Researchers continue to study internal parasites and their evolution, with a focus on preventing resistance to deworming drugs

Mark Twain probably wasn’t talking about equine parasites when he said, “It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.”

Still, few sentiments ring truer when it comes to the rapidly changing world of parasite control in horses.

It wasn’t long ago we knew regular rotational deworming as the best way keep internal worms at bay. While that was true to the best of our knowledge at the time, it was what we didn’t know—and what researchers sought to find out—that’s led to our current understanding of parasites and how to manage them.

But even that is ever-evolving.

“It’s important to keep researching parasites because all horses are constantly exposed to them, they can cause significant disease, and, as living biological organisms, they keep evolving and changing,” says Martin K. Nielsen, DVM, DVSc, PhD, Dipl. EVPC, ACVM, the Schlaikjer Professor of Equine Infectious Disease at the University of Kentucky Gluck Equine Research Center, in Lexington.

So, he and other researchers carry out new studies to advance their knowledge about equine parasites, the drugs that help manage them, ways to control them sans medications, and more. Here’s what some of that recent research has shown us.

Key Focus: Anthelmintic ­Resistance

One of the main topics scientists are focusing on is anthelmintic resistance (AR), which occurs when parasites survive deworming treatment and then pass that resistance on to subsequent generations. And, says Ray Kaplan, DVM, PhD, Dipl. ACVM, EVPC, it’s a problem all horse owners should understand.

A bit of background: While dewormers are likely some of the most common treatments owners administer to horses, parasites typically don’t cause serious disease in adult horses