Equine Health (Horses)

Parasites in Horses

Internal parasites are the greatest single cause of colic in horses and are often a causative or contributing factor in many respiratory, digestive, and performance problems. Though parasites are a constant problem for horse owners, the severity of the problem can be lessened with a regular preventive worming program, formulated by your veterinarian.

The four most common internal parasites found in horses are bots, strongyles, ascarids, and pinworms.


Bots are the larvae (immature worms) of the botfly. Since these flies are common to the horse’s environment, it is almost impossible for a horse not to be infected.

During the warm months of late summer and early fall, adult botflies lay their eggs on the hair of various parts of the horse, especially the chest, forelegs, throat, and nose. Stimulated by the horse’s licking, the larvae hatch and enter the horse’s mouth, where they settle in the tissues of the gums, cheek and tongue. After about a month, the larvae migrate to the stomach, where they attach to the stomach lining. It is not unusual for several hundred bots to attach to the stomach, causing irritation, interfering with digestion, and obstruction to the opening of the small intestine.

Bot larvae are passed in the feces after about eight to ten months. They burrow into the ground and pupate. They become adult flies in about a month, ready to start the cycle again by laying their eggs on the horse.

It is reasonable to assume that most horses become infected and should be treated from the time botflies or “nits” are seen on the horse until about a month after the first hard frost. Botflies are killed by freezing temperatures. Several commercial anti-bot preparations are on the market and some are relatively toxic. It is wise to consult your veterinarian as to the type of drugs and frequency of treatment against bots as a part of your overall parasite control problem.


The term strongyle refers to a large group of closely related species of internal parasites. Strongyles are also called blood worms. They are very dangerous because the immature worms migrate through blood vessels of the intestine, and produce intestinal inflammation which may result in “fatal” colic. Horses of all ages are infected.

The strongyle’s life cycle begins in the intestinal tract where the female lays eggs that are passed in the feces. Under proper environmental conditions (including warmth and moisture), the eggs hatch into larvae in the manure. Under cold and dry conditions, the eggs can survive unhatched for long periods, to emerge when conditions are right.

The infective larvae migrate onto grass blades, where they remain until grazing horses ingest them. They then develop into young parasites in the intestines, and migrate for 6-7 months along the walls of the arteries, liver, and intestinal wall, eventually returning to the large intestine as young adults. The period of migration can be up to 300 days for some species of strongyle larvae. Adult worms in the large bowel lay eggs that are passed in the feces, completing the life cycle. A female strongyle can lay up to 5000 eggs per day.

Horses with strongyles may lose condition, weaken, and have diarrhea. They may become anemic due to the parasites’ blood sucking. Horses in good physical condition may have a large number of strongyle larvae that can create arterial aneurysms (a balloon-like defect) which can cause sudden death if the artery ruptures at the aneurysm.

Veterinarians diagnose strongyle infection from microscopic observation of eggs in the feces. Blood tests are often used to assess the seriousness of an infection. Frequent deworming treatments are recommended to reduce the risk of serious problems from these parasites and should be decided upon following consultation with your veterinarian.

Ascarids, or large roundworms

Ascarids (large roundworms) affect young horses more than mature ones. The 6- to 12-inch long worms can number in the hundreds in the horse’s small intestine, interfering with the young horse’s nutrition. Colic, coughing, and diarrhea may also result from ascarid infection.

Foals acquire infective ascarid eggs from feces that other horses have passed. Infective eggs, swallowed in contaminated hay or water, hatch in the intestinal tract. The young worms burrow through the intestinal wall, taking about a week to make their way to the lungs. From there the young worms travel up the trachea to the mouth, to be swallowed a second time. They mature in the intestine in two to three months, then lay eggs that are passed in the feces to start the cycle anew. Female ascarids can lay up to 200,000 eggs per day.

Foals should be first treated at 8 weeks of age, then every 6 to 8 weeks until they become 2-year-olds, for adequate control of ascarids.


Though less dangerous than other internal parasites, pinworms are annoying to the horse because they cause severe anal itching. Adult worms crawl part way out of the anus to deposit their eggs on the adjacent surface. The eggs hatch outside of the horse’s body and become infective in a few days, although they can survive unhatched for several months. The parasite is taken into the animal through contaminated water, grain, hay, or grass. Young worms mature in the large intestine in three to four months, then begin the cycle anew.

A characteristic of pinworm infection is rubbing of the tail and anal region, causing broken tail hairs and bare patches around the tail.

Pinworms can be treated successfully with the same drugs that are effective against strongyles and ascarids.

An Ounce of Prevention

Breaking the life cycle of parasites is as important as administering dewormers. Manure should be removed daily in stalls and weekly in pastures. Pastures and paddocks should be well drained and not overpopulated. Fly control programs help with bot prevention and general well-being.

Wise horse owners will keep a close eye on their horses, watching for such telltale signs as loss of condition, dull hair coat, tail rubbing, and diarrhea. Routine examination of fecal samples under a microscope will enable the veterinarian to detect inapparent infections.

An effective parasite control program involves each and every animal on the farm. Ideally, intensive treatments should be scheduled at regular intervals from birth until death of the animal. Your veterinarian should be consulted to help you establish a program that will be practical for your management conditions.

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