The University of Georgia College of Veterinary Medicine

Georgia Veterinary Scholar Program

Georgia Veterinary Scholar	Faculty Mentor

Erin Menigo University of Georgia Class of 2007	Dr. Ray Kaplan

Combination Therapy to Overcome Dewormer Resistance in Small Strongyles of Horses

Cyathostomes, also referred to as small strongyles, are the most prevalent and important equine parasitic pathogen. Cyathostomes are treated frequently with anthelmintics and therefore the principle targets of parasite control programs. The efficacy of oxibendazole and pyrantel paomate, two commonly used anthelmintics, has been steadily decreasing in recent years. This is due to an increase in the levels of resistance in cyathostomes. The objective of this study is to determine if the combined use of oxibendazole and pyrantel paomate at the full-recommended dosages will result in a clinically significant increase in efficacy as compared to the use of the drugs individually.

Fecal egg count reduction (FECR) tests were performed on 8 different horse farms in Georgia and South Carolina. FEC were performed using the Stoll technique. For inclusion in the study, farms must have > 10 horses with a FEC of greater than or equal to 40 eggs per gram (EPG). Horses are separated into allocation blocks of four on the basis of their FEC. Within these blocks, the horses are assigned randomly to one of three treatment groups: oxibendazole, pyrantel paomate or a combination of the two. Both drugs were administered at labeled dosages using an oral paste formulation. Feces were collected on the day of treatment and again 9-13 days after treatment. Pre and post treatment FEC were compared to determine the FECR and efficacy of treatment using the following formula: [(pre FEC-post FEC)/pre FEC]100.

Though the data is not complete, it appears so far that the combination of the oxibendazole and pyrantel paomate demonstrates a greater FECR than does each drug individually. Data from five farms and a total of 60 horses have been collected thus far. The average FECR of oxibendazole, pyrantel paomate, and the combination is: 77.1%, 69.4%, and 93.4% respectively. 1/5 farms had resistance to both drugs individually and the combination treatment increased the FECR by 53.2% over the most effective individual drug. On the other farms (4/5 farms) where at least one drug was either effective (>90%FECR) or demonstrated suspected resistance (80-90% FECR), the combination increased mean FECR by 4.2% (range 0-9.9%). On one farm that had suggested resistance to both drugs individually, the combination treatment increased FECR to 99.9%. Excluding the one farm with high levels of resistance to both drugs, the mean % reduction for oxibendazole and pyrantel paomate individually was 87.3% and only 3/8 individual treatments yielded FECR > 90%. However, mean FECR increased to 95.3% when the combination was used, and ¾ farms had reductions of > 98.5%. This suggests that on most farms using the oxibendazole/pyrantel paomate combination causes clinically significant increases in efficacy give that the target level of effective treatment is 90%. Given the high prevalence of resistance to these drugs, the chance of effective treatment appears to be greatly increased by using the combination. These data suggest that the combination of oxibendazole and pyrantel paomate offers an alternative therapy to ivermectin and moxidectin, the only drugs to which cyathostomes have not developed resistance. Data collection on additional farms is still in progress.