Keeping Horse Worms at Bay

Editor's note: This article is part of's ongoing coverage of topics presented at the 2012 Kentucky Equine Research Conference, held May 17-18 in Lexington, Ky.

Parasite resistance is a popular topic of discussion in the equine veterinary community. Some horse owners cling to traditional deworming methods while others embrace new recommendations from veterinarians and researchers.

At the 2012 Kentucky Equine Research Conference, held May 17-18 in Lexington, Ky., Martin K. Nielsen, DVM, PhD, Dipl. EVPC, an assistant professor in equine parasitology at the University of Kentucky's M.H. Gluck Equine Research Center, discussed the concept of sustainable equine parasite control and made some deworming recommendations.

What is Sustainable Parasite Control?

"Parasites can't be avoided," he said, adding that we can, however, minimize disease risk while trying to prevent parasites from becoming resistant to all forms of dewormers.

Nielsen began by breaking down the phrase "sustainable parasite control" and discussing the ultimate goals:

  • Sustainable--While most dewormers were effective when first introduced, drug resistant parasites are a growing problem. Nielsen relayed that we now need to focus on using the drugs that are still effective to treat specific parasites.
  • Parasite--We know we want to control parasites, but which ones are the most important to focus on? He discussed the most important parasites to consider later in the presentation.
  • Control--"We need to know what we want to achieve when we deworm horses," Nielsen said. "We want to avoid parasitic disease, not eradicate parasites."

Nielsen said sustainable parasite control's goal isn't to eliminate worms: "We've tried for 50 years to be parasite free. We still have all the different parasites. The only major difference is the drug resistance. It's just as natural for horses to have parasites in their gut as it is to have bacteria in their gut."

To that end, Nielsen discussed a study showing that large parasite burdens in horses don't necessarily cause disease. Using young, apparently healthy horses, he determined the animals had average parasite burdens of about 150,000 worms each, with individual burdens ranging from 121 worms to 1,398,572 worms.

Types of Parasites

Roundworms--Parascaris equorum, commonly known as roundworms, are essentially ubiquitous in foals, Nielsen said. Roundworms are the most dangerous parasite in foals, he said, as they are known to cause impactions in the small intestine, generally around weaning time. The prognosis for survival is typically reserved for affected foals, and surgery is often required, he noted.

Roundworms are resistant to ivermectin and moxidectic, Nielsen said, and evidence suggests emerging resistance to pyrantel salts as well. Nielsen noted there are "rumors" of resistance to benzimidazoles, but no published evidence confirms these rumors.

Small Strongyles--Nielsen said there are about 50 different species of small strongyles (also known as cyathostomins), which are considered ubiquitous in horses of all ages. Small strongyles are known to cause acute, profuse diarrhea, mainly in young horses, he said.

"It's alarming that small strongyles are so resistant," Nielsen said, noting the worms are resistant to benzimidazoles and pyrantel salts and are beginning to show resistance to ivermectin and moxidectin. He also said many researchers and veterinarians believe daily dewormer contributes to small strongyles' resistance.

Large Strongyles--Unlike small strongyles, large strongyles are relatively rare in managed farm settings, Nielsen said. If present, however, these worms--particularly Strongylus vulgaris--can cause severe colic, thromboembolism (blood clots in the arteries), and intestinal damage or death.

Large strongyles have no reported anthelmintic resistance, Nielsen noted.

Tapeworms--Anoplocephala perfoliata, or tapeworms, have a 20 to 80% prevalence in horses, depending on geographic location, Nielsen said. These parasites can cause spasmodic colic, intussusceptions (where the intestine telescopes back onto itself), and ileal impactions, he added.

"We don't know if there's any anthelmintic resistance in tapeworms yet," he noted, as means to measure resistance in tapeworms haven't been developed.

So How Should we Treat Parasites?

With different levels of anthelmintic resistance among parasites, how should horse owners consider deworming their charges? For adult horses, Nielsen recommended fecal egg counts to determine the each horse's specific deworming requirements.

He also recommends an annual fecal egg count reduction test to see which dewormers work on specific farms and which aren't as effective.

Nielsen noted that owners should deworm young horses more frequently than adult horses, and suggested a deworming schedule:

  • Two to 3 months of age: Treat horses for ascarids with a benzimidazole;
  • Six months of age: Take a fecal sample to determine whether to treat for ascarids or strongyles, and then treat accordingly;
  • Nine months of age: Treat horses for strongyles using an efficient drug; and
  • One year of age: Treat horses for tapeworms and strongyles.

Geographic location might impact the exact deworming schedule for young horses, so consider discussing these deworming plans with a veterinarian.

Take-Home Message

While we can't completely, eliminate equine parasites, Nielsen recommends horse owners consider implementing fecal egg counts and fecal egg count reduction tests as part of a sustainable parasite control program.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.