Equine Pinworm Oxyuris equi Not Resistant to Pyrantel Pamoate or Ivermectin

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.

Key findings of the study were that significant decreases in adult pinworms were noted after administration of either pyrantel pamoate or ivermectin. Treatment efficacies against the fourth stage larvae were 91.2% and 96.0% for pyrantel pamoate and ivermectin, respectively.

According to the study authors, “The current study demonstrated acceptable adulticidal and larvicidal efficacy of both pyrantel pamoate and ivermectin paste formulations against O. equi and did not support the existence of macrocyclic lactone or pyrimidine resistance in the pinworm populations evaluated."

The study, "Efficacy of pyrantel pamoate and ivermectin paste formulations against naturally acquired Oxyuris equi infections in horses," is scheduled to be published in an upcoming edition of the journal Veterinary Parasitology.

The abstract is currently available on PubMed.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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