Anthelmintic efficacy on UK Thoroughbred stud farms.

Abstract: Anthelmintic drugs have been applied indiscriminately to control horse nematodes for over 40 years. We undertook a comprehensive study to investigate efficacy of the four available broad-spectrum anthelmintic drugs on 16 Thoroughbred stud farms using the faecal egg count reduction test. Efficacy against strongyles was determined by calculating the percentage of reduction in faecal egg count between the group mean at Day 0 and Days 14-17 post-treatment and the 95% lower confidence intervals estimated by non-parametric bootstrapping. Individual strongyle faecal egg count reduction tests (n=429) were performed in which 179, 131, 89 and 30 horses were administered ivermectin, moxidectin, pyrantel and fenbendazole, respectively. Moxidectin was efficacious in all tests (faecal egg count reduction range: 99.8-100%; 95% lower confidence intervals range: 96.8-100%) and reduced efficacy of ivermectin (faecal egg count reduction range: 85.7-100%; 95% lower confidence intervals range: 65-100%) was observed in one group of yearlings. Reduced pyrantel efficacy was observed in five groups of yearlings (faecal egg count reduction range: 0-73%; 95% lower confidence intervals range: 0-59.5%), but pyrantel was found to be efficacious when administered to mares (faecal egg count reduction range: 98-99.4%; 95% lower confidence intervals range: 91.8-99.3%). Low efficacy of fenbendazole was always observed (faecal egg count reduction range: 0.4-41%; 95% lower confidence intervals not calculable). Two further methods for estimating efficacy were applied and outputs obtained using all methodologies were in agreement. Efficacy against Parascaris equorum was assessed on four farms: fenbendazole had acceptable efficacy (faecal egg count reduction range: 97.5-99.9%; 95% lower confidence intervals range: 96.3-99.1%), but reduced efficacy of ivermectin was observed (faecal egg count reduction range: 25.5-91.2%; 95% lower confidence intervals range: 6.7-82.4%). Strongyle faecal egg count were analysed at approximately 2 week intervals for up to 12 weeks after anthelmintic drug administration to determine the egg reappearance period for moxidectin, ivermectin and pyrantel. The egg reappearance period for all three anthelmintic drugs was shorter than previously observed. Overall, our results indicate that ivermectin and moxidectin administration provided acceptable efficacy at 14 days; however, egg reappearance period results suggest that these products are working less effectively than measured previously. As shortened egg reappearance period is believed to be an early indicator of resistance, this highlights the issue of impending multi-drug resistance in strongyles on stud farms.