9 Most Common Internal Horse Parasites

and pasturepoor environments. Horses inadvertently ingest a mite that carries the infective stage of the parasite. The parasite then matures into a worm and attaches to areas of the digestive system specifically the end of the small intestines called the ileum and the start of the large intestines called the cecum4 . These attachment sites are clinically significant because a large tapeworm burden in this area can lead to gastrointestinal motility problems and lifethreatening forms of colic. Once attached the tapeworm sheds eggfilled segments to be excreted in manure.

Tapeworm burden cannot be diagnosed on fecal egg count but the worm is very susceptible to a dewormer called praziquantel which should be given at least once yearly in the late fall or early winter.

Ascarids
Ascarids are a major cause of colic in foals and weanlings5 but natural immunity is developed as a horse ages past about a year old. Ascarid eggs are remarkably hardy and remain viable in the environment. Once ingested by the foal they hatch and release larvae. These larvae migrate through

the liver lungs then trachea where they are coughed up and reingested to mature in the small intestines before releasing eggs. Because of this circuitous lifecycle foals most at risk of small intestinal obstruction by large amounts of maturing ascarids are usually around 5 months old. This obstruction often manifests as abdominal distension and colic behavior such as pawing and rolling.
A dewormer that causes the worms to die at once can lead to obstruction as well so a careful deworming program specific to foals should be developed.
Pinworms