Numerous parasites can infect the digestive system of horses (see Digestive Disorders of Horses: Common Gastrointestinal Parasites of Horses). The most common ones are described in the following text.

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Common Gastrointestinal Parasites of Horses Common and Scientific Names How Contracted Signs Treatment and Prevention Habronema species Horses ingest larvae of house or stable flies that contain larvae of the stomach worms. Worms produce tumor-like enlargements in stomach; signs usually absent unless enlargements lead to obstruction or rupture. Some medications may be effective. Horse bots (Gasterophilus species) Ingestion of bot larvae (see text for details). Stomach and sometimes mouth irritation. Treatment includes medication. Bot control programs can be effective if done on a regional basis. Large strongyles (Strongylus vulgaris, S. edentatus, S. equinus) Infection is passed by eating larvae on grass or bedding, which migrate through the body. Signs include weakness, emaciation, colic, diarrhea, and anemia; diagnosis is by finding eggs in feces. Control programs limit pasture contamination by preventing fecal excretion of eggs. Regular deworming. Ascarids (Parascaris equorum) Affects mostly foals; infection sources are pastures, paddocks, and stalls contaminated with eggs from foals of previous year. Migrating larvae can produce respiratory signs (“summer colds”). Heavily infected foals show unthriftiness, loss of energy, occasionally colic with obstruction of the intestine by the worms. Diagnosed by presence of worms or their eggs in feces. Regular treatment of foals with antiparasitic medications beginning at 1 month of age recommended on farms where infection is common. Prevention includes daily administration of pyrantel tartrate in grain. Pinworms (Oxuris species) Most common in horses less than 18 months old. Egg masses around anus appear as white crusty mass. Signs include horse rubbing anal and tail regions, causing bare patches. Medications provide effective treatment. Small stomach worm (hairworm, Trichostrongylus axei) Normally a problem only in horses on pastures also used by ruminants (cows, sheep, or goats). Stomach inflammation resulting in weight loss. Detected after culture of feces. Some antiparasitic drugs are effective. Small strongyles (Cyathostomes, multiple species) Infection passed by eating larvae on contaminated pasture. Often few signs; heavy infections may lead to poor absorption of nutrients and weight loss. Several medications available, although some worms may be resistant to particular drugs. Strongyloides westeri Transmitted from mares to foals in milk. Possible cause of early diarrhea in foals. Infection of foals can be prevented by treatment of mares with ivermectin 24 hours after foaling. Tapeworms (Anoplocephala magna, A. perfoliata, Paranoplocephala mamillana) Transmitted by eating soil mites that are the intermediate host. Diagnosed by finding eggs in feces, which may require multiple tests. Several medications provide effective treatment (see text for details).

Ascarids

Horse ascarids (Parascaris equorum) are stout, whitish worms up to 12 inches (30 centimeters) long that primarily affect foals. Foals get the worms by ingesting eggs, which can remain viable for years in contaminated soil. The main sources of infection are pastures, paddocks, or stalls contaminated with eggs from foals of the previous year.

In heavy infections, migrating larvae may produce respiratory signs (“summer colds”). Infected foals can show poor condition, lack of energy, or colic. Intestinal obstruction and perforation have also been reported.

Diagnosis is based on finding eggs in feces. If infection is suspected, but the worms have not matured enough to begin producing eggs, deworming medication can be given. This will cause large numbers of immature worms to be passed in the feces.

On farms where the worms are common, most foals become infected soon after birth. Treatment with a deworming medication should be started when foals are about 8 weeks old and repeated at 6- to 8-week intervals until they are yearlings. Some medications can also prevent ascarid infection if given daily once foals begin eating grain regularly.

Horse Bots

Horse bots are found in the stomach. They are the larvae of bot flies, Gasterophilus species. Three common species are distributed worldwide, and a number of others are found in parts of Europe, Africa, and Asia. The adult flies lay eggs that stick to the hairs of the horse's body. When the horse grooms itself, wetting the mature eggs, the larvae then hatch and are carried by the tongue into the mouth. The larvae stay embedded in the tongue or the lining of the mouth for about 1 month, after which they pass to the stomach, where they attach themselves. The main disease effects are caused by the larvae, which attach by oral hooks to the lining of the stomach, causing wounds and ulceration.

Bots cause a mild stomach inflammation and potentially ulcers, but large numbers may be present without any signs. The larvae migrating in the mouth can cause inflammation and may produce pain during eating. The adult flies may annoy horses when they lay their eggs. Sometimes the yellow to cream-white bot eggs (1 to 2 millimeters long) on the horse's hairs can be seen. Specific diagnosis of infection is difficult; gastroscopy (viewing the inside of the stomach with an endoscope) is the most effective means of diagnosis.

Treatment usually involves appropriate antiparasitic drugs. The drug ivermectin is very effective against oral and stomach stages of bots and, when used as part of a routine parasite control program, provides effective bot control throughout the season. Moxidectin is effective against stages living in the stomach. Although there is no satisfactory method for protecting exposed horses from attack by the adult flies, bot control programs can greatly reduce fly numbers and larval infections. To control bots, your veterinarian will recommend a treatment program based on your local environment and bot fly season, the degree of infection, and the medical history of your horse. If you recognize bot eggs on your horse, you can clean the area with warm water. This causes the bots to hatch but reduces the likelihood of the horse ingesting the larvae.

Large Strongyles

The large strongyles of horses belong to a group of parasites known as roundworms. They are also known as blood worms or red worms. Horses become infected when they ingest larvae in forage contaminated by feces. Once inside the horse, the larvae become active in the intestine and migrate extensively (in blood vessels and other organs) before developing to maturity in the large intestine. In the intestine, they cause anemia due to blood loss, weakness, weight loss, and sometimes diarrhea. One strongyle, Strongylus vulgaris can damage the anterior mesenteric artery and its branches, which interferes with the flow of blood to the intestines. This can cause colic, tissue death, twisting of the intestine, intussusception (telescoping on itself) of intestinal tissues, bleeding, or possibly intestinal rupture. The central nervous system can also be affected by these parasites.

Typically horses become infected with both large strongyles and small strongyles (see Digestive Disorders of Horses: Small Strongyles). Diagnosis is based on the detection of eggs in the feces. When treating this infection, your veterinarian will often recommend both antiparasitic medication and measures to control the presence of the parasites in the horse's environment. Parasite control programs are designed to minimize the level of pasture contamination and thus reduce the risks associated with migrating larvae. Routine antiparasitic treatments do this by preventing excretion of strongyle eggs in the feces.

Small Strongyles

Over 40 species of small strongyles can infect domestic horses. The adult worms live within the large intestine. Unlike the large strongyles, small strongyles do not migrate outside of the intestinal wall, and they are generally less damaging. Healthy horses may have large numbers of small strongyles without signs of infection. In heavier infections, however, disruption may be extensive enough to disturb digestion and absorption of nutrients, resulting in loss of condition, inflammation of the mucous membranes of the large intestine, and diarrhea.

Larval cyathostomiasis is a syndrome of sudden weight loss, often with severe diarrhea, associated with the mass emergence of previously inactive strongyle larvae from the intestinal wall. It is seen in temperate areas in late winter and spring, particularly in young ponies and horses (those less than 5 years old). This condition is not common in the United States, but it occurs more frequently in Europe. The bright red larvae can sometimes be seen in the feces and are helpful in making a diagnosis. Biopsy of the large intestine also may assist in diagnosis.

Adult strongyles are easily removed from the intestines by a variety of antiparasitic drugs. Small-strongyle larvae in the intestinal tissues are much more difficult to effectively remove with medication. Horses already infected with these parasites may not respond to treatment if inflammation is too severe. In these cases, the prescribed treatment often includes supplementing antiparasitic drugs with corticosteroids and other appropriate supportive treatment.

Routine or interval antiparasitic treatments can minimize the level of pasture contamination, thereby reducing the risks associated with the accumulation of larvae and adult worms. Alternatively, infection may be prevented by daily administration of prescription medication. The interval between routine treatments depends on several factors including the drug; the horse's age, health, and value; and the level of risk of disease. Removal of feces from paddocks and pastures aids in control and may also reduce the number of antiparasitic treatments required. As is the case with other infective worms, your veterinarian may recommend a treatment program that involves medication, supportive care for the horse, and steps to reduce the presence of the worms in the horse's environment.

Tapeworms

Tapeworms (cestodes) may be found in the small or large intestine but may also be in the stomach. Infection usually occurs when horses eat pasture mites that become infected by ingesting the tapeworm eggs. Larvae develop and mature in the horse's gastrointestinal tract.

In light infections, no signs of disease are present. In heavy infections, digestive disturbances can be seen. Horses may lose weight and condition and become anemic. Ulceration may occur where the tapeworm attaches to the intestinal lining. Intestinal perforation, abdominal infection, and colic may be seen in some infections. Colic is more likely in horses with tapeworm infections, and it often recurs.

Diagnosis is made by identifying tapeworm eggs in the feces. Because the eggs are not always present, multiple fecal samples and multiple tests may be required to make a definite diagnosis.

Your veterinarian will recommend medical treatment based on the species of tapeworm. Anoplocephala species can be effectively treated with pyrantel salts and praziquantel. The drug praziquantel appears to be effective in removing -Paranoplocephala mamillana, while pyrantel salts are not. In facilities where tapeworms are common, signs of tapeworm infections can be prevented by pyrantel salts routinely administered daily during the grazing season, or praziquantel administered within an interval deworming program. Follow your veterinarian's recommendations as to an appropriate treatment program for your horse.

Protozoa

Protozoa are single-celled, microscopic parasites. Several types can infect the horse's digestive tract.

Cryptosporidiosis

Cryptosporidiosis is an uncommon infection in foals caused by the parasite Cryptosporidium parvum. The source of infection is egg cysts (oocysts) containing the parasite. The oocysts are excreted in the feces of infected horses or other infected species (such as rodents or farm cats). Simultaneous infections, especially with rotavirus and coronavirus, are common; studies suggest that diarrhea is more severe in these mixed infections. Cryptosporidiosis is usually not fatal unless complicated by other factors such as other infections, energy deficits from inadequate intake of colostrum and milk, a weakened immune system, or chilling.

Signs include mild to moderate diarrhea that persists for several days regardless of treatment. Feces are yellow or pale, watery, and contain mucus. The diarrhea may result in weight loss and emaciation. In most cases, the diarrhea improves on its own after several days. Varying degrees of apathy, poor appetite, and mild dehydration are present. More severe dehydration, weakness, and collapse are rare. A diagnosis is based on laboratory detection of oocysts in feces.

A number of drugs have been tested for their effectiveness against Cryptosporidium, but at this point, few if any medications are available. As with other intestinal illnesses, cleanliness and isolation of sick animals are the best routes of control.

Infections in domestic animals may be a source of infection for susceptible people, particularly those with weakened immune systems, in whom disease can be severe. The infection is transmitted predominantly from person to person, but direct infection from animals and waterborne infection from contamination of surface water and drinking water by domestic or wild animal feces can also be important.

Giardiasis

Giardiasis is a chronic, intestinal protozoal infection that is seen worldwide in most domestic and wild mammals, many birds, and people. Though infection is common in dogs and cats, it is rare in horses (see Digestive Disorders of Dogs: Giardiasis).

Last full review/revision July 2011 by Peter D. Constable, BVSc (Hons), MS, PhD, DACVIM; Gordon J. Baker, BVSc, PhD, MRCVS, DACVS; Joseph A. DiPietro, DVM, MS; Walter Ingwersen, DVM, DVSc, DACVIM; John E. Madigan, DVM, MS; James N. Moore, DVM, PhD; Michael J. Murray, DVM, MS; Sofie Muylle, DVM, PhD; Stanley I. Rubin, DVM, MS, DACVIM; Susan D. Semrad, VMD, PhD, DACVIM; Josie L. Traub-Dargatz, DVM, MS, DACVIM