Equine Dewormers and Strongyles Research

Editor’s note: Intensive use of many anthelmintic drugs in recent years has been met with increasing drug resistance by internal parasites, and in particular small strongyles (cyathostomines). This months, we will look at various studies that have attempted to define the prevalence of this resistance phenomenon worldwide.

Strongyle parasite infection, particularly larval cyathostominosis of small strongyle encystment in the intestinal lining, has significant medical ramifications on the equine intestinal tract. There is the potential to lead to weight loss, diarrhea and colic. 

In an attempt to update dewormer efficacy on strongyles in the United States, a study—the first in more than 15 years—evaluated response by these intestinal parasites to various anthelmintics [Nielsen, M.K.; Branan, M.A.; Wiedenheft, A.M.; Digianantonio, R.; Scare, J.A.; Bellaw, J.L.; Garber, L.P.; Kopral, C.A.; Phillippi-Taylor, A.M.; Traub-Dargatz, J.L. Anthelmintic efficacy against equine strongyles in the United States. Veterinary Parasitology 2018].

Fecal egg count reduction testing was performed the day of dewormer treatment and  14 days later. The results:

• Macrocyclic lactones (ivermectin and moxidectin) had 95% efficacy for all the horses.
• Pyrimidine/benzimidazoles had efficacy in only 43.5% of the horses.

Anthelmintic efficacy was associated with body weight, amount of dewormer given and operation size, while geographic regional differences across 28 states had no association. This study underscores the presence of increasing anthelmintic resistance to pyrimidine and benzimidazole deworming medicationd. 

This study underscores the presence of increasing anthelmintic resistance to pyrimidine and benzimidazole deworming medications.