Efficacy of major anthelmintics for reduction of fecal shedding of strongyle-type eggs in horses in the Mid-Atlantic region of the United States.

This research examined the effectiveness of different deworming drugs in reducing strongyle-type parasite eggs in horse feces in the Mid-Atlantic region of the United States. The study found reduced effectiveness in some drugs and high efficacy in others, providing evidence of the presence of drug-resistant parasites in this region.

Objective and Methodology of the Study

• The primary aim of this study was to evaluate the effectiveness of commercially available deworming drugs (anthelmintics) in reducing the number of cyathostomin (a type of parasitic worm) eggs in horse feces in the mid-Atlantic region of the United States. This region comprises areas in southeastern Pennsylvania, northern Delaware, and northeastern Maryland.
• A total of 989 horses from 67 different farms were treated with various anthelmintics under investigation—fenbendazole, oxibendazole, pyrantel pamoate, ivermectin, and moxidectin—at recommended dosages for each drug.
• The researchers used fecal egg count reduction testing to determine the efficacy of these dewormers in horses that had a high presence of cyathostomin eggs in their feces—equal to or more than 200 eggs per gram—on the day of treatment. 272 horses met this criterion.

Results and Findings

• The study found that some anthelmintics showed decreased efficacy in controlling the parasite load. Fenbendazole, oxibendazole, and pyrantel pamoate reduced strongyle-type eggs in horse feces by less than 90%. In terms of horses showing more than 90% reductions, the percentages were extremely low—6% for fenbendazole, 21% for oxibendazole, and 43% for pyrantel pamoate-treated horses.
• On the other hand, the macrocyclic lactones (ivermectin and moxidectin) demonstrated high efficacy in reducing the egg count in all sampled horses.
• These findings seem to indicate the existence of deworming-drug-resistant cyathostomins in the mid-Atlantic region of the United States, potentially hampering the effectiveness of parasitic control programs in horses.

Implications of the Study

• The evidence of decreased anthelmintic efficacy in controlling cyathostomins in certain regions of the United States presents a challenging issue for parasite control programs in horses. This resistance could lead to ineffective treatments and a rise of health-related problems in horses related to parasitic infections.
• The findings of this study could inspire a more sustainable approach for parasite control programs, highlighting the need to rotate the use of different classes of anthelmintics or consider other non-drug methods of parasite control.