Effect of ivermectin treatment on eosinophilic pneumonia and other extravascular lesions of late Strongylus vulgaris larval migration in foals.

Eighteen parasite-free pony foals were infected orally with 500 third stage larvae of Strongylus vulgaris. At 56 days after infection, six ponies were treated with intramuscular ivermectin (22, 23-dihydroavermectin B1); six were treated with oral ivermectin; and six were not treated. Necropsy was done 91 days after infection to study the pathologic effects of migrating S. vulgaris larvae and to determine the efficacy of ivermectin in attenuation of S. vulgaris-induced lesions. Larval migration induced eosinophilic inflammation of the liver, spleen, mesenteric, colic and cecal lymph nodes, and small and large intestine. Previously unreported parasitic lesions included eosinophilic pneumonia with eosinophilic granulomas and pulmonary lymphoid nodules. S. vulgaris larvae were observed in eosinophilic granulomas in the lung, epicardium, liver, and intestinal serosa. Injectable and oral ivermectin formulations were equally effective in reduction of these lesions.