The elimination of equine strongyles and hematological and pathological consequences following larvicidal doses of thiabendazole

Twelve horses wer divided into three groups and given various doses of a mixed species strongyle inoculum, representing light, moderate, and heavy infctions. Three weeks after the larval inoculations, three animals from each group were given larvicidal doses of thiabendazole (TBZ) (440 mg kg^?1 on two consecutive days); one animal from each group served as a non-medicated control. Treatment was repeated three weeks later. One treated animal from each group was designated for long-term study; others were necropsied to study adult and larval parasite loads.Six of the twelve animals with strongylosis developed moderate eosinophilia.TBZ given at 440 mg kg^? on two consecutive days caused depression, lethargy, and anorexia which lasted for five days. Eosinopenia, lymphopenia, and neutrophilia occured in treated animals, and lasted for three days. During the course of TBZ treatment, one horse died from what appeared to be a mis-dosing or an anaphylactic reaction.At necropsy, active thrombi of the anterior mesenteric artery were seen in parasitized animals, but not in those treated with TBZ. Five out of seven medicated horses were completely free of adult and larval strongyle parasites. One had a few STrongylus edentatus larvae and another had small strongyles. No Strongylus vulgaris larvae or adults were recovered from any horse treated with TBZ.