Effectiveness of fenbendazole against later 4th-stage Strongylus vulgaris in ponies.

Twelve pony foals (reared worm-free) were inoculated with Strongylus vulgaris. Approximately 8 weeks later, 4 of the foals were given fenbendazole (10% suspension) at a dosage rate of 10 mg/kg of body weight daily for 5 days and 4 foals were given the suspension at a dosage rate of 50 mg/kg daily for 3 days; the remaining foals were given a placebo. All treatments were administered by stomach tube. Fenbendazole was 99.6 and 97.9% effective in the 2 treatment groups, respectively, in eliminating later 4th-stage S vulgaris larvae located near the origin of major intestinal arteries. On microscopic examination of the ileocolic artery from fenbendazole-treated foals, a few larval remnants were found beneath the tunica intima in small organized mural thrombi overgrown with endothelium. It would appear that larvae are rapidly destroyed after administration of fenbendazole. A pony foal reared on pasture and with arteriographic evidence of arteritis of the cranial mesenteric and ileocolic arteries was treated with fenbendazole (10% suspension) by stomach tube at a dosage rate of 50 mg/kg of body weight daily for 3 days. By arteriographic examination made 4 weeks later, there was evidence of regression of the lesion, and at necropsy done a week later, there was no arteritis or larvae in the lumen of those arteries.