Evaluating efficacy and strongyle egg reappearance period after ivermectin treatment in a Hungarian stud farm

Horses remain globally at constant risk of strongylid infections, with cyathostomins being particularly prevalent and abundant. It is essential to routinely monitor the efficacy of anthelmintics against equine nematodes. This study aimed to evaluate the efficacy of ivermectin against equine strongyles and estimate the strongyle egg reappearance period (ERP) following ivermectin treatment for the first time in Hungary. Fecal samples were collected from 57 Thoroughbred mares before treatment and at 2, 4, 5, 6, and 7 weeks post-treatment at a Hungarian National Stud Farm. Fecal egg counts (FECs) were determined using the Mini-FLOTAC technique. Fecal Egg Count Reduction (FECR) was calculated at two weeks post-treatment to evaluate efficacy using a Bayesian hierarchical model, and ERP was estimated following current guidelines. The FECR calculations did not indicate any evidence of resistance to ivermectin. Data suggested an ERP of 6 weeks, which is a notable reduction from historic data. Evaluation of two defined age groups suggested a longer ERP in the older group (12–20 years old). The decrease in ERP has emerged as a likely consequence of treatment-intensive deworming practices employed worldwide and represents a loss of anthelmintic performance. These findings underscore the need for implementing FEC-based management strategies in Hungary to preserve anthelmintic efficacy and reduce strongylid infection pressure on pastures.