Current data show resistance among cyathostomins (small strongyles) to the three main groups of anthelmintic (antiparasitic) drugs commonly used to deworm horses. Martin Nielsen, DVM, PhD, Dipl. ACVIM, Schlaikjer professor of equine infectious diseases in the Gluck Equine Research Center at the University of Kentucky (UK), spoke about the topic at the university’s Equine Research Showcase on Feb. 9.

“The small strongyle parasite, which is the type of parasite that all horses get, is the primary parasite category, the one that we are always deworming for,” Nielsen said.

The first class of anthelmintic drugs introduced and administered to combat small strongyles was the benzimidazoles in the 1960s. We see the highest level of widespread resistance in small strongyles to this class of drugs because it’s been used for the longest period of time, Nielsen said.

“It is very rare to find these products still working,” Nielsen said.

Researchers first identified internal parasite resistance to pyrantel salts in 1996, and Nielsen said we’re likely to find small strongyles resistant to pyrantel dewormers today.

Macrocyclic lactones (ivermectin, moxidectin) are our “last resort,” he said, with good efficacy, but some early signs of emerging resistance have been reported, said Nielsen. In 2020, routine data collected from a farm in Central Kentucky confirmed resistance to macrocyclic lactones among small strongyles.

Nielsen shared data showing that macrocyclic lactones were 100% effective against small strongyles in a group of U.S.-bred yearlings, but the same dewormers administered to a group of imported Irish bred yearlings didn’t remove small strongyle parasites.

“There’s no doubt, looking across this data, that we have clear-cut, proven resistance to ivermectin in these imported parasites,” Nielsen said. “The problem is, if we have resistance to everything, what can we deworm with?”

He said researchers have also evaluated moxidectin’s efficacy against resistant parasites. Despite this drug being potentially more potent than ivermectin, it neither overcame resistance nor provided better efficacy.

Nielsen closed with a note that ivermectin and moxidectin resistance is occurring in small strongyles. This case was only discovered due to the meticulous testing procedures in place on this particular farm. Without regular deworming efficacy testing, drug-resistant parasites will go undetected, and farms will be left without interventions. Good parasite control starts and ends with testing the dewormers being used yearly, he said.

Author Sydney Carter, a junior majoring in equine science and management and minoring in journalism, is a communications and student relations intern with UK Ag Equine Programs.