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Endoparasites - Strongyles, Large

Strongylus vulgaris: Bloodworm, Large strongyles. Also included in the term "large strongyles" are Triodontophorus spp. as well as Strrongylus equinus and Strongylus edentatus.

General Description: These worms are stout-bodied. Strongylus vulgaris is the smallest, 2 cm. long. S. equinus reaches 5 cm. and is the largest of the three. When freshly collected, all are reddish brown due to the ingestion of blood.

Life Cycle: The eggs of the 3 species develop to infective third stage larvae on pasture. Moisture is critical for larval survival, and warmth speeds development to as little as 3 days. Third- stage ingested larvae drop their protective sheath in the small intestine. After this point, the Strongylus species have different developmental patterns:

  • . The third-stage larvae penetrate the intestinal mucosa, where they moult to fourth-stage larvae in several days. Penetrating nearby blood vessels, the larvae wander through the arteries for 2 weeks before reaching the anterior mesenteric artery, where they remain for 4 months. After moulting to immature adults, S. vulgaris returns via the arteries to the large intestine and burrows into the lumen. Six to 8 months after the original infection eggs begin to pass in the faeces.
  • Strongylus equinus. After burrowing into the submucosa and moulting to fourth-stage larvae, these parasites migrate to the liver where they wander for 6-7 weeks. Emerging from the liver, they molto to immature adults in various abdominal organs, then return to the large intestine. About 9 months after infection mature adults lay eggs.
  • Strongylus edentatus. Larvae penetrate the intestine and migrate via the portal vein to the liver, where moulting occurs. After 9 weeks, the fourth-stage larvae wander in the peritoneum, causing the formation of nodules. Migrating to the large intestine, the strongyle larvae form nodules in the gut wall, which they rupture to enter the colon lumen.

Location: Large intestine and cecum.

Geographical Distribution: Economically important wherever horses are raised.

Significance: The large strongyles are the most important parasites of horses. Of the three species, S. vulgaris is the most pathogenic.

Effect on Host: Larvae irritate the walls of the small intestine and the arteries. Strongylus vulgaris larvae cause roughening of the arterial walls, providing sites for clotting. These clots may break off and lodge in other vessels, cutting off the blood supply to various body parts. The weakened arteries may develop aneurisms, or more correctly, verminous arteritis. These aneurisms may burst, causing death. Adult large strongyles are "plug feeders," which feed by ingesting plugs of mucosa and capillaries. Heavy feeding of this type produces intestinal damage, anaemia, fluid loss into the intestine, and blood protein loss. Intestinal damage causes diarrhoea, fever, edema, anaemia, anorexia, depression, weight loss, and dehydration. Lameness may result from hind leg circulation being blocked by blood clots. Strongylus equinus and S. edentatus cause liver damage and peritonitis. The majority of colic in horses has been attributed to the lesions caused by the migratory stages of S. vulgaris.

Diagnostic Information: After adult worms are present, eggs may be found in the faeces. Larvae are needed for species identification. Diagnosis is difficult during the prepatent, migratory phase.

Control: Treatment with an effective anthelmintic is necessary. In particular, since foals are very susceptible, brood mares should be treated and moved to clean pastures. Overcrowding must be avoided. Removal of faeces, ploughing to break up dense ground cover, and frequent faecal checks will aid in Strongylus contr


Large numbers of Strongylus adults in horse´s intestines   Strongylus - egg   Migratory tracts in lining of blood vessel
S. vulgaris showing mouth grasping tissue   Strongylus larva in subperitoneal cyst   Dissection of verminous arteritis




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