Parasiticidal Resistance Reported in New Study

Kentucky researchers report that roundworms and small strongyles, two common equine intestinal parasites, are developing resistance against most of the commercially available worming products. What's worse, no new drugs against either of these parasites are forthcoming on the market.

"Since drug resistance by small strongyles and roundworms has been increasing since the late 1950s, the purpose of this study was to obtain field data regarding the efficacy of commercial wormers to obtain more information on the development of drug resistance," explained Eugene Lyons, PhD, from the University of Kentucky's Gluck Equine Research Center.

Drug resistance by small strongyles and roundworms has been increasing since the late 1950s.
Between June 1 and December 20, 2007, foals on five farms located in Central Kentucky were monitored in this field study. Efficacy of fenbendazole, oxibendazole, pyrantel pamoate, and ivermectin were evaluated by calculating the reduction in the number of foals with roundworm eggs or small strongyle eggs in their feces before versus after treatment.

"Our results showed that there was a significant reduction in roundworm eggs after treatment with fenbendazole and oxibendazole, but not ivermectin or pyrantel pamoate," summarized Lyons. "In addition, only ivermectin reduced the number of small strongyle eggs--the remaining three drugs were ineffective."

Lyons explained that while ivermectin still appears to be effective against small strongyles, research using worm count data in addition to egg count data needs to be done in order to evaluate why eggs of these parasites are returning more quickly than previously after treatment.

"In this study, we calculated eggs counts only one to two weeks post-worming. This might have been too early and providing misleading information because other studies have suggested that small strongyle eggs counts are returning more quickly than they used to post-treatment," said Lyons. (See Study: Small Strongyles Developing Resistance to Ivermectin for more on this.)

Parasite resistance to these drugs is a serious concern for horse owners, veterinarians, and the scientific community in general.

The study, "Evaluation of parasiticidal activity of fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate in horse foals with emphasis on ascarids (Parascaris equorum) in field studies on five farms in Central Kentucky in 2007," was published in the July 2008 edition of Parasitology Research.

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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