Internal Parasites in Horses

All horses and ponies are exposed to internal parasites or worms throughout their lives. More than 150 species of internal parasites can afflict horses, according to the University of Missouri Extension. Without treatment, worms can kill a horse. Many can be prevented with regular worming medications and by reducing parasites in the barn and pasture.

  1. Strongyles

    • Two types of stongyles affect horses: large strongyles and small strongyles. Large strongyles are the most harmful internal parasites for horses, according to North Dakota State University Extension Service. Large strongyles live in the horse's large bowel and abdominal organs. Small strongyles make their homes in the horse's large bowel. Strongyles worms deposit their eggs in the horse's digestive system so that they are excreted with the horse's feces.

    Ascarids or Roundworms

    • Ascarids and roundworms live in the lungs, liver and small intestines. They can grow up to 1 foot in length, according to "Horse Owner's Veterinary Handbook." Horses less than 2 years old are most vulnerable. These parasites don't feed on the horse's blood, but damage the horse by crawling around in the tissues. Adult worms lay eggs in the horse's intestines so that their eggs pass out with the horse's feces.

    Threadworms

    • These tiny, thin worms burrow through the horse's skin to get to the internal organs and eventually settle in the horse's small intestines. Foals are most at-risk of getting infected. One species can be transmitted to foals through the mare's milk up to 40 days after she has foaled.

    Stomach Bots

    • The bee-like bot fly lays eggs on a horse's legs, nose or throat, depending on the species. These eggs are nearly impossible to remove from a horse's hair. The eggs hatch in a few days and horses swallow the larvae through self-grooming, or the larvae burrow into the skin of the mouth and go down the horse's throat. The larvae then crawl to the stomach to feed on the stomach wall. Horse stomachs can rupture from bot infestations. Horses of all ages are prone to bot infections.

    Pinworms

    • Pinworms live most of their lives in a horse's colon, except for when they are eggs and newly hatched larvae. They deposit microscopic, sticky eggs in the horse's perineum, the skin around the vulva for mares and skin around the anus for geldings and stallions. The eggs cause intense itching in the areas around the rectum, vulva and tail. When the horse rubs its rump against a wall, tree trunk or fence, it leaves eggs behind. Pinworms can cause colic.

    Tapeworms

    • Tapeworms first inhabit forage mites that live in horse pastures and eat horse dung. Horses consume the mites and tapeworms while grazing. Adult tapeworms live where the large and small intestine meet. Their eggs are excreted in horse dung. Tapeworms can cause colic or fecal impaction, according to "Horse Owner's Veterinary Handbook."