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Horse Parasites & Worms

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Horses carry more than 100,000 internal parasites on average. These parasites consist of more than 60 species. Because horses graze, they tend to consume eggs from various worms. Combined with a host of external parasites, your horse can become miserable very fast. Whether internal or external parasites, they can cause weight loss, diseases, diarrhea, colic, poor growth, pneumonia and even death.

Bloodworms and Small Strongyles

Bloodworms, as their name implies, feed off blood. They're extremely dangerous to horses because their larvae migrate through the arteries and cause aneurysms and blood clots, killing the horse. Adult bloodworms live in the intestines and suck blood, causing anemia. They're called "large strongyles" in equine medicine. Small strongyles, meanwhile, live in the intestine and can damage the large intestine's wall. These worms pose no health risk to humans.


Pinworms cause irritation around the anus and in the large bowels. Horses with pinworms eat more food but look unhealthy. They are not the type of pinworms humans get; there is no health risk to humans.


Roundworms, or ascarids, can damage your horse's lungs as the larvae migrate. They wind up in and damage various organs. They can cause impaction colic -- that is, their mass can become so great in the gut that they can block the stomach or intestines. They can cause a potbelly or bloated look, poor coat or an otherwise unhealthy appearance, and they can lead to pneumonia and death. These worms pose no health risk to humans.


Threadworms (Strongyloides westeri) primarily attack nursing foals through the mare's milk. These worms cause diarrhea that can kill a young horse. Threadworms can live in the soil and penetrate a foal's skin to infect the horse with cutaneous larva migrans, a skin disease. Threadworms can penetrate your skin and infect you with the same disease, which makes it imperative that you work to reduce threadworms in the environment.

Fly Bots

Fly bots are larvae from certain flies that lay their eggs on horses' coats. When a horse bites or scratches himself using his mouth, the horse may ingest the eggs. These eggs hatch in your horse's stomach. The larvae grow there, feeding off your horse's nutrients. A horse may have stomach irritation, colic or impaction leading to death. Flies can lay their eggs on humans, and their larvae can burrow into your skin. It is important to reduce flies around your horse.

Biting Flies

Biting flies include deer flies, horse flies, stable flies, sand flies, biting midges or no-see-ums, black flies and horn flies. These flies not only bite, painfully, but many carry stomach worms and diseases such as tularemia, equine infectious anemia, blue tongue and anaplasmosis.

Lice, Mites, Ticks and Mosquitoes

Lice are biting and bloodsucking tiny parasites transmitted to a horse by a fly, another horse or infested bedding or equipment. They can cause anemia, itching, sores, weight loss and general poor health. Five different mites that attack horses are classified as mange mites or chiggers. It's important to control these mites as they can infect humans. Ticks, meanwhile, can carry diseases to your horse, such as equine piroplasmosis, anaplasmosis and tularemia. They can cause discomfort due to their bites and blood-sucking. Mosquitoes carry diseases such as West Nile virus, eastern equine encephalitis, western equine encephalitis and other encephalomyelitis.

Parasite Management

You can control most internal and external parasites with proper stable and pasture management. Keep feces picked up and composted in a covered bin; rotate pastures; and use pasture drags to keep the soil churned up to keep it dry. Put your horse on a regular worming schedule, recommended by your veterinarian, and use horse-safe insecticides to keep external parasites off. Grooming your horse will help eliminate fly bot eggs. Eliminate standing water where mosquitoes lay their eggs and keep your horse in clean and dry conditions.