Merck Manual

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Common Gastrointestinal Parasites of Horses

Common Gastrointestinal Parasites of Horses

Common and Scientific Names

How Contracted

Signs

Treatment and Prevention

Stomach worms (Habronema species and Draschia megastoma)

Horses ingest house or stable flies (or their larvae) 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 2 months of age and repeated every 6 to 8 weeks for the first year. Prevention includes daily administration of pyrantel tartrate in grain.

Pinworms (Oxyuris equi)

Most common in horses less than 18 months old.

Egg masses around anus appear as white to yellow 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 (cyathostomins, 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).