Prevalence of anthelmintic resistant cyathostomes on horse farms.
OBJECTIVE
To determine prevalence of anthelmintic resistance in cyathostome nematodes of horses in the southern United States.


METHODS
Cross-sectional study.


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


METHODS
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
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.
Department of Infectious Diseases, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, USA.
Ray M
RM
Kaplan
Thomas R
TR
Klei
Eugene T
ET
Lyons
Guy
G
Lester
Charles H
CH
Courtney
Dennis D
DD
French
Sharon C
SC
Tolliver
Anand N
AN
Vidyashankar
Ying
Y
Zhao
D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology / Q000627:therapeutic use*; D001562:Benzimidazoles / Q000494:pharmacology / Q000627:therapeutic use; D004351:Drug Resistance; D005243:Feces / Q000469:parasitology; D005260:Female; D005273:Fenbendazole / Q000494:pharmacology / Q000627:therapeutic use; D005431:Florida; D005845:Georgia; D006734:Horse Diseases / Q000188:drug therapy* / Q000469:parasitology; D006736:Horses; D007559:Ivermectin / Q000494:pharmacology / Q000627:therapeutic use; D007629:Kentucky; D008145:Louisiana; D008297:Male; D010270:Parasite Egg Count / Q000662:veterinary; D021261:Parasitic Sensitivity Tests / Q000662:veterinary; D015995:Prevalence; D011716:Pyrantel Pamoate / Q000494:pharmacology / Q000627:therapeutic use; D013022:South Carolina; D013319:Strongyle Infections, Equine / Q000188:drug therapy*; D013323:Strongylus; D016896:Treatment Outcome
D000871:Anthelmintics; D001562:Benzimidazoles; C011321:oxibendazole; D005273:Fenbendazole; D007559:Ivermectin; D011716:Pyrantel Pamoate