Internal parasites of horses have been recognized for centuries. But until the early 1900s, methods for the control of equine endoparasites lacked a scientific basis. For example, in the 1600s one recommendation was to incise (cut with a surgical instrument) the horse’s palate with the intent that the ingested blood would kill any internal parasites. Beginning in the 1940s and extending to the 1980s, new classes of antiparasitic compounds were developed approximately every 10 years.

Currently in the United States, only benzimidazoles (fenbendazole and oxibendazole), tetrahydropyrimidines (pyrantel pamoate and pyrantel tartrate), and macrocyclic lactones (ivermectin and moxidectin alone or combined with praziquantel) are commercially available for parasite control in horses.

The major endoparasites of horses include bots, large strongyles, small strongyles or cyathostomes, ascarids, and tapeworms. Large strongyles (Strongylus spp.) are one of the most significant equine parasites. The larval stages can cause disease due to migration in blood vessels and abdominal organs. Drug resistance is not evident in the case of large strongyles.

Cyathostome larvae do not migrate parenterally like Strongylus spp., but encyst in the mucosa and submucosa of the horse’s large intestine. Intestinal disease can be induced by cyathostomes when large numbers of larvae excyst from (exit) the lining of the large intestine, a condition called “larval cyathostomiasis.” Resistance to fenbendazole, oxibendazole, and pyrantel pamoate is now common among cyathostomes. Also, both ivermectin and moxidectin have become less effective against immature (L4) cyathostomes in the lumen of the large intestine; thus, the life cycle is shortened