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 liquid drench) on encysted cyathostomes was tested in a controlled study using 12 adult ponies with naturally acquired cyathostome infections. Six treated ponies and six non-treated controls were held in separate stalls for a period of 5 weeks. Cyathostome burdens, which included lumenal larvae, adults and encysted larvae, were determined at necropsy. The viability of encysted larvae, based on morphologic integrity, was assessed by observation of mural transillumination and by the histologic appearance of 12 larvae per pony. Efficacy against adult cyathostomes was 99.9%. Lumenal cyathostome larval numbers were reduced by 87%. Numbers 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 viability of encysted larvae were not observed. The data strongly indicated that ivermectin has little demonstrable effect on encysted equine cyathostomes.