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Wyoming veterinarian explains cycle, control of strongyles in horses

by Wyoming Livestock Roundup

While many health concerns are emerging in the horse management realm, like Equine Viral Arteritis (EVA) and Contagious Equine Metritis (CEM), Gunda Gamble of G Bar G Vet Clinic in Riverton says internal parasites remain a basic concern.

After attending vet school in Germany, Gamble says she’s noticed horse owners in the U.S. know a lot about their horses, but she still sees many people bring horses to the clinic with dull hair coats or a loss of muscle tone.

“One thing you can’t check out is the load of worms in the horse from a fecal sample,” she says. “We always assume the load of parasites is way higher than we could possibly imagine.”
According to Gamble, worms are the main cause of colic. “When you think colic, think worms,” she says. “But that’s not the time to deworm the horse.”

She says to look at a horse’s gums when they’re healthy. “They should be bright pink, while cows and cats have gums that are pale pink. If a horse’s gums are light pink, think large strongyles.”

She mentions large and small strongyles, or bloodworms, as particularly prevalent in horses.

“Strongyle larvae don’t stay in intestines – they migrate through the intestinal wall and up the wall of the artery clear to the aorta, traveling the distance from the intestines to the spinal column,” she explains. ”They cause the arterial wall to become contorted, making it rough so blood running to intestines is exposed to that rough wall, which causes blood clots.”

That consequence can also cut off the blood supply to parts of intestines. “You’ll see little sections of intestines that are blue, and that means several inches of the intestines are not working right,” says Gamble. “It swells, picking up infection from bacteria brought in by digestion. That weakened wall will pick up bacteria, which will take it right to the bloodstream, causing aneurisms, blood clots, diarrhea, ill thrift and colic.”

“Small strongyles are the worst in horses because they eat the eggs off the ground, which is the difference between horses and cows,” notes Gamble. “Cows get parasites in the summer when there’s at least two inches of grass growth. You can deworm once in the fall with a pour-on and they will not get reinfected in a drylot.”

She adds pour-ons don’t work in horses because their hair follicles do not allow the dewormer to penetrate into their system. “They have zero effect on horses,” she says.

Regarding small strongyles, Gamble says horses are reinfected the next time they put their head down to the ground.

“It’s healthy for a horse to eat off the ground rather than from a raised manger, but they’re picking up tens of thousands of eggs with every bit they ingest in a drylot,” she continues. “Even in the winter. A lot of worm eggs are tenacious and can live for years. There’s a capsule on them that protects them from heat and extreme cold.”

Because of that, she says keeping pastures and stalls clean is important, as well as rotating pastures. She adds that dogs can’t get worms from eating horse manure, but rather they pick up tapeworms from eating carcasses, of which mice and rabbits are the worst.

Differing from large strongyles, small strongyles get into intestinal wall and make a calcium capsule around themselves, numbering in the hundreds of thousands. “That causes inflammation throughout the intestines, gut motility can’t work right and over a period of time have impactions with an ill thrifty horse not able to digest properly.”

Gamble says one way to tell if a horse has a large parasite load is if you see manure with long strands of fiber left in it. “You should not be able to see what your horse is eating – grain or fibers of hay or grass. If you do, you have a tooth issue and peristalsis issue,” she says of digestive movement.

“Small strongyles are the hardest to diagnose, because they don’t pass through the system when they’re embedded in the intestine,” she says.

When considering a deworming program for horses, Gamble says don’t always go with ivermectin, and don’t go with chewing tobacco, like some used to do. “Before dewormers were readily available some would feed chewing tobacco, which would cause gut spasms – a massive gut ache that would cause them to shoot the worms out. That will only get rid of some of the worms.”

“The most effective way to treat small strongyles is to feed every horse once a year a Safeguard block, which are readily available in feed stores. Just be careful you don’t have get the one with rumensin,” says Gamle. The blocks are labeled for cattle, but one per horse amounts to a five-day deworming program.

“They eat it kind of fast. If you put it in pasture with a group of horses, once a month throw another one out,” she says. “It’s a good dewormer.”

She says most horses love the Safeguard blocks, but she’s had one that wouldn’t touch it.

“If the horse won’t eat it an alternative is liquid Panacur in a jug, which compares to Panacur paste. Pull out 50 cc’s and squirt it in the mouth.”

“Deworming every four to eight weeks is not overdoing it,” says Gamble of keeping a horse herd free of large and small strongyles.

Gunda Gamble was present at the early February Fremont County Farm and Ranch Days, held in Riverton. Christy Hemken is managing editor of the Wyoming Livestock Roundup and can be reached at

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