Worms in horses produce a range of symptoms that can appear similar in some circumstances. That’s why it’s crucial for horse breeders and owners to get a professional diagnosis from a veterinarian before deciding on a course of de-worming measures.

Following are some common symptoms of worms in horses and the types of worms that may produce them.

Colic — Horse colic is defined as abdominal pain, but it’s usually a sign of some other illness than a condition in itself. Colic is the number-one cause of premature death in horses, which is why preventing colic caused by parasites is so important. Worms can cause two kinds of colic in horses: spasmodic colic and colic that requires surgery. Both kinds cause severe pain (imagine yourself with a bad bellyache) that will make the horse roll back and forth in the hay to try to relieve itself. Both Small Strongyles and Tapeworms have been found to cause spasmodic colic, something akin to intestinal cramps in humans. In large numbers, Large Redworms have been found to cause blood clots, ulcers and intestinal blockages, resulting in colic that requires surgery.

Diarrhea — Many kinds of parasites cause diarrhea in horses, as do other things. Small Redworm infection frequently is one cause of diarrhea in horses, especially among young horses in the spring. Veterinarians treating horses with diarrhea often it find hard to locate the cause, since it’s impossible to detect the buildup of worm larvae in the intestinal walls. Diarrhea is dangerous because horses can quickly become dehydrated and go into shock. A veterinarian should see a horse with diarrhea immediately.

Weight loss — Sometimes young horses with large infestations of Small Redworms won’t develop diarrhea. Instead they’ll lose weight and fail to thrive (stunted growth) because the parasites are consuming their blood and nutrients. To date, no one knows why some horses respond in this way to a larger burden of Small Roundworms.

Potbelly — Another sign of worm infestation is when foals and young horses develop a potbelly while their ribs show through. A rough, dull coat can accompany this condition. Large Roundworms often are the cause.

Mouth sores — There are many reasons for mouth sores in horses, including bots from the botfly. Female botflies lay their eggs on the horse’s coat and the horse licks up the eggs when it licks its coat. Typically the eggs enter into the stomach and eventually pass out through the feces. However in warm weather and with moisture present, the eggs can hatch on the horse’s coat and migrate to the mouth. There the larvae will cause sores around the teeth and on the tongue.

Stomach ulcers — Internal stomach ulcers often are caused by irritation from the ingested larvae of botflies. In cases of extreme burden, bots can cause stomach rupture. A horse that goes off its feed because of mouth sores may also have stomach ulcers and should be seen by a veterinarian.

One disease related to horse worms can already be fatal by the time symptoms are observed in a horse. This disease is known as larval cyathostomosis. It’s caused by a sudden eruption in the horse’s gut of thousands of Small Strongyle larvae that have burrowed into the intestinal walls and “cocooned” themselves in mucosal cysts. The damage caused by this disease quickly results in chronic diarrhea, which in turn can lead to dehydration and shock; dramatic weight loss; low-grade colic; and overall weakness.

The severity of the illness depends entirely upon the level of parasite burden in the horse. In milder cases, the horse may show only a weight loss for which there is no explanation, or it may simply not perform as expected. In severe cases, death often results.

Once the larvae mature into adult worms, they begin shedding eggs that can be detected in a fecal egg count. Assuming it has survived the symptoms of larval cyathostomosis the horse can then be treated for Small Strongyles.

Symptoms of Horse Worms
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