Resistant Parascaris equorum on Thoroughbred and Standardbred farms to ivermectin and moxidectin was identified in Ontario, Canada.

Rolph de Gannes, an equine veterinarian practicing in Southern Ontario, Canada, had the impression that ivermectin was not removing the equine roundworm Parascaris equorum in foals and weanlings on a Thoroughbred farm. Because resistance of P. equorum to ivermectin and moxidectin has previously been reported, de Gannes, together with J. Owen D. Slocombe, DVM, PhD and Mary C. Lake, RVT from the Ontario Veterinary College, performed 12 clinical trials in 2002 and 2003.

In total, 76 foals from three separate farms (two Thoroughbred and one Standardbred) were included in the study. Foals were included in the clinical trials if they were positive for P. equorum on fecal analysis.

In all foals a fecal sample was obtained two to three days before chemical deworming (i.e., pre-treatment, PRT) and again 12-18 days post-treatment (POT). Foals were treated with ivermectin paste, moxidectin gel, fenbendazole paste, pyrantel pamoate paste, or nothing (control). The weight of each foal was determined using a weigh band and each anthelmintic (dewormer) was administered according to the manufacturer's recommended dosages