Prevalence of anthelmintic resistant cyathostomes on horse farms

Ray M. Kaplan Department of Infectious Diseases, College of Veterinary Medicine, University of Georgia, Athens, GA 30602.

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Thomas R. Klei Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803.

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Eugene T. Lyons Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546.

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Guy Lester Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32611.
Present address is the Division of Veterinary and Biomedical Sciences, Murdoch University, Murdoch 6150, Western Australia.

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Charles H. Courtney Department of Pathobiology, College of Veterinary Medicine, University of Florida, Gainesville, FL 32611.

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Dennis D. French Departments of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803.

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Sharon C. Tolliver Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546.

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Anand N. Vidyashankar Department of Statistics, College of Arts and Sciences, University of Georgia, Athens, GA 30602.

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Ying Zhao Department of Statistics, College of Arts and Sciences, University of Georgia, Athens, GA 30602.

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Abstract

Objective—To determine prevalence of anthelmintic resistance in cyathostome nematodes of horses in the southern United States.

Design—Cross-sectional study.

Animals—786 horses on 44 farms and stables in Georgia, South Carolina, Florida, Kentucky, and Louisiana.

Procedure—Fecal egg count (FEC) reduction tests were performed on 44 large farms and stables. Horses on each farm were treated with an oral paste formulation of fenbendazole, oxibendazole, pyrantel pamoate, or ivermectin at recommended label dosages. A mixed linear model was fitted to the percentage reduction in FEC, accounting for differences among farms, states, ages, treatments, and treatment by state interactions.

Results—By use of a conservative measure of resistance (< 80% reduction), the percentage of farms with anthelmintic-resistant cyathostomes was 97.7%, 0%, 53.5%, and 40.5% for fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate, respectively. Mean percentage reductions in FEC for all farms were 24.8%, 99.9%, 73.8%, and 78.6% for fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate, respectively. Pairwise contrasts between states for each treatment revealed that in almost all instances, there were no significant differences in results between states.

Conclusions and Clinical Relevance—The prevalence of resistance found in this study was higher than that reported previously, suggesting that anthelmintic resistance in equine cyathostomes is becoming a major problem. Furthermore, data from these 5 southern states, which are geographically and physiographically distinct, were remarkably similar. This suggests that drug resistance in cyathostomes is highly prevalent throughout the entire southern United States and probably nationwide. (J Am Vet Med Assoc 2004;225:903–910)

Abstract

Objective—To determine prevalence of anthelmintic resistance in cyathostome nematodes of horses in the southern United States.

Design—Cross-sectional study.

Animals—786 horses on 44 farms and stables in Georgia, South Carolina, Florida, Kentucky, and Louisiana.

Procedure—Fecal egg count (FEC) reduction tests were performed on 44 large farms and stables. Horses on each farm were treated with an oral paste formulation of fenbendazole, oxibendazole, pyrantel pamoate, or ivermectin at recommended label dosages. A mixed linear model was fitted to the percentage reduction in FEC, accounting for differences among farms, states, ages, treatments, and treatment by state interactions.

Results—By use of a conservative measure of resistance (< 80% reduction), the percentage of farms with anthelmintic-resistant cyathostomes was 97.7%, 0%, 53.5%, and 40.5% for fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate, respectively. Mean percentage reductions in FEC for all farms were 24.8%, 99.9%, 73.8%, and 78.6% for fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate, respectively. Pairwise contrasts between states for each treatment revealed that in almost all instances, there were no significant differences in results between states.

Conclusions and Clinical Relevance—The prevalence of resistance found in this study was higher than that reported previously, suggesting that anthelmintic resistance in equine cyathostomes is becoming a major problem. Furthermore, data from these 5 southern states, which are geographically and physiographically distinct, were remarkably similar. This suggests that drug resistance in cyathostomes is highly prevalent throughout the entire southern United States and probably nationwide. (J Am Vet Med Assoc 2004;225:903–910)

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