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Horse deworming guidelines undergo changes after 40 years

by Wyoming Livestock Roundup

For over 40 years, horse producers have been urged to follow a routine program of deworming their horses every two months. 

Ray Kaplan, professor of parasitology at the University of Georgia College of Veterinary Medicine Department of Infectious Diseases, said this routine rotational program people followed was designed to control bloodworms, which were a significant threat to horses 40 years ago. 

“Bloodworms have become very rare over the years,” Kaplan said. “In fact, they are practically extinct in causing disease in horses. Small strongyles are now more of a parasitic threat.” 

“This shift in parasites, combined with more drug resistance in horses from wormers given over the years, has lead us to believe the recommendations from over 40 years ago no longer make sense,” the veterinarian said. 

Regulatory changes

Kaplan, along with a group of colleagues from the veterinary industry, served on the American Association of Equine Practitioners Parasite Control Guidelines committee in 2013. The group was given the task of revising the outdated regulations and creating a new set of guidelines for horse owners to follow. 

The new regulations encourage horse owners to have a fecal count conducted at least once a year.

“The focus is also on parasite control as a holistic part of the farm, rather than a deworming program that relies solely on chemicals,” Kaplan continued. 

If horse owners can focus on the environment their horses are in, preventing fecal contamination and not overgrazing their pastures, they will have healthier horses that don’t need dewormed that often, Kaplan explained. 

“The parasites horses have now are not that horrible. Most horses do pretty well without frequent deworming,” he added.

Fecal counts

Nathan Voris, a senior veterinarian with Zoetis Equine Technical Services, explained fecal counts. 

“As long as the horse maintains the same level of health, a horse that is a low shedder of parasites tends to be a low-shedder throughout its life,” he explained. “A horse that is a high shedder, will tend to be a high shedder throughout its life.”

“By identifying high shedding horses, a deworming program can be put together for that horse that doesn’t overexpose it to medications when its not necessary,” Voris continued. 

The idea of annual fecal count exams is to identify horses that have undergone a change in their health that has also changed their fecal health.

Both veterinarians said that when a fecal count is conducted, the technician looks mostly at how many strongyles are present. For mature horses, 500 eggs per gram is considered a high fecal count, 200 to 500 is considered moderate and under 200 is considered low.

Voris said the best time to take fecal counts is in the spring and fall. If high shedders are found, they should be treated to keep the parasite load down on pastures. 


“Give a paste wormer before the horse eats, so it doesn’t have a wad of feed in its mouth,” he said. “It’s more likely to get a full dose. Take time and be patient if the horse is reluctant to take its paste.”

“What people don’t realize is the purpose of deworming is to control parasites,” Kaplan explained. “However, using a drug that doesn’t work is giving you no results.”

“Horses with severe parasite problems may be on a deworming program, but it won’t do any good if they are resistant to the drugs. That is why it is important to involve your veterinarian in the parasite control program,” he said.

Holistic approach

There are also ways to control parasites without using drugs. 

“That is where pasture management becomes very important,” Kaplan said. “It is a critical part of preventing horses from becoming contaminated with worms.” 

“Fewer parasites will be ingested by controlling fecal contamination. Make sure and provide the horse with enough grass and nutrition. If a pasture looks like a golf course, the horse will ingest worm larvae,” he stated. 

Picking up manure pads from the pasture can significantly reduce ingestion of worm larvae by horses. 

In warmer temperatures, Kaplan said it can take as few as four days for manure pads to start breaking down, driving worm larvae into the grass for the horse to eat. 

When it is colder, it can take up to two weeks. 

Manure management

“Horses will segregate pastures to areas where they defecate and where they graze. But, if the pasture is overgrazed, hunger will win out, and they will graze closer to their feces,” he said. “Ninety percent of parasite larvae is in the bottom four inches of grass.”

Composting manure is lethal to larvae eggs because of the high temperature reached during the composting process. Kaplan said the composted manure can become an excellent fertilizer without having any eggs in it. 

However, spreading un-composted manure when temperatures are cold can spread parasite larvae all over the pasture. 

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to

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