Quality routine FECR testing is recommended in horses.

By Mark Andrews

A valuable reminder of the danger of importing anthelmintic resistance is given in a recent report.

A case of macrocyclic lactone (ML) resistance in a group of Thoroughbred yearlings imported from Ireland to the United States is described by Martin K Nielsen, who is the Schlaikjer Professor of Equine Infectious Disease at the University of Kentucky’s Gluck Equine Research Center, and colleagues. 

The findings emphasize that the global movement of horses has the potential to quickly spread ML-resistant parasites around the world.

Anthelmintic resistance to benzimidazole and pyrantel is widespread among the cyathostomins (small redworms). The macrocyclic lactones (such as ivermectin and moxidectin) are generally effective, but there have been occasional reports of resistance.

Thoroughbred yearlings imported from Ireland to the US were found to have evidence of ML resistance when fecal egg count reduction (FECR) tests were carried out. The three batches of imported yearlings had FECR test results ranging from 93.5 percent to 70.5 percent. (FECR test is the standard method of assessing resistance in cyathostomins. If the FEC is not reduced by 95 percent or more after treatment with a macrocyclic lactone such as ivermectin, that is interpreted as evidence of resistance.) When the test was repeated after a further ivermectin treatment in two of the groups the FECR results were even lower.

Related: Deworming Strategies for Healthier Horses

In contrast, three groups of US-bred yearlings had FECR test results of 99-100 percent after treatment with ivermectin.

Full details of the study are published in the International Journal for Parasitology: Drugs and Drug Resistance. 

The authors point out: “This case clearly illustrates the importance of quality routine FECR testing, which immediately informed the farm manager of the situation and allowed him to react in time by keeping the populations completely separate, thereby avoiding an introduction of the resistant parasites to the entire facility.”

They strongly recommend that equine operations heed this threat to equine health, and routinely monitor anthelmintic efficacy on a yearly basis. 

For more details, see Importation of macrocyclic lactone resistant cyathostomins on a US thoroughbred farm

• M K Nielsen, M Banahan, R M Kaplan
• Int J Parasitol Drugs Drug Resist (2020)14:99-104.
• doi: 10.1016/j.ijpddr.2020.09.004

Published with the kind permission of Mark Andrews, Equine Science Update.

Photo: iStock/Iillisphotography