THE MERCK VETERINARY MANUAL
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Large Strongyles in Horses

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The large strongyles of horses are also known as blood worms, palisade worms, sclerostomes, or red worms. The three major species are Strongylus vulgaris (up to 25 mm), S edentatus (up to 40 mm), and S equinus (up to 50 mm). (See alsoTriodontophorus spp, Small Strongyles in Horses.) Under favorable conditions, the larvae develop to the infective stage within 1–2 wk after the eggs are passed. Infection is by ingestion of infective larvae, which exsheath in the intestine and migrate extensively before developing to maturity in the large intestine. The prepatent period is 6–11 mo. The larvae of S vulgaris migrate extensively in the cranial mesenteric artery and its branches, where they may cause parasitic thrombosis and arteritis. Larvae of the other two species may be found in various parts of the body, including the liver, perirenal tissues, retroperitoneal tissues, and pancreas. These species do not produce lesions in the mesenteric arteries. Mixed infections of large and small strongyles are the rule.

Adult large strongyles have large buccal capsules and are active blood feeders; they ingest mucosal plugs as they move about in the intestine. The associated blood loss may lead to anemia. Weakness, emaciation, and diarrhea are also common. S vulgaris is important because of the damage it does to the cranial mesenteric artery and its branches. As a result of the interference with the flow of blood to the intestine and thromboembolism, any of several conditions may follow, including colic; gangrenous enteritis; or intestinal stasis, torsion or intussusception, and possibly rupture. Cerebrospinal nematodiasis (see Nematodes Causing CNS Disease) can cause a variety of lesions and signs depending on the part of the CNS affected.

Diagnosis of mixed strongyle infection is based on demonstration of eggs in the feces. Specific diagnosis can be made by identifying the infective larvae after fecal culture. Serologic diagnosis based on a rise in β-globulins has been recommended but is not specific for S vulgaris. Parasitic arterial lesions have been demonstrated using arteriography in ponies and small horses. Efforts have been made to utilize molecular methods and antibody measurements for prepatent diagnosis of S vulgaris infections, but these techniques are not yet available for standard use.

Colic due to arterial lesions has been successfully controlled by anthelmintic treatments. Ivermectin and moxidectin at standard dosages are effective against the larval stages (L4 and L5) of S vulgaris; fenbendazole and oxfendazole, at dosages higher than that for adult parasites, are also effective against larval infections. Daily administration of pyrantel tartrate effectively prevents establishment of arterial stages of S vulgaris. A number of anthelmintics, including the benzimidazoles, pyrantel, and ivermectin, are active against adult large strongyles. Large strongyle infections have been eliminated from closed herds with ivermectin treatment.

Parasite control programs are designed to minimize the level of pasture contamination and thereby reduce the risks associated with migrating larvae. Routine anthelmintic treatments do this by preventing fecal excretion of strongyle eggs. (Also see Small Strongyles in Horses.)

Last full review/revision October 2014 by Thomas R. Klei, PhD

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