Drug Resistance from Daily Dewormers

Are daily dewormers contributing to the development of drug-resistant parasites? A study from the North Carolina State University (NCSU) suggests they might be. The study used fecal worm egg count reduction tests (FWECRT) to evaluate small strongyle resistance to oral Strongid paste dewormer (pyrantel pamoate 6.6 mg/kg) in 16 horses boarded at a large farm that were already receiving a daily dewormer (pyrantel tartrate). Baseline fecal worm egg counts (FWEC) were considered unacceptably high in five of the horses. The FWEC of two horses in the study actually increased after treatment with pyrantel pamoate, while several others had suboptimal levels of reduction. The study concluded, "Daily administration of pyrantel tartrate was clearly inadequate for prophylactic control of strongyles on the farm. In addition, strongyles resistant to pyrantel pamoate were identified in seven of the nine horses tested."

"In certain circumstances they (daily dewormers) may be useful, but I think people rely on them too much," said Dianne Little, BVSc, MSpVM, MRCVS, Dipl. ACVS, a research associate in the Carolina Colic and Digestive Disease program at NCSU. "They ignore other aspects of parasite control such as reducing the horse's parasite intake from pasture." She added that the use of low doses of dewormer such as that present in feed blocks have been shown to cause resistance to dewormers in sheep. "Effective control of strongyles, particularly cyathostomes (small strongyles), is becoming increasingly difficult in horses as anthelmintic (dewormer) resistance becomes more commonly recognized," Little said.

She explained, "If worms are exposed to low doses of a drug over a period of time, some develop mechanisms to survive treatment." The surviving parasites are able to pass along the drug resistance to the next generation. Eventually enough worms become resistant that treatment fails and resistance is detected.

To help minimize reliance on anthelmintics, Little said it's important to reduce the amount of parasites to which horses are exposed. She suggested owners follow pasture hygiene protocols such as composting manure or not spreading it on pastures, resting pastures, collecting manure from pastures, and cross-grazing with sheep or cattle. Little added that minimizing the number of deworming treatments and dosing according to the horse's body weight is also essential.

"We should be at least monitoring fecal worm egg counts for horses that are receiving daily dewormers," she explained. Before incorporating any daily dewormer into your control program, Little recommends performing a FWECRT with pyrantel pamoate to detect drug resistance. In addition, FWEC should be performed biannually to monitor control. For more information on FWEC tests see www.TheHorse.com/emag.aspx?ID=5193.

Researchers in the study, which was published in the Jan. 1, 2006, edition of the Journal of the American Veterinary Medical Association, were Little; Emily Brazik, DVM; and Jan Luguire, DVM.

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for TheHorse.com .

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