Parasite Control: An Update

Parasite Control: An Update

For strongyles, use ivermectin or moxidectin alone or in combination with praziquantel twice a year—consider treating in the spring and fall.

Photo: Kristen M. Janicki, MS, PAS

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.

Heavy infections with adult ascarids (Parascaris spp.) can cause intestinal blockage and rupture because of their bulk. These too have become resistant to ivermectin, moxidectin, and pyrantel pamoate.

The final group of equine endoparasites, tapeworms (Anoplocephala spp.), can also result in intestinal hypertrophy, blockage, intussusception, and rupture; they do not exhibit drug resistance.

Parasite treatment schedules have been based on the life cycle of the parasites since the early 1900s. In the mid-1960s, it was suggested that horses should be dewormed for strongyles every six to eight weeks. This frequent deworming was thought 1) not to provide time needed for Strongylus spp. to mature, 2) to help decrease potential cyathostome egg deposition on pastures, and 3) not to allow time for ascarids to mature.

High strongyle fecal egg counts indicate contamination of pastures and increased potential for ingestion of infective larvae by grazing horses. Thus, profiling the number of eggs per gram (EPG) in feces has been used in updating deworming schedules. Since Strongylus spp. are now rarely encountered, a deworming schedule can be more flexible. Unfortunately, there is no direct relationship between EPG values and cyathostome numbers.

A suggested deworming program is as follows:

  • Establish a strongyle EPG profile for individual horses rather than deworming all horses; a study of 1,114 Thoroughbred mares showed that one fecal sampling per horse was sufficient for establishing a strongyle EPG profile.
  • For strongyles, use ivermectin or moxidectin alone or in combination with praziquantel twice a year—consider treating in the spring and fall. While benzimidazoles and pyrantel may be ineffective on cyathostomes, they may be efficacious in treating other parasite species.
  • Treat foals every eight weeks of age for ascarid infection until they become yearlings; oxibendazole is currently considered the drug of choice followed by fenbendazole.
  • Control Strongyloides with ivermectin or oxibendazole and tapeworms with praziquantel or pyrantel pamoate/tartrate.

Contact—E.T. Lyons, PhD——859/218-1115—University of Kentucky Maxwell H. Gluck Equine Research Center, Lexington, Kentucky

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.

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Equine Disease Quarterly

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