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Worms are parasites that affect horses and other animals including humans. Horses are afflicted with a number of different internal parasites in various stages of growth and in various parts of the body. Because parasites are so prevalent in the horse's environment and easily transmitted, it is virtually impossible to rid horses of all parasites or to prevent re-infection.

With the horse as host to these parasites, the worms pass through the horse's system, damaging blood vessels, arteries, and organs, interfering with growth and development, creating nutritional deficiencies, and predisposing the horse to other diseases and conditions.

Strongyles are a species of roundworm, also known as bloodworm, that is arbitrarily divided into two groups, known as large strongyles and small strongyles. The larvae of large strongyles migrate through the horse's circulatory system, damaging blood vessels as they move from organ to organ before reaching the intestine, where they lay eggs that are shed in the feces. Damage to arteries leads to thrombosis (blood clots), embolism, and development of aneurysms.

The larvae of small strongyles lie dormant in the intestinal wall and eventually become encysted. Extensive damage to the intestinal wall occurs if large numbers of small strongyles emerge simultaneously.

Ascarids, also called large roundworms, are often found in young horses. They can grow to be twelve inches long and can be found in the hundreds in the small intestine, leading to poor nutrition and causing coughing, colic, and diarrhea. If they reach the lungs, they may cause pneumonia.

Threadworms, also known as strongyloides, are usually the first intestinal parasite in young foals. Acquired through the dam's milk or from eating the mare's feces, they migrate to the lungs and then return to the intestines where they mature into adult worms. By sixteen weeks, most foals develop resistance to threadworms and maintain minimal infection.

Pinworms, usually acquired by drinking contaminated water or eating infested hay or grass, are less dangerous to a horse's health, but extremely annoying because of the intense anal itch they cause.

Tapeworms, said to infect up to 40% of horses in the United States, are spread through mites that are found on plants located in pastures. The mites consume the tapeworm eggs found in infected manure. Grazing horses swallow the mites and become infected. Malnourishment, colic, and digestive problems may be the result of tapeworms.

Bots are the immature larvae of the botfly. Most commonly found in the early fall and late summer, adult female botflies lay eggs on the horse's hair and, as the horse licks itself, the larvae attach themselves to the tongue, gums, and cheeks.

After three to four weeks, the bot larvae travel to the stomach and attach to the lining where they cause irritation and may block the opening to the small intestine, as well as interfere with digestion. The larvae can also cause colic, ulcers, and perforation of the intestine, resulting in peritonitis.

Lungworms are sometimes found in horses that have contact with burros and donkeys. Infected foals seldom show symptoms. The adult worms live in the lungs and lay eggs in the breathing tubes. The eggs move toward the larynx and are swallowed and passed in the feces, where they hatch within a few hours, are ingested by the grazing horse, penetrate the wall of the bowel, and are carried to the lungs. A persistent cough and labored breathing are signs of infestation.

Stomach worms or habronema live in colonies in the wall of the horse's stomach. Eggs that pass in the feces are picked up by flies, which serve as intermediate hosts. When the fly feeds on the wounds or around moist areas of the horse's body, the larvae escape from the mouth parts of the flies and may be swallowed by the horse.

A large number of habronema can produce severe gastritis. Tumor-like enlargements may occur in the wall of the stomach and, if these tumor-like enlargements rupture, peritonitis usually ensues. In addition to internal problems, habronema cause summer sores and conjunctiva.

Hairworms, or small stomach worms, often infect horses that are in close contact with cattle. Living deep in the wall of the stomach, a severe infestation produces gastritis, ulcers, weight loss, and anemia.


  • Weight loss
  • Anemia
  • Colic
  • Diarrhea
  • Poor coat condition
  • Slowed growth
  • Loss of condition
  • Severe itching in anal region
  • Digestive problems
  • Malnutrition
  • Coughing
  • Ulcerations in mouth
  • Ulcers
  • Intestinal and bowel obstruction


Intestinal parasites of many varieties are ever-present in the horse's environment and are among the most serious and common health problems affecting horses. They gain access to the horse through ingestion, from the bites of flies and insects, and by burrowing through the horse's skin.

Because the environment and the horse's body provide conditions that promote the various stages of the worm's life, it is up to the horse owner to keep horses relatively untroubled by parasites through a well-established, consistent program of worm/parasite control. 


The American Association of Equine Practitioners (AAEP) makes the following suggestions regarding the best ways to rid horses and their environments of worms:

  1. Pick up and dispose of manure droppings in the pasture at least twice a week.
  2. Mow and harrow pastures regularly to break up manure piles and expose parasite eggs and larvae to the elements.
  3. Rotate pastures by allowing other livestock, such as sheep or cattle, to graze the pasture, thereby interrupting the life cycles of parasites.
  4. Group horses by age to reduce exposure to certain parasites and maximize the deworming program geared to that group.
  5. Keep the number of horses per acre to a minimum to prevent overgrazing and reduce fecal contamination per acre.
  6. Use a feeder for hay instead of feeding on the ground.
  7. Remove bot eggs quickly and regularly from the horse's coat to prevent ingestion.
  8. Rotate deworming agents, not just brand names, to prevent chemical resistance.
  9. Consult your veterinarian to set up an effective and regular deworming schedule.


Treatment of internal parasites is best accomplished by a knowledgeable veterinarian who is familiar with all deworming agents available and able to prescribe a deworming program for a particular horse or group of horses. Because of the life stages of parasites from egg to larvae to worm, no deworming agent will be completely effective in ridding a horse of all parasites.

Dewormers, with the exception of ivermectin, attack only the adult worms in the intestinal tract and do not affect the encysted larvae. Also, drug-resistant worms are a continuing problem. According to authorities, it is important to consider several important points in administering an effective deworming program:

  • All horses on the premises should be dewormed at the same time.
  • Deworming should be done at regular intervals, using a program developed by a knowledgeable veterinarian.
  • Use a dewormer that is highly effective for the species in question, and in the correct dosage.
  • Treatment of expectant mares is necessary to prevent foals from being burdened with parasites shortly after birth.
  • All labels should be read and understood completely before using any deworming product, and care must be taken to avoid overdosing the horse.

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