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Comparison of a single dose of moxidectin and a five-day course of fenbendazole to reduce and suppress cyathostomin fecal egg counts in a herd of embryo transfer–recipient mares

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  • 1 College of Veterinary Medicine, University of Georgia, Athens, GA 30302.
  • | 2 Zoetis Equine Technical Services, 10610 E David Allen Rd, Columbia, MO 65201.
  • | 3 Equine Medical Services, 5851 Deer Park Rd, Columbia, MO 65201.
  • | 4 Equine Medical Services, 5851 Deer Park Rd, Columbia, MO 65201.
  • | 5 Department of Infectious Diseases, University of Georgia, Athens, GA 30302.

Abstract

Objective—To compare larvicidal regimens of fenbendazole and moxidectin for reduction and suppression of cyathostomin fecal egg counts (FEC) in a transient herd of embryo transfer–recipient mares.

Design—Randomized, complete block, clinical trial.

Animals—120 mares from 21 states, residing on 1 farm.

Procedures—An initial fecal sample was collected from each mare; mares with an FEC ≥ 200 eggs/g were assigned to treatment groups. Eighty-two horses received fenbendazole (10.0 mg/kg [4.5 mg/lb], PO, q 24 h for 5 days) or moxidectin (0.4 mg/kg [0.18 mg/lb], PO, once); FEC data were analyzed 14, 45, and 90 days after treatment.

Results—Mean FEC reduction was 99.9% for moxidectin-treated mares and 41.9% for fenbendazole-treated mares 14 days after treatment. By 45 days, mean FEC of fenbendazole-treated mares exceeded pretreatment counts; however, FECs of moxidectin-treated mares remained suppressed below pretreatment values for the duration of the 90-day study. Fecal egg counts were significantly different between groups at 14, 45, and 90 days after treatment.

Conclusions and Clinical Relevance—Failure of the 5-day regimen of fenbendazole to adequately reduce or suppress FEC suggested inadequate adulticidal and larvicidal effects. In contrast, a single dose of moxidectin effectively reduced and suppressed FEC for an extended period. Given the diverse geographic origins of study mares, these results are likely representative of cyathostomin-infected mares in much of the United States, confirming previous findings indicating that fenbendazole resistance in cyathostomins is widespread and that moxidectin remains an effective treatment for control of these important parasites.

Abstract

Objective—To compare larvicidal regimens of fenbendazole and moxidectin for reduction and suppression of cyathostomin fecal egg counts (FEC) in a transient herd of embryo transfer–recipient mares.

Design—Randomized, complete block, clinical trial.

Animals—120 mares from 21 states, residing on 1 farm.

Procedures—An initial fecal sample was collected from each mare; mares with an FEC ≥ 200 eggs/g were assigned to treatment groups. Eighty-two horses received fenbendazole (10.0 mg/kg [4.5 mg/lb], PO, q 24 h for 5 days) or moxidectin (0.4 mg/kg [0.18 mg/lb], PO, once); FEC data were analyzed 14, 45, and 90 days after treatment.

Results—Mean FEC reduction was 99.9% for moxidectin-treated mares and 41.9% for fenbendazole-treated mares 14 days after treatment. By 45 days, mean FEC of fenbendazole-treated mares exceeded pretreatment counts; however, FECs of moxidectin-treated mares remained suppressed below pretreatment values for the duration of the 90-day study. Fecal egg counts were significantly different between groups at 14, 45, and 90 days after treatment.

Conclusions and Clinical Relevance—Failure of the 5-day regimen of fenbendazole to adequately reduce or suppress FEC suggested inadequate adulticidal and larvicidal effects. In contrast, a single dose of moxidectin effectively reduced and suppressed FEC for an extended period. Given the diverse geographic origins of study mares, these results are likely representative of cyathostomin-infected mares in much of the United States, confirming previous findings indicating that fenbendazole resistance in cyathostomins is widespread and that moxidectin remains an effective treatment for control of these important parasites.

Contributor Notes

Ms. Geeding's present address is Veterinary Teaching Hospital, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803. Maren Mason was a third-year veterinary student at the time of the study.

Supported by a grant from Zoetis. This manuscript represents data collected through a Zoetis veterinary student summer research project.

The authors thank Deb Amodie for statistical analysis.

Address correspondence to Dr. Voris (nathan.voris@zoetis.com).