiabendazole (TBZ)         90-100       10-75

                  MBZ + TCF                   95-100       65-95

                  OFZ + TCF                   95-100      95-100

                  TBZ + TCF                   90-100      95-100

Oranophospates    Trichlorfon (TCF)           90-100      95-100

Phenylguanidines  Febantel (FBT)              95-100      95-100

                  FBT + TCF                   95-100      95-100

Pyrimidines       Pyrantel-pamoate             60-70      90-100

                  tartrate (2)

(1) F = feed, P = paste, T = stomach tube.

(2) Infective larvae are prevented from entering the tissues.
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Author:Parker, Rick
Publication:Equine Science, 3rd ed.
Date:Jan 1, 2008
Previous Article:Chapter 14 Health management.
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