Prevalence and clinical implications of anthelmintic resistance in cyathostomes of horses.
OBJECTIVE
To determine the prevalence and clinical implications of anthelmintic resistance in cyathostomes of horses.


METHODS
Prospective study.


METHODS
80 horses on 10 farms in a 5-county region of northeast Georgia.


METHODS
On each farm, horses were stratified in descending order according to pretreatment fecal egg count (FEC), blocked into groups of 4, and then randomly assigned to 1 of 4 treatment groups: no treatment (controls), and treatment with pyrantel pamoate, fenbendazole, or ivermectin. Fecal samples were collected 24 hours prior to treatment and 2, 4, and 6 weeks after treatment for determination of FEC. Mean percentage of reduction in FEC was then calculated for each treatment group. For horses from each farm, the efficacy of each anthelmintic was categorized on the basis of mean percentage of reduction in FEC at 2 weeks after treatment (< 80% reduction = ineffective; 80 to 90% reduction = equivocal; and > 90% reduction = effective).


RESULTS
Pyrantel pamoate was effective at reducing FEC in horses from 7 farms, ineffective in horses from 2 farms, and equivocal in horses from 1 farm. Fenbendazole was ineffective at reducing FEC in horses from 9 farms and equivocal in horses from 1 farm. Ivermectin was effective at reducing FEC in horses from all 10 farms.


CONCLUSIONS
Results suggest that cyathostome resistance to fenbendazole is highly prevalent, and resistance to pyrantel pamoate is high enough to warrant concern. Resistance to ivermectin was not detected. On the basis of these data, it appears that ivermectin continues to be fully effective in horses. However, too few farms were used in this study to determine the prevalence of cyathostome resistance to ivermectin. Therefore, the efficacy of ivermectin should continue to be monitored closely.
Tarigo-Martinie
J L
JL
Department of Medical Microbiology and Parasitology, College of Veterinary Medicine, University of Georgia, Athens 30602, USA.
Wyatt
A R
AR
Kaplan
R M
RM
D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology / Q000627:therapeutic use*; D004351:Drug Resistance; D005243:Feces / Q000469:parasitology; D005260:Female; D005273:Fenbendazole / Q000494:pharmacology / Q000627:therapeutic use; D005845:Georgia / Q000453:epidemiology; D006736:Horses; D007411:Intestinal Diseases, Parasitic / Q000188:drug therapy*; D007559:Ivermectin / Q000494:pharmacology / Q000627:therapeutic use; D008297:Male; D010270:Parasite Egg Count / Q000662:veterinary; D015995:Prevalence; D011446:Prospective Studies; D011716:Pyrantel Pamoate / Q000494:pharmacology / Q000627:therapeutic use; D013319:Strongyle Infections, Equine / Q000188:drug therapy*; D013320:Strongyloidea / Q000187:drug effects*; D016896:Treatment Outcome
D000871:Anthelmintics; D005273:Fenbendazole; D007559:Ivermectin; D011716:Pyrantel Pamoate