EVALUATION OF IVERMECTIN AT AN ELEVATED DOSE AGAINST ENCYSTED EQUINE CYATHOSTOME LARVAE

The efficacy of a high dose of ivermectin (1.0 mg per kg Eqvalan liqui d drench) on encysted cyathostomes was tested in a controlled study us ing 12 adult ponies with naturally acquired cyathostome infections. Si x treated ponies and six non-treated controls were held in separate st alls for a period of 5 weeks. Cyathostome burdens, which included lume nal larvae, adults and encysted larvae, were determined at necropsy. T he viability of encysted larvae, based on morphologic integrity, was a ssessed by observation of mural transillumination and by the histologi c appearance of 12 larvae per pony. Efficacy against adult cyathostome s was 99.9%. Lumenal cyathostome larval numbers were reduced by 87%. N umbers of encysted cyathostome larvae, identified by transillumination of the large intestine, were reduced by 35%. However, this reduction was not statistically significant (P>0.05) and differences in viabilit y of encysted larvae were not observed. The data strongly indicated th at ivermectin has little demonstrable effect on encysted equine cyatho stomes.