Parasites: Internal Parasites of Horses

Internal parasites are small organisms that live a portion of their life cycle in a host animal — in this case, the horse.

They live in internal organs, body cavities, and tissues while gaining their nutritive source by feeding on the host animal.


Above is a picture of Bots in a horses stomach.


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Parasites: Ascarids

Ascarids in horses

Parascaris equorum, the horse roundworm. A large yellow to white parasite (females may be up to 15 inches long). Ascarids may appear in the feces of infected horses. Adult horses can develop resistance to this parasite; thus, roundworms primarily infect young horses less than 2 years of age.

Clinical signs include: unthriftiness, potbelly, rough hair coat, and slow growth. Some young horses develop nasal discharge accompanied by a cough as a result of larvae migration.

See Ascarids in

Parasites: Bots

Bot larvae in horses

Bots are a type of intestinal parasites that are larvae of the botfly, Gasterophilus. Female botflies lay their eggs by attaching them to the hairs of the horse.

Different species lay their eggs on different parts of the horse’s body.

  • Gasterophilus nasalis lay their eggs between the jaw bones.
  • Gasterophilus hemorrhoidalis lay their eggs on the short hairs of the lip.
  • Gasterophilus intestinalis lay their eggs on the forelimb and shoulder.

Botfly larvae probably cause minimal damage to …

Parasites: Large Strongyles


Strongyles are grouped as either large or small. The three primary species of large strongyles that infect the horse are:

  • Strongylus vulgaris
  • Strongylus endentatus
  • Strongylus equinus

The adult form of all strongyles (large or small) live in the large intestine. The larvae of large strongyles migrate through various parts of the body. Strongylus vulgaris, the bloodworm, will burrow into and migrate in the walls of the arteries that are the primary blood supplier to the small and large intestines. The …

Parasites: Small Strongyles

The small strongyle is considered to be the most common internal parasite of horses. The adult form of all strongyles (large or small) infects the cecum and ventral colon of the horse.

When large numbers of larvae invade the intestine, the horse may become clinically sick, showing signs of:

  • Fever
  • Diarrhea
  • Loss of appetite
  • Weight loss
  • Colic
  • Possibly death

With chronic infections, recurrent colic is a major indicator of the parasite. Small strongyles can be diagnosed on fecal flotation