Pyrantel pamoate and ivermectin paste formulations appear to be effective against the equine pinworm Oxyuris equi despite allegations that resistance was developing, reports Craig R. Reinemeyer, DVM, PhD, president of East Tennessee Clinical Research Inc., in Rockwood, Tenn.

In light of the anecdotal reports suggesting that O. equi were not being removed following treatment with anthelmintics and because the possibility of macrocyclic lactone resistance to O. equi had not been formally evaluated, Reinemeyer and colleagues put the worms to the test.

Unlike other equine internal parasites, pinworms are a bit trickier to study because the adult worms do not lay eggs that can be counted in feces. Instead, the adult female worms residing in the large intestine protrude from the anus, deposit sticky eggs on the anus and perianal skin, then migrate back into the large intestine. That is, pinworm eggs are not routinely passed in feces and therefore can not be counted using conventional methods.

To assess the efficacy of anthelmintics against equine pinworms, 21 naturally infected horses were treated with pyrantel pamoate, ivermectin, or nothing (control). Fourteen days following treatment, the large intestinal contents were collected and the numbers of adult and fourth stage larval O. equi were counted