Determining Treatment to Control Two Multidrug-Resistant Parasites on a Texas Horse Farm

A study was undertaken at the Texas A&M Horse Center to evaluate and compare the effectiveness of three anthelmintics—ivermectin, fenbendazole, and a combination of ivermectin and pyrantel pamoate—on fecal egg count reductions of cyathostomes and Parascaris equorum in 30 naturally infected foals. The foals were randomized into three treatment groups, with individuals being rerandomized after each 8-week observation period. The treatments of ivermectin and fenbendazole were given at the manufacturer's recommended doses, and the pyrantel treatment was given at two times the manufacturer's recommended dose. Fecal egg counts were performed at the time of treatment and at 2-week intervals after treatment for a total of 8 weeks. Each foal received a total of three treatments during the course of the study. Fecal egg counts were performed by a modified McMaster's test, with a sensitivity of 25 eggs per gram of feces, and by the modified Wisconsin double centrifugal flotation technique, with a sensitivity of 0.2 eggs per gram of feces. Fecal egg reduction percentages were calculated. Analysis of the results showed that ivermectin, either used alone or with pyrantel, was a more effective anthelmintic for cyathostome (small strongyle) control than fenbendazole. Fenbendazole and pyrantel showed a higher initial reduction in Parascaris egg counts when compared with the ivermectin-only-treated group, but this difference lessened over time. The use of the combination treatment showed the best results for controlling both parasites, indicating that a combination of anthelmintics may be necessary to control parasites on some equine farms.