How climate change may affect how you deworm your horse

If average global temperatures continue to increase, the development of drug resistance among internal parasites common to horses may accelerate.

From extreme weather events to increased incidence of certain diseases, climate change poses a number of well-known threats to horse health. Now new research adds another concern to the list: accelerating drug resistance among internal parasites.

The study, a joint effort between the Grasslands Research Center in New Zealand and the University of Kentucky, was “an exercise of climate change science meets equine parasitology,” says Martin Nielsen, DVM, PhD.

Specifically, the research teams used computer models and simulations to predict whether climate change is likely to affect the lifecycle of cyathostomins---internal parasites commonly known as small strongyles---and what that might mean for the development of resistance to anthelmintics, the chemicals used to control them.

Resistance develops among parasite populations because random genetic mutations enable individual worms to withstand the effects of particular chemical classes of dewormers. These mutations are relatively rare, but overzealous and indiscriminate use of dewormers kills off susceptible worms, leaving behind only those that are resistant. The survivors then reproduce and resistant worms become a growing segment of the population. If these practices continue, the number of invulnerable worms grows with each lifecycle and, eventually, such a large proportion of the parasite population is resistant to that particular chemical class that those particular dewormers are effectively useless in that geographic location for decades.

Surveillance deworming—an approach that relies on fecal egg-count tests to identify horses in a herd that need to be treated with particular chemicals—can slow the rate at which resistance develops, but horse owners have been slow to adopt this approach over the conventional “every-horse-every-eight-weeks” routine.

For their study, the researchers combined two existing sets of computer models---climate change models that presented six different climate possibilities to cover a range of possible outcomes, and parasite models that presented two different treatment scenarios: Treating horses every two months and treating horses every six months.

“The climate prediction models can output temperature and precipitation data,” says Nielsen. “And the parasite simulation model is set up for importing exactly that kind of data. So one model’s output is the other model’s input.”

The resulting simulations, which produced predications through the year 2100, indicated that continued climate trends will bring about longer periods of the warmer, wetter weather that is ideal for parasite reproduction. This, in turn, will lead horses to carry larger parasite loads.

And although the model didn’t predict an increase in deworming frequency on the part of veterinarians and horse owners, that is one possible reaction to increased worm burden that needs to be avoided, says Nielsen. “That will just make things worse,” he says. “The best approach is selective therapy, where only some of the horses are treated (based on their fecal egg counts).”

Even if the expected warming of the climate is somehow avoided, says Nielsen, it’s important for horse owners to start thinking differently about parasite control to protect the efficacy of available drugs: “Accept that your horses have worms and that is ok; Keep pastures healthy and avoid overgrazing; [Utilize] mixed or alternate grazing with ruminants.”

Reference: “Climate change is likely to increase the development rate of anthelmintic resistance in equine cyathostomins in New Zealand,” International Journal for Parasitology: Drugs and Drug Resistance, December 2020