Worms can cause serious health problems for horses. Get to know the most common species of equine worms and plan how to treat.

The following is a list of the most common species of equine worms.


Strongyles are found in the large intestines where the females lay eggs that leave the body via manure. The eggs hatch and when larvae grow they climb grass where they are consumed by the horse. They cause damage when they move through the intestine walls into vital organs and arteries.

Large Roundworms

Large Roundworms are found in the small intestine of the horse. The female lays eggs in the intestines which are excreted in manure. The eggs are consumed while the horse is grazing and hatch in the stomach and intestines.They migrate into the blood where they are transported to the liver and lungs, here they cause digestive upset and organ damage.


Pinworms mature in the large intestine and rectum of the horse. Eggs are consumed by horses from contaminated feed, water, bedding etc.This worm causes irritation so rubbing of the tail is a common symptom. Eggs can be transported onto any surface the horse touches as well as passed in manure.


Bots are the immature version of the bot fly. Female bot flies lay yellow eggs on the hairs of the horse that  hatch into first stage larvae after 2-5 days. Theses larvae migrate to the mouth and burrow into the soft tissue there.

Three weeks later, second stage larvae emerge and travel down the throat, attach to the stomach lining and develop into third stage larvae. They obstruct the flow of food and irritate the stomach lining and can cause stomach rupture. These larvae then pass through the horse and hatch from manure.


Tapeworms attach to the horses intestinal lining. They take nutrients from the horses diet and damage tissues. Eggs are passed with the manure of infected horses onto pasture and are ingested by forage mites.The tapeworm develops in the body of the mite and gets ingested by a grazing horse. It is released and 6-10 weeks later is an adult that attaches to the horse’s intestine and continues the cycle.

Signs and symptoms your horse has worms.

Signs and symptoms include: decreased weight, reduced appetite, colic, lethargy, susceptibility to infections, pneumonia, organ damage and death.

These are just the most common species of equine worms which you are likely to deal with. It is important to know these, as most dewormers only kill certain types of worms.