Equine strongylids: Ivermectin efficacy and fecal egg shedding patterns

Equine cyathostomins are ubiquitous in grazing horses around the world and a main target in parasite control programs. Anthelmintic resistance has been reported with increasing frequency in these parasites over the past decades, and recent findings of fulminant resistance to the macrocyclic lactone class have raised severe concerns. This study aimed to evaluate ivermectin efficacy in cohorts of yearlings and mares present on four different farms in Central Kentucky. Strongylid egg counts were determined with an automated egg counting system, and the percent fecal egg count reduction (FECR) was calculated using a hierarchical Bayesian analysis. Novel principles were used for classification of groups to either no signs of anthelmintic resistance, evidence of resistance, or inconclusive. Furthermore, an epidemiological analysis was conducted evaluating the possible influence of pregnancy status, time of foaling, and year of arrival on mare strongylid egg shedding levels. A total of 102 yearlings and 247 mares were enrolled in the study. Evidence of ivermectin resistance was documented in one group of yearlings with a mean FECR of 91.2% and a 95% credible interval of 84.0–95.8. The results from one mare group and one additional yearling group were deemed inconclusive, whereas the remaining five groups displayed no evidence of ivermectin resistance. Strongylid shedding in the mares was not positively associated with any of the evaluated factors. This study is the first to demonstrate ivermectin resistance in US bred horses, and the findings emphasize the need for routine monitoring of anthelmintic efficacy on horse farms.