Next phase in the war on worms

“For a long time, parasitology has been really reliant on seeing eggs in poop,” says Dr. Emily Jenkins, a researcher at the Western College of Veterinary Medicine (WCVM).

The current method of detecting intestinal parasites in a horse is to take a fecal egg sample and count the number of eggs per gram in the horse’s feces. “Low shedders” are classed as having 200 eggs per gram of feces. The middle ground for shedding is anywhere between 200 and 500 eggs per gram, while “high shedders” are identified by 500 or more eggs per gram of feces.

However, there’s a catch: this method will only hint at the presence of intestinal parasites, but not the specific species as nearly all parasites lay eggs nearly identical in appearance. And even if the horse has intestinal parasites, they may not be shedding eggs.

Jenkins and her graduate student, Toni-Anne Saworski, are hoping to develop a better, more accurate way of detecting and diagnosing intestinal parasites. In their study, they’re using the DNA of parasites in fecal samples to identify parasite species shed by horses before and after treatment with deworming medication.

They will focus on using both fecal egg counts and fecal egg reduction tests to note the presence of eggs and whether anthelmintic drugs (dewormers) are effective.

Similar to how certain bacteria are developing resistance to antimicrobial drugs, some species of equine intestinal parasites are becoming increasingly resistant to the deworming drugs used to destroy them.

“This project is going to look at the DNA that’s inside the worms to figure out exactly what species they are, figuring out which ones are the most disease-causing, which ones are actually infecting our horses, and then hopefully, [we will] be able to see how owners’ current deworming methods are working,” says Saworski.

A similar study to Jenkins’s current one was performed 10 years ago. At that time, the only research tool available to identify the parasites present were microscopic methods.

“It’s great to have a baseline from 10 years ago,” says Jenkins. “But also, just [to] have so much better tools to bring to bear on this question.”

The WCVM study stems from previous research done by Drs. Jocelyn Poissant and John Gilleard at the University of Calgary’s Faculty of Veterinary Medicine. Poissant and Gilleard used samples from feral and domestic horse herds to validate DNA metabarcoding approaches and to identify strongyles and other parasites in horses.

Their work is the basis for the “equine nemabiome,” which describes the community of living parasites among a horse herd.

“[The study] was worked on by professors at the University of Calgary [and] used to study parasites that infected wild horses,” says Saworski. “This is the first time it’ll be used on domestic horses.”

In their study, the WCVM researchers are collecting fecal samples from local domestic horse herds, including herds used in a previous study on intestinal parasites. They will then characterize the nemabiome among the herds, and then compare ascarid and strongyle fecal egg counts (FECs) and strongyle species composition before and after treatment with ivermectin dewormers. 

By detecting the DNA of parasites in fecal samples from horses pre- and post-treatment, the WCVM team will develop herd-level “parasite profiles” and determine which parasite species are the most resistant.

Researchers have successfully used the nemabiome approach in sheep where there is evidence of resistance in Haemonchus contortus, a common parasite found in the species. Jenkins says that sheep infected with this parasite are dying from anemia, so there is very clearly a resistance issue.

“Now we can take this approach which has been piloted in sheep really successfully … and apply it to horses to hopefully catch this process earlier before reaching the point where we have something that’s really clinically an issue and very resistant,” says Jenkins.

Many horse owners deworm their horses on a regular basis, whether they need it or not. This practice is partially contributing to the issue of anthelmintic resistance.

“They’ll [horse owners] just give dewormers once a year, twice a year, four times a year, depending on what their routine is,” says Saworski. “But of course, similar to antibiotic resistance, this leads to anthelmintic resistance.

“We’re wasting product, we’re worming when we don’t have to, maybe we’re not being as efficient as we want.”

“Interestingly, every grazing animal has some degree of parasitic exposure because they graze,” says Jenkins. “I personally suggest that for the herd in question you actually calculate which 20 per cent of horses are contributing most of the worms and just treat them.”

She adds that veterinarians are particularly worried about younger horses (under a year old) since they are most likely to have higher levels of parasites.

A parasite of particular concern is Strongylus vulgaris (large strongyles or equine bloodworm), which is one of the many species whose eggs are identical to other parasites. The equine bloodworm is especially dangerous because it can cause verminous arteritis — a potentially fatal but rather uncommon disease. It’s caused by S. vulgaris larvae that block blood flow to the horse’s gut by throwing off clots. Verminous arteritis can also lead to colic.

Jenkins says there is also evidence in Western Canada of potential resistance in ascarids, or roundworms.

“We’re sort of in an arms race with parasites, so older products are no longer effective,” says Jenkins. “The industry has already shifted to something like moxidectin, which is more effective against those larval stages.

“You don’t need to reach for dewormers, except maybe for those really high shedders. And that way, we’ll preserve our drugs to use longer in the war against worms.”

The Townsend Equine Health Research Fund (TEHRF) is providing financial support for this study.

Jessica Colby of Montmartre, Sask., is a University of Regina journalism student. She worked at the WCVM as a summer research communications intern in 2021.