Worms in Horses 101: Signs, Risks & Deworming Basics

Published on: 5/15/2025

Key Takeaways

Around 90% of equine worms come from the environment, putting horses at risk for poor nutrient absorption, colic, lung damage, weight loss and reduced performance without a proper deworming program.
Pinworms cause intense tail itching when females deposit eggs around the anus, and contamination can spread through bedding, wraps, grooming tools and fences.
Ascarids are especially dangerous for young horses as larvae migrate through the liver and lungs before maturing in the intestine, leading to coughing, poor growth and potential colic.
Bots enter the mouth when horses lick bot eggs off their legs and attach to the stomach lining, where they remain for months and can cause internal irritation.
Large and small strongyles, as well as tapeworms, continue to be major threats, with strongyles damaging intestinal arteries and tapeworms contributing to colic and digestive issues that often go undetected on fecal tests.

Did you know that 90% of worms in horses come from the environment they are exposed to every day? Infected horses spread eggs or larvae through their manure, which is ingested by other horses while grazing. The eggs or larva mature into adult worms in your horse’s intestinal system. Those worms can be small and large strongyles, tapeworms, pinworms, roundworms (ascarids), lungworms, bloodworms, eye worms and threadworms.

Equine parasites present a number of health risks to horses, including:

- Poor nutrient absorption
- Increased risk of colic and pneumonia
- Lung damage
- Decreased performance
- Stunted growth
- Weight loss
- Poor coat condition
- Help control your horse's risk for parasites; ensure they are on an effective equine deworming program.

For a better understanding of worms in horses, review the descriptions below.

Horse Worm Facts

Pinworms

Pinworm eggs are picked up by horses from contaminated feed, water, bedding, and may also be present on tail wraps, grooming materials, and even fence posts and stalls.

The female pinworm deposits eggs around the anus, secreting a substance which can cause intense itching. This may lead to tail rubbing and even injury to the tail and rump. Washing the perianal region may help relieve the itching, but all materials used should be discarded or washed in hot water with soap. Pinworms may be difficult to diagnose in manure samples, but the eggs can usually be picked up on scotch tape pressed to the skin near the anus.

Ascarids (large roundworms)

The adult stages of the large roundworm is found in the small intestine, where the female passes large numbers of eggs into the manure. In about two weeks, these eggs become infective and the horse picks them up while grazing. The larvae migrate into the blood vessels and are carried to the liver and lungs. The immature worms are coughed up and swallowed, maturing in the small intestine to complete the life cycle.

Bots

Bots are the immature maggot stages in the life cycle of the bot fly, the adult of which resembles the honeybee in general appearance. The females lay their eggs by attaching them to the hairs of the front legs, throat, and under line. As the horse licks itself, the larvae attach themselves to the lips and tongue and burrow into these tissues. After about three weeks they attach themselves to the lining of the stomach, where they may remain for several months, causing additional damage.

Large Strongyles (blood worms)

Adult strongyles are found firmly attached to the walls of the large intestine, where the females pass large numbers of eggs into the manure. These eggs hatch and the larvae climb blades of grass and are swallowed. The larvae then migrate to the large arteries which supply the intestines. As the artery walls are damaged, blood clots form and break away, causing colic.

Small Strongyles (Cyathostomins)

Ubiquitous parasites, infecting nearly all grazing horses. Relatively mild pathogen, except when present in large numbers.

Tapeworms

Difficult to diagnose, it is estimated that 40% of the horses in the U.S. are affected. Tapeworms are contracted during grazing, when the horse ingests the intermediate host, a mite found on plants. Once infected, tapeworms contribute to digestive problems, colic and malnourishment. Tapeworms are often undetected using normal fecal flotation methods.

Symptoms of a Heavy Worm Burden

It’s important to be able to tell when your horse is carrying a large worm burden so you can quickly call a vet. Sick horses will often show very clear signs such as a poor, dull coat, unusual weight loss (or inability to gain weight), and stomach upsets, illness and serious colic. Colic is a major cause of death in horses, so worm burden should be taken very seriously. If you see the slightest sign of any of these symptoms, it’s probably time to call your vet. It’s always better to be safe.

Frequently Asked Questions

How often should I deworm my horse?

Most horses need dewormed at least once or twice a year. It's commonly recommended that horses be dewormed in the late fall with a trusted deworming product, and again in the spring.

Do I need a fecal egg count test?

To ensure your horse stays healthy, we recommend a fecal egg count reduction test, which accurately identifies your horse's parasite burden.

Can worms in horses be prevented?

They can't be 100% prevented, but the risk can be greatly reduced by following a consistent pasture managmement plan, regularly testing your horses with fecal egg counts, and practicing deworming. Good pasture management practices will always promote healthy horses.

Whether for horse, livestock, or pet, Valley Vet Supply is your one-stop shop for trusted care. Explore more of our horse dewormers category at ValleyVet.com