Dewormer Resistance in Small Strongyles

Small strongyles are some of the most problematic internal parasites in horses. Infected horses can experience a wide range of symptoms, from rough hair coat, poor growth, and suboptimal performance to life-threatening chronic diarrhea, colic, and severe weight loss. The most effective means of controlling small strongyles is the regular use of deworming medications. Unfortunately, some small strongyles are resistant to dewormers.

Recently, 10 horse farms in northeast Georgia were recruited for a study to determine the prevalence of dewormer resistance in small strongyles. On each farm, horses were assigned to receive Strongid, Panacur, Eqvalan, or a placebo paste. Fresh fecal samples were collected before and after treatment to count strongyle eggs. Treatment was considered effective if there was greater than a 90% reduction in egg count. A reduction of 80-90% was considered equivocal, while less than 80% reduction was considered ineffective and an indication that drug-resistant small strongyles were present.

Resistance to Panacur was identified on 90% of the farms tested. Twenty percent of farms had evidence of resistance to Strongid, but there was no evidence of resistance to Eqvalan on any farm. However, only a small number of farms were tested, so it is possible that ivermectin resistance exists elsewhere. In fact, ivermectin-resistant parasites have been found in sheep and goats. Regardless, horse owners should be able to continue using ivermectin, moxidectin, and even pyrantel products for effective control of small strongyles. The authors point out that there does not appear to be any benefit to rotating dewormers with each treatment. Instead, it is recommended that dewormers be rotated annually.

Journal of the American Veterinary Medical Association, 218(12), 1957-1960, 2001.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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