Veterinary session

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.

Fecal samples were collected at a Hungarian National Stud Farm from 53 Thoroughbred mares and then were immediately treated with ivermectin (0.2 mg/kg, Noromectin Equine Paste, Norbrook Laboratories Ltd). Fecal Egg Counts (FECs) were determined at 2, 4, 5, 6, and 7 weeks post-treatment using the Mini-FLOTAC technique. The fecal egg count reduction (FECR) was calculated at two weeks post-treatment to evaluate efficacy using a Bayesian hierarchical model, and the ERP was estimated following current guidelines.

In the present study, 41 (77.36%) out of 53 were strongyle egg count positive pre-treatment, the average eggs per gram (EPG) was 385. The FECR was observed to be 99.9% (CI=99.8% – 100%) in the 2nd week, 98.61% (CI=98.2% – 98.9%) in the 4th week, 93.25% (CI=92.4% – 94%) in the 5th week, 79.2% (CI=77.8% – 80.7%) in the 6th week, and 71.1% (CI=69.3% – 72.9%) in the 7th week.

The FECR did not indicate any evidence of resistance to ivermectin. Data suggested an ERP of 6 weeks, which is notable reduction from historic data. The decrease in ERP has emerged as a likely consequence of treatment-intensive deworming practices employed worldwide and represents a lack of anthelmintic performance.These findings underscore the need for implementing FEC-based management strategies to preserve anthelmintic efficacy and reduce the strongylid infection pressure on pastures.