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Pet Owner Version

Gastrointestinal Parasites of Horses


Thomas R. Klei

, PhD, School of Veterinary Medicine and Louisiana Agriculture Experiment Station, Louisiana State University

Reviewed/Revised May 2019 | Modified Oct 2022
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Numerous parasites can infect the digestive system of horses ( see Table: Common Gastrointestinal Parasites of Horses Common Gastrointestinal Parasites of Horses Common Gastrointestinal Parasites of Horses ). The most common ones are described in the following text.


Stomach Worms (Habronema and Draschia)

The stomach worms Habronema muscae, H microstoma, and Draschia megastoma are common in horses. The adult worms are small (6-25 mm long). Draschia are found in tumor-like swellings in the stomach wall, while the other species are found on the surface of the stomach lining. The eggs or larvae are ingested by larvae of house or stable flies, which serve as intermediate hosts. Horses are infected by swallowing flies that contain infective larvae or by free larvae that emerge from flies as they feed around the lips.

A bloody gastritis may result from heavy infections with adult worms. Draschia produces the most severe lesions--tumor-like enlargements up to 10 cm (4 inches) in diameter. These are filled with pus and a large number of worms and are covered by an intact stomach lining, except for a small opening through which the eggs pass. Rarely, these masses rupture and cause fatal inflammation of the abdomen (peritonitis). Larvae of Habronema and Draschia have been found in the lungs of foals associated with bacterial abscesses ( see Rhodococcus equi Pneumonia Rhodococcus equi Pneumonia Pneumonia is an infection of the lungs and airways that causes disturbance in respiration and deficiency of oxygen in the blood. It is common in foals up to 6 months of age. The causes of foal... read more ).

Diagnosis is difficult, because the eggs are not easily detected in feces using standard methods. Molecular methods have recently been developed for this purpose but are not be useful for routine use. Worms and eggs may be found by collecting stomach fluid. Most antiworming medications (anthelmintics) have not been tested against Habronema or Draschia, although some are effective against their cutaneous larvae and against adults of H muscae.


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. As a result, most worms are maturing when the foals are approximately 4 to 5 months old. 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.

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 larvae hatch, they crawl or are carried by the tongue (after grooming) 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. The larvae may also cause wounds in the mouth that can drain discharge of mucus and pus.

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 but can be made by identifying the larvae in the horse's feces.

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.

Large Strongyles

The large strongyles of horses belong to a group of parasites known as roundworms. They are also known as blood worms, palisade worms, sclerostomes, 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 cranial mesenteric artery and its branches, which interferes with the flow of blood to the intestines. This can cause colic, tissue death, impaired intestinal motility, 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 below). 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.

Small Strongyles

More than 40 species of small strongyles (called cyathostomins) 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 affected by larval cyathostomiasis 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.

Removal of feces from paddocks and pastures aids in control and may also reduce the number of antiparasitic treatments required. Some small strongyles are resistant to some types of medications, so the effectiveness of the deworming protocol is assessed by performing periodic fecal exams. 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.

Small Stomach Worm (Trichostrongylus)

The small stomach worm (hairworm) of horses, Trichostrongylus axei, is also found in cattle, so it is generally only a problem in horses kept on pastures with cows. Details of the life cycle in horses have not been carefully studied, but it is known that the larvae penetrate the lining of the stomach. These worms produce a longterm, bloody inflammation of the stomach (catarrhal gastritis), which may result in weight loss. The affected area may be rather small and irregular, or it may grow to involve much of the stomach and cause ulcers.

Diagnosis can be difficult, because the eggs look like those of other worms. Growing the eggs in the lab until they turn into larvae may be necessary. Several effective medications are available for treatment.


Strongyloides westeri is a worm found in the small intestine in foals. Adult horses are rarely infected, but mares often have larval stages within their tissues that are activated when the mare gives birth and are then transmitted to foals in the milk. However, it is not clear if S westeri infection causes diarrhea in foals. The diagnosis can be made by finding the eggs in feces. Several medications are available to treat the infection. Prevention of passing the worms to foals during nursing can be done by treating the mare within 24 hours of foaling.


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 or infections may occur where the tapeworm attaches to the intestinal lining. 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. Follow your veterinarian’s recommendations as to an appropriate treatment program for your horse.


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


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, calves, 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.

Foals with normal immune systems do not usually show any signs. If present, signs of cryptosporidiosis 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. Longterm infections can occur in Arabian foals with inherited combined immunodeficiency. 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 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 less common in horses. Giardia protozoa have been reported to be found in 0.5% to 20% of fecal samples from horses. Younger animals are more likely to be affected.

Giardia protozoa live in the small intestine, where they attach and multiply. They produce cysts that are passed in the feces. Transmission occurs when a horse eats these cysts, either through contact with an infected animal or a contaminated environment. Shedding of cysts by an infected animal may be continuous over several days and weeks but is often intermittent. The cysts survive well in the environment. It is currently unclear whether the same species of Giardia can infect both domestic animals and people. It appears that some Giardia species can infect a variety of mammals, whereas others only infect a single species.

Giardia protozoa decrease the absorption of nutrients and water from the small intestine, which can result in diarrhea and weight loss. Giardiasis is diagnosed by identifying the protozoa in the feces. Your veterinarian will recommend an appropriate treatment.

Giardia cysts in the feces are a source of infection and reinfection, particularly those in crowded environments. Prompt removal of feces limits environmental contamination. Your veterinarian can recommend appropriate disinfectants to reduce the spread of the protozoa in the environment.

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