Internal Horse Parasites

Internal Parasites That Can Affect Your Horse

american quarter horse in field


Mosquitoes and face flies are obvious external pests that your horse has to deal with, but there are unseen 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, cause diarrhea and weight loss, and can 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 a problem. In fact, having some intestinal parasites is normal for a horse. However, if a horse is weak, sick, young, very old, or has other health problems, internal parasites can add to the health issues. Severe intestinal parasitism leads to signs of malnutrition such as a distended belly, weight loss, dull or frizzy hair coat, dull attitude, and diarrhea.

It is common for horses to harbor several different types of intestinal parasites at the same time. Good pasture management and a regular parasite management program can prevent severe parasitic infections. Here are the most common types of equine intestinal parasites.

  • 01 of 07


    This is a class of equine parasites and can be further divided into large and small strongyles.

    Large strongyles, sometimes called "bloodworms" or "redworms," are likely the most dangerous equine intestinal parasite because they can cause the most damage. Larvae of these parasites sit on blades of grass where a horse can easily ingest them. After the larvae are ingested, they burrow through the lining of the digestive tract and migrate through the blood vessels that supply the intestines. The larvae stay in these blood vessels for a few months, further developing into adult parasites. Once adults, the parasites migrate back to the intestine where they attach to the wall and release eggs which the horse passes in manure, to reinfect the pasture and start the life cycle over again.

    Larvae migrating in the intestinal blood vessels can damage the vessels, cause bleeding and clots, arterial damage, and serious arterial blockages which can even lead to death.

    Small strongyles are generally thought not to be as pathogenic, however, over the past few decades, researchers have been documenting cases of resistance in these parasites to commonly used dewormers. Small strongyles have the ability to encyst in the wall of the intestine. This makes it harder for deworming drugs to kill them and is, therefore, a handy survival mechanism for this parasite. Typically this parasite will encyst over the winter and when spring arrives, the worms will emerge from the cysts in large numbers. This causes severe irritation to the lining of the intestine and can result in diarrhea and colic in the horse.

  • 02 of 07

    Roundworms or Ascarids

    Equine Ascarid parasites
    Ascarids By eXtensionHorses - ascarid1, CC BY-SA 2.0,

    Ascarids are long, round, pale gray worms that can grow up to a foot long. They live in the small intestine of horses. Because of their large size, they can cause impaction colic. These parasites can wreak havoc in foals especially, causing them to appear malnutritioned, with a potbelly appearance and poor hair coat. As a horse matures, it tends to build a natural immunity to this particular type of parasite.

  • 03 of 07


    Image of pinworms.
    Pinworm (Oxyuren) in the institute for parasitology of the Ludwig-Maximilians-University Munich. Image Credit: /Getty Images

    If you see your horse scratching the base of its tail on a tree or fence post, 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. Considered the most benign of the equine intestinal parasites, pinworms are easily treated with routine deworming products. The most harm they do is the resultant bald tail from the horse scratching.

  • 04 of 07


    Micrograph of lungworms.
    Light micrograph of Lungworm (Capillaria aerophila): nematode parasite which infests the respiratory tracts of certain mammals Image Credit:Les Stocker/Oxford Scientific /Getty Images

    These worms live within the horse's lungs. Lungworms cause irritation in the bronchial tubes that can lead to coughing. Severe infections may result in bronchitis, secondary bacterial infection, and pneumonia. If a horse has a cough that does not seem to be associated with another sickness or dusty feed, your veterinarian may suspect lungworm. This parasite is most often found in donkeys and mules, but horses can be infected if they share a pasture.

    Continue to 5 of 7 below.
  • 05 of 07


    Light micrograph of parasitic tapeworm
    Light micrograph of parasitic tapeworm Image Credit:Science Photo Library - ERIC GRAVE/Brand X Pictures /Getty Images

    Tapeworms, as they're name suggests, are flat, segmented, tape-like parasites. They are pale, yellowish, and 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. Tapeworms generally aren't as dangerous to a horse's overall health as roundworms or strongyles can be. However, in large numbers, tapeworms still rob nutrients from the horse. Tapeworms are treated with a specific type of dewormer called praziquantel. Not all over-the-counter dewormers contain this drug, so it's best to consult your vet if you suspect tapeworms.

  • 06 of 07


    Bot Fly Larva
    Illustration of a single larva of the bot fly. Image Credit:Science Picture Co /Getty Images

    Bots are not an intestinal parasite but rather a fly. Botflies lay eggs on your horse's front legs. The eggs are itchy and irritating and when the horse scratches himself, he ingests the eggs. Once swallowed, the eggs hatch and the larvae attach themselves to the lining of the horse's stomach where they can cause woulds and ulcerations. After a period of time, the larvae develop further and then release themselves from the stomach lining and are passed in the horse's manure. Once on the ground, the larvae burrow and further pupate into adult flies to restart the life cycle. In general, bot larvae are not harmful in small numbers. Horse owners can purchase bot fly knives at the tack store; these metal grooming instruments are used to scrape bot fly eggs off the inside of a horse's front legs to prevent ingestion.

  • 07 of 07


    A single threadworm.
    Threadworm, coloured scanning electron micrograph (SEM). This is a nematode worm that is a parasite of the small intestines of numerous animals. Image Credit: Science Photo Library/STEVE GSCHMEISSNER/Getty Images

    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 mostly a problem only in young foals. Owners of pregnant mares may consider treating the mare for threadworms before she foals. Threadworms may be passed from mare to foal through the mare's milk. The worms can cause damage to the horse's lung tissue, the lining of the digestive system and cause skin irritation. Foals with severe infestation may suffer from diarrhea, which may not respond to treatment, and dehydration. As the foal ages, it develops natural immunity to this parasite, which is rarely a health issue in older horses.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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  1. Gastrointestinal Parasites of Horses. Merck Veterinary Manual.

  2. Prevent Parasites, Reduce Problems for Your Horse. University of Illinois College of Veterinary Medicine.

  3. Internal Parasites in Horses. American Veterinary Medical Association.

  4. Lungworm Infection in Horses. Merck Veterinary Manual.

  5. Strongyloides (Threadworm) in horses. USDA National Institute of Food and Agriculture.