Because parasite resistance to dewormers is developing in the United States, owners and trainers should deworm less often and only use products they know are effective in their herd, suggested Craig R. Reinemeyer, DVM, PhD, president of East Tennessee Clinical Research, Inc. in Rockwood, Tenn.

In a recent study, Reinemeyer and his colleagues tested whether a resistant strain of Parascaris equorum, the ascarid of horses, could be controlled with pyrantel pamoate (Teva Animal Health). They infected 14 naive foals with eggs from a strain of P. equorum that they knew was resistant to macrocyclic lactone dewormers (ivermectin and moxidectin) and then randomized the foals into two groups; one received pyrantel pamoate and one went untreated. Afterward, they counted the ascarid eggs in manure every day and then the number of worms recovered from foals at necropsy. Treated foals had significantly lower egg counts and significantly fewer adult ascarids than the untreated ones.

"Pyrantel pamoate (13.2 mg/kg) showed 97.3% efficacy against a known-resistant strain," said Reinemeyer, who added that resistance is becoming a serious problem "especially in those areas, such as Lexington and Ocala, where breeding and foaling of Thoroughbreds is highly centralized."

Reinemeyer attributes the resistance to exclusive and overuse of a single class of anthelmintic (dewormer) (macrocyclic lactones). "Because moxidectin is not approved for use in foals in the U.S., most cases will be observed after the use of ivermectin," he said