Reduced Efficacy of Fenbendazole and Pyrantel Pamoate Treatments against Intestinal Nematodes of Stud and Performance Horses

Nematodes are an important cause of disease and loss of performance in horses. Changes in the parasitic fauna of horses have occurred in the past few decades, making cyathostomins the major parasites in adult horses, while large strongyles have become less prevalent. Parascaris spp. remains the most important parasite infecting foals and weanlings. Anthelmintic resistance is highly prevalent in cyathostomins and Parascaris spp. worldwide and it must be factored into treatment decisions. To assess anthelmintic efficacy in Northern Italy, we sampled 215 horses from 17 sport and horse-breeding farms. Fecal egg count reduction tests (FECRT) were used to assess anthelmintic efficacy. Copromicroscopic analysis was performed using MiniFLOTAC before treatment with fenbendazole, pyrantel pamoate or ivermectin, and repeated 14 days post-treatment. Strongyle-type eggs were detected in 66.91% of horses (CI95% 61.40-73.79%), while Parascaris spp. was detected in 2.79% (CI95% 1.94-5.95%). Reduced efficacy against cyathostomins was observed for fenbendazole in 55.56% of the treated animals (CI95% 41.18-69.06%), and for pyrantel pamoate in 75% of animals (CI95% 30.06-95.44%). Ground-based actions must be set in place to promote the uptake of state-of-the-art worm control plans that will prevent clinical disease while minimizing the selection pressure of resistant parasites.

Automated summary

Changes in the parasitic fauna of horses have led to cyathostomins becoming the major parasites in adult horses, while large strongyles have decreased in prevalence. Parascaris spp. remains the most important parasite in foals and weanlings. Anthelmintic resistance is prevalent worldwide in cyathostomins and Parascaris spp., impacting treatment decisions. Testing in Northern Italy found high rates of strongyle-type eggs in horses and lower rates of Parascaris spp., with reduced efficacy of fenbendazole and pyrantel pamoate against cyathostomins. Ground-based actions are needed to prevent clinical disease and reduce selection pressure of resistant parasites.