Worms in Horses

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Worms in horses are common in horses of all ages, but some worms infect younger horses, particularly foal’s, more than others. While there are many different types of worms that infect horses of all ages, the foal will have its first encounter with the roundworm (Ascarid). Roundworms infect the foal’s body through the mouth. The foal swallows the parasite by ingesting either soil or water. Once inside the body, the roundworm will develop within 12 weeks. Once developed, the eggs hatch and the larvae begin their journey to the foal’s small intestine by traveling through the liver and lungs.

At this point, the roundworm is already dangerous. During this process, the foal is susceptible to pneumonia. Once the larvae reach the intestines, they will feed on the contents of the foal’s intestines, leeching vital nutrients daily. In addition to depleting the foal’s nutrient stores, the roundworm can also cause intestinal blockages and ruptures. Symptoms of roundworm in horses include: The roundworm can reach up to 20 inches in length if left untreated.

Roundworms in horses can be treated with two types of deworming chemicals called antiparasitics or anthelmintics. Some of these chemicals work by paralyzing the parasite, allowing the horse to expel them and other chemicals prevent nutrient consumption or limit reproductive capabilities in the parasites. This kills them or stops the life cycle. There are also a number of commercial antiparasitic compounds on the market. The more common classes include avermictins/milbimycins, benzimidazoles, and pyrimides. Anitparasitics can be purchased in paste, feed, additives, and gel form. When administered properly, all treatments are effective. Dosage is based on the horse’s weight.

Symptoms of roundworms include retarded growth, rough hair coat, pneumonia, cough/nasal discharge, pot belly, colic, and death.

Other types of worms common to horses include: threadworms, bloodworms, the botfly (infects the mouth), tapeworms, pinworms, and lungworms. Threadworms can be detected very easily. If the horse has a sudden bout with diarrhea, chances are he has threadworms. Bloodworms enter the horses body through ingested food. They travel via the blood vessels to the aorta. The aorta feeds the intestinal tract. They will mature in the intestinal tract and then make their way to the intestines to lay eggs. Bloodworms can cause inflammation, aneurysms, and colic.

Tapeworms are common during grazing season and infect the intestines and pinworms infect the colon of the horse. Lungworms infect the bronchi and the trachea and they cause constant coughing. The symptoms of worms in horses are varied. If your horse has any of the following symptoms, contact a vet.

Symptoms of Worms in Horses

Tail rubbing
Stomach ulcers
Weight loss
Cough/nasal discharge
Poor appetite
Rough coat
Recurrent colic


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Author: The Top Worm

1 thought on “Worms in Horses

  1. My two horses (one a 23 yr old retired thoroughbred and the other a 6 yr old warmblood) are paddocked together on my five acre property.Normally I am very diligent with worming, but this time have let it reach about 16 weeks in between. Both have recently been disinterested in their food and their manure has been more loose than normal. They do not appear to be in pain but do seem a bit lethargic. Could this be caused by worms and what type?

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