Common Equine-Affecting Internal Parasites to Be Aware Of

It’s fall! This means many equestrians are probably going to be deworming their horses to protect them against internal parasites. But do they know what they are deworming for?

In this post, we’ll run through some quick facts about six common internal parasites that affect the equine species. On the lineup are ascarids, pinworms, large strongyles, tapeworms, small strongyles, and stomach bots!

Ascarids

Ascarids (aka roundworms) are most common in young horses under two years of age. Many believe that older horses develop an immunity to ascarids.

Ascarids are found on numerous surfaces in the equine environment. According to PennState Extension, ascarids are found on “many surfaces such as buckets, troughs, and even the udder of the mare”.

The worms migrate through the intestine and burrow themselves in the intestinal wall. The parasites will then travel to the liver and then the lungs. Ascarids return to the small intestine when the horse coughs up and swallows them again. Gross, I know.

Once back in the GI tract, the parasites mature and lay eggs that will be excreted in the horse’s manure. Each worm can excrete 200,000 eggs per day!

Symptoms of ascarids in horses include poor coat quality, pot belly, diarrhea, pneumonia, liver damage, or slow growth. Treatment involves using pyrantel pamoate or fenbendazole to kill adult roundworms.

Check out a picture of ascarids in a horse’s stomach here. WARNING: IF YOU’RE SQUEAMISH, DON’T CLICK!

Pinworms

Pinworms are minimally harmful and one of the easiest to spot. Once ingested, infective pinworm eggs travel to the colon and reach sexual maturity after about 5 months.

When the horse is relaxed or sleeping, (disgustingly) female pinworms crawl out of the horses rectum to shed their eggs. They deposit their eggs and a sticky substance on the perianal region before crawling back into the rectum.

Infective pinworm eggs are carried out with manure or rubbed off by the horse (hence why tail rubbing is the main symptom). As horses change locations, the eggs and adults travel with them. Treatment involves administering ivermectin and pyrantel pamoate.

Check out an image of a horse’s rubbed tail due to pinworms here.

Large Strongyles

There are three main types of large strongyles: Strongylus vulgaris, Strongylus endentatus, and Strongylus equinus. The one that most people focus on and causes the most severe disease, is Strongylus vulgaris (bloodworms).

The development of these larvae is highly influenced by temperature. Strongyle larvae grow faster at higher temperature and slower in cooler temperatures. These parasites also handle cold weather quite well, due to the protective sheath that surrounds them.

Horses consume these larvae when they graze, allowing them to travel to the intestinal tract. Once there, the parasites can migrate to other organs and arteries, such as the cranial mesenteric artery. This artery is the main supply of blood to the intestines and GI tract.

Once in the cranial mesenteric artery, the parasites can cause colic, blood vessel ruptures, thrombosis (blood clots that block veins or arteries), and tissue death.

Fortunately, this parasite is not as common as it used to be and is currently rare in horses. Properly managed herds and pastures need not be too concerned. If infestation occurs, fenbendazole, ivermectin, and moxidectin are all effective against the migrating larvae of large strongyles.

Check out a cool diagram of Strongylus vulgaris’ life cycle in the horse here.

Tapeworms

Tapeworms (like other parasites we have mentioned) are consumed by horses as they graze. However, unlike other parasites, tapeworms exist inside mites that already exist in the soil. The mites eat the tapeworm eggs and the horse eats the mites during grazing.

Once consumed by the horse, these parasites develop into adults in 2 months. Tapeworms then travel to the ileocecal junction (the area between the small intestine and the cecum). Once at the junction, tapeworms begin to multiply and clog up this region.

While most horses can handle tapeworms well, some horses may develop colic or impaction, which can lead to death if not caught early on.

Unfortunately, horses never acquire immunity to tapeworms so all age groups and categories are susceptible. Treatment for tapeworms includes administration of combinations of moxidectin and praziquantel.

Check out a diagram of a tapeworm’s life cycle in the horse here.

Small Strongyles

Small strongyles (aka cyathostomes) are the number one parasite problem in equines and the most damaging. Like large strongyles, small strongyles larvae are consumed by horses as they graze.

After being consumed, the parasites travel to the intestinal tract to either mature or become dormant and eventually emerge. The parasites’ eggs are passed in the feces and become infective in several days to several weeks.

Some of the main effects that small strongyles cause are diarrhea, weakness, colic, acute weight loss, and anorexia. Parasites may also continue development or go into a state of dormancy known as hypobiosis.

Once they reach this stage, they become encysted in the cecum or large intestine for around 3 months up to 3 years. As more and more parasites build up, the horse is at risk of cyathostominosis.

Cyathostominosis is the mass emergence of arrested stages of encysted larvae that has a high fatality rate of 50%. Symptoms of this disease include diarrhea, serious colic, and death. Treatment consists of a 5-day consecutive administration of fenbendazole or moxidectin.

Check out a cool diagram of small strongyles’ life cycle in the horse here.

Stomach Bots

The final parasites we will discuss are stomach bots! Stomach bots are the larvae of adult bot flies (you may be familiar with such insects). Female botflies lay their eggs on horses and the eggs are ingested by horses through licking/scratching.

Once in the mouth, larvae attach and burrow into the tongue and gums to incubate for about 3 weeks. Disgusting, I know. After incubation, the parasites are swallowed and attach themselves to the lining of the stomach.

According to Succeed Veterinary Center, symptoms of bot fly infection include “weight loss, lack of appetite, changes in body condition and hair coat, changes in behavior, difficulty maintaining condition, stereotypies such as cribbing or wood chewing, or underperforming.”

The bots spend 7 to 10 months in the stomach before being passed in manure to make more flies. Then the cycle repeats! Treatment of bots consists of deworming with moxidectin/ivermectin in the fall after the last killing frost and again in the spring before the insects reemerge.

You can also sponge your horse with warm water and vinegar to help encourage eggs to hatch and come off, thereby preventing ingestion.

Check out an image of bot fly eggs on a horse here.

Thank you so much for reading our post about common equine internal parasites! Let us know if any other information surprised you!

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