Types of Internal Horse Parasites (Worms)

Learning about the different types of internal horse parasites (worms) is the first step in controlling them. This task is a responsibility that each horse owner must be prepared to take on. According to OSU cooperative extension services,

    Internal parasites continue to be a significant threat to the health of horses. Internal parasites are small organisms that live a portion of their life cycle in a host animal. They live in internal organs, body cavities, and tissues and gain their nutritive source by feeding on the host animal. The horse is affected by many different species of parasites. The nature and extent of damage varies with the parasite.

    Parasite infestation causes loss of nutrients or blood from the host and serious economical and medical problems in managing horses. While it is obviously expensive to feed parasite-burdened horses, depletion of nutrients and blood can cause severe loss of condition, decrease growth, and reduce reproductive and athletic performance.

On this page we will first look at parasites in horses and their life cycle, then we will address treatment plans and how to deal with them in Controlling common internal horse parasites .

The four most common types of internal horse parasites are

  • Strongyles (large/small)
  • Ascarids(horse roundworm)
  • Strongyloides(threadworm)
  • Stomach Bots


Strongyles are grouped as either large or small, with the small strongyles (cyathostomes) considered to be the most common. Often they are called red worms and are readily visible in an infested horses droppings.

The lifecycle of the small red worm is as follows:

    Adults lay eggs in large intestine – passed out in feces – eggs hatch/develop 3 stages (3rd stage infective) – horse eats infected worms – worms migrate to large intestine – larvae (worms) burrow into the lining of large intestine (they can lie dormant for quite some time) – adults lay several thousand eggs in large intestine and the cycle starts again.

Severely infested horses may show clinical signs such as sudden onset diarrhea or colic. Horses may also be infected and show no major signs. Though you might notice a problem in areas such as decreased feed efficiency, rates of weight gain or poor performance.

Large strongyles follow the same lifecycle as small strongyles except the larvae migrate further than the large intestine.

    Strongylus vulgaris – the bloodworm – will migrate/burrow into the walls of the arteries that are the primary blood supply to the small and large intestine. This migration can cause a disruption in the flow of blood to the intestines by the formation of clots. Around 120 days, the larvae then move back to the large intestine where maturation is completed.

    Strongylus endentatus and strongylus equinus have similar life cycles except, instead of the arteries, the larvae migrate to the liver. Though it results in damage to the liver, it is still not as dangerous as the bloodworm.

    The effective use of horse wormers has greatly reduced the prevalence of large strongyles, which is the horse parasite that does the most damage.

Ascarid (horse roundworm)

The horse roundworm is a very large yellowish white parasite that may pass out in the droppings. (Females can be up to 15 inches long) Usually adult horses develop immunity to this parasite. Roundworms primarily infect horses below 2 years of age.

The life cycle (rather scary) is as follows.

    Horse consumes water/feed/pasture containing infective eggs – eggs hatch – larvae burrow into small intestine – migrate through veins to the liver, the heart, then lungs – migrate around in air spaces in lung – coughed up then swallowed – mature in large intestine – lay eggs and cycle (3 months) is complete.

Physical damage can occur to the liver and lung tissue due to inflammation and scarring. The adult round worm can cause physical damage ranging from mild digestive upset to sever colic. Some clinical signs are typically potbelly, rough hair coat and slow growth.

Strongyloides (threadworm)

The threadworm is a horse parasite that infects foals as early as 4 days of age. They quickly develop immunity and lose the intestinal infection of adult parasites by 60 to 90 days. However, the life cycle of the threadworm can be completed in less than 2 weeks, which can result in severe infestation. The most significant medical problem this causes in foals is diarrhea that may not respond well to treatment. They can quickly become dehydrated from this diarrhea if the issue isn’t dealt with. One of the best ways to prevent it is to worm the broodmare within 24 hours of foaling.

Stomach Bots

Stomach bots are not really worms but instead are the larvae of the botfly. Female botflies lay their eggs by attaching them to the horse’s hair. (those little yellow “things” that don’t come of when you brush your horse. Different species lay their eggs on different parts of the horse. Legs, haw, lips, etc.)

The life cycle is as follows:

    Larvae attach/burrow into the tongue and gums – incubate there for 3 weeks – are swallowed and attached to the lining of the stomach (approximately 9 months) – pass out with manure – larvae pupate into adult flies – cycle starts over.

The adult flies are active from late spring to the killing frost in the fall. There whole cycle depends on the larvae over wintering in the stomach and then being passed out in the spring.

There are other species of horse parasites that might cause some trouble:

  • Lungworms
  • Pinworms (which often cause tail rubbing)
  • Tapeworms (which has been shown to perhaps cause certain types of colic in horses)

Most of these three species of horse parasites don’t seem to create as serious of a health threat due to the lower incidence of infestation or there lifecycle is not as harmful. You can have your vet check for the presence of tapeworms/lungworms and he can recommend the proper treatment. Pinworms are killed by Ivermectin

For more information on common horse parasites, please contact us.

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