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Parasite or worm infections in horses

Internal equine parasites, commonly known as “worms,” are very common, and all horses are at risk of infection. Horses are affected by many different species of worms, and their life cycle and the symptoms they cause may vary. The life cycle of most parasites incorporates stages on the pasture (eggs and larvae), and horses get infected by ingesting the eggs or larvae while grazing. After ingestion, the eggs or larvae will develop into adults in the digestive tract or other organs of the horse and eventually produce eggs that can be shed in the environment through the feces.

Not all parasite or worm infections in horses cause clinical signs; many horses with low parasite burden will not show any signs of sickness, but some horses will get symptoms that can vary from mild (weight loss; mild, intermittent colic) to extremely severe (surgical colic, death).

What are the most common equine parasites?

Small strongyles

Small strongyles, also called “small redworms,” include approximately 50 different species and are the most common worms in horses and the main concern in parasite control. They have a worldwide distribution, and most horses are infected with small strongyles or have been infected at some point in their life. The adult worms reside in the lower intestines of the horse and consistently produce eggs that are excreted onto the pasture. This is how the worms are transmitted to other horses. Adult worms measure 5–25 mm and are very thin. It is rare to see them on horse feces because of their small size. Parasite control programs in adult horses are mainly developed around the treatment and prevention of small strongyles.

Large strongyles

Large strongyles are the most harmful worms for horses. They can cause serious symptoms, and severe infection can be life-threatening. Their life cycle includes a larval or immature stage during which they migrate in the blood vessels and can cause anemia, blood clots, and even death. The most common worm in this group, called Strongylus vulgaris, measures around 2 cm in the adult form. Fortunately, most dewormers are effective against large strongyles, and these parasites have not yet developed resistance to the drugs in dewormers. Thus, regular deworming has made large strongyles much less common.


Ascarids are a family of parasites also known as “roundworms.” The roundworm in horses is called Parascaris equorum. The adult worm is particularly large, measuring up to 30 cm long and 6 mm wide. They are especially important to control in young foals and can cause severe health problems, including impactions (blockages of the intestine) that often require surgery. The Parascaris equorum life cycle includes a larval or immature stage during which the parasite migrates to the lungs and can cause respiratory symptoms like coughing. The eggs of this parasite are quite resistant and can survive for months to years in the environment.


Tapeworms are flat-bodied, segmented worms, and the most common species in horses is Anoplocephala perfoliata. The adult worms are mainly located in the upper intestines, and their presence may cause severe digestive tract disorders resulting in colic, which can require surgery. Horses become infected by ingesting forage mites while grazing. Forage mites actually serve as an intermediary host for tapeworm larvae. The adult tapeworm measures 3–4 cm long and is 8–14 mm wide.


Bot flies, Gasterophilus, live for only a few weeks, during which they reproduce and deposit eggs directly onto horses’ hair coats, often on the legs. Subsequently, horses ingest the eggs while grooming, and the eggs develop into larvae (bots) in the mouth. They then migrate to the stomach, where they attach to the lining and can remain for months. Bots measure around 2 cm; they have a round body, and their mouth has hooks that allow attachment to the stomach lining. Severe infestation can cause symptoms like weight loss and colic.

What are the symptoms of a parasite or worm infection in horses?

Parasite infection can cause a variety of symptoms depending on the species involved and the severity of the infection.

Learn more about the symptoms of a parasite or worm infection in horses.

How do horses get infected with parasites or worms?

Horses can get infected by parasites in different ways, depending on the type of worm involved and its different life stages. Overall, the main route of infection is through ingestion of infective eggs or larvae while grazing on the pasture.

Learn more about strategies to limit the exposure of horses to parasites and prevent infection.

Why should horses be de-wormed?

Most worms can cause health issues if left untreated. Symptoms vary from mild (weight loss, intermittent mild colic) to extremely severe (surgical colic, death).

While it is virtually impossible to get rid of all worms in all horses, the population of parasites and the shedding of eggs should be controlled and limited to a point where horses are unlikely to develop symptoms.

Learn more about current recommendations for parasite control programs in horses, including targeted deworming.