1. Home

Internal Horse Parasites

Internal Parasites That Can Affect Your Horse


Mosquitoes and face flies are obvious external pests that your horse has to deal with, but there are internal pests that can profoundly affect your horse's well being. Internal parasites can do significant damage to your horse's internal organs. They steal nutrients from the food the horse eats and can cause havoc with the intestinal wall, lungs or cause clots in the blood vessels carrying nutrients to the intestines.

If a horse is otherwise healthy and well fed, a moderate parasite load might not be obvious. But, if the problem gets out of hand, the horse may suffer signs of malnutrition such as a distended belly, weight loss, have a dull or frizzy hair coat, dull attitude, diarrhea, and may show colic symptoms. Especially if the horse is very young or old, internal parasites can result in death.

Unfortunately, horses can harbor several types of worms at the same time. Good pasture management and a regular de-worming program can prevent severe parasite infestations. Many different types of internal parasites can take up residence in your horse. Here are the most common kinds.

Redworms, Bloodworms or Strongyles

These worm-like vampires suck blood. Stronglyes are the most common type of internal parasite. After the horse ingests larvae that have attached to plants the parasites attach themselves to the intestinal wall, where they can damage the lining, cause bleeding, clots and damage to the intestinal wall which can lead to colic symptoms, arterial damage, and serious arterial blockages which can even lead to death. Like many internal parasites, the life cycle is completed when eggs are shed in the manure, which then hatch, attach themselves to plants and are then ingested.

White Worms, Round Worms or Ascarids

My most horrifying encounter with ascarids was when large quantities were expelled from a weanling foal I rescued. They are long, round, pale grey worms that can grow up to a foot long. Because of their size they can cause impaction colic. The foal I rescued would develop a cough the after I wormed her—possibly from the dying off worms trying to escape through any exit. (The rescue had a happy ending as I sold the foal as a yearling and she went to a lovely home with acres of pasture and a brand new barn.)


If you see your horse scratching the base of its tail and there's no obvious external reason, suspect pinworms. Pinworms live in the large intestine and lay their eggs around the anus. Horse pinworms are not the same as human pinworms, so no need to worry about you or other pets becoming infected.


These worms live within the horse's lungs. They are most common in areas of heavy rainfall. Lungworms cause irritation in the bronchial tubes that can lead to coughing, bronchitis, secondary bacterial infection, pneumonia and death. Donkeys are particularly susceptible to lung worm and if pastured with horses, the infestation can be spread to all through ingesting the eggs with the pasture grass.

Tape Worms

Less common, but no less damaging are tape worms. Tape worms, as they're name suggests, are flat, tape like parasites made up of segments. They are pale yellow-ish white in color. The end-most segments of the worms are filled with eggs. When the horse passes manure, the egg filled segments are dropped into the pasture. These eggs are then consumed by a mite—which can then be eaten by a horse as it grazes. Tape worms steal the nutrients from the food the horse eats leading to poor condition. Tapes don't always respond to common de-worming medication methods, so it's best to consult your vet if you suspect tape worms.


In late summer and early fall, your horses may be pestered by the bee-like bot fly. Learn more about bots and bot flies.


A their names suggest these are small threadlike worms about 5/16 of an inch long. Threadworms enter the horse either by ingestion or by penetrating the skin. These worms are a problem in foals, and should be controlled in mature horses to avoid spreading infestations to young horses. Threadworms may be passed from mare to foal through the mare's milk. The worms can cause damage to the horse's lung tissue, lining of the digestive system and cause skin irritation. Foals with a severe infestation may suffer from poor condition, colic and diarrhea.


  • Merial
  • Merck Manual - Health-management Interaction: Horses
  • Hayes, M. Horace, and Peter D. Rossdale. Veterinary notes for horse owners: an illustrated manual of horse medicine and surgery. 17th ed. New York: Prentice Hall Press, 1987. Print. "The Merck Veterinary Manual." The Merck Veterinary Manual. N.p., n.d. Web. 17 Feb. 2012. .
  1. About.com
  2. Home
  3. Horses
  4. Horse Care
  5. Internal Parasite Control
  6. Internal Horse Parasites

©2014 About.com. All rights reserved.