Internal Equine Parasites

Internal parasites, or “worms” as they are often called, can pose a significant risk to your horse’s health. Not only do they rob your horse of vital nutrition, but they are a major cause of equine colic.

Identification Leads to Prevention


Pinworm larvae.

Pinworm eggs.

Pinworm eggs.

Large and small strongyles.

Large and small strongyles.

Bot fly larvae in the stomachs of two horses.

Bot fly larvae in the stomachs of two horses.



Ascarid egg.

Ascarid egg.

A large tapeworm.


A Fecal Exam is the best way to identify most internal parasitic infections. We do fecal exams in the clinic, and usually have the results to you within 48 hours.

Pinworms often are more easily found via the “Scotch Tape” test. Tapeworm eggs are usually excreted in the tapeworm segments, so they’re often not found on routine fecal exams. The best way to test for Tapeworms is through a blood test for tapeworm antigen.

Based on the results of the fecal exam, we can customize a deworming program for your horse.

Types of Parasites

Large Strongyle larvae can live in the mesenteric artery, and can damage or block this artery, cutting off the blood supply to the intestinal tract. Small strongyle larvae encyst in the intestinal walls. When they hatch out in the spring, they cause severe inflammation of the intestinal walls and thus colic.

Bot fly larvae migrate thru the tongue and esophagus after they are ingested, and attach themselves to the lining of the stomach, where they stay for up to 11 months. In large numbers, they contribute to gastric (stomach) ulcers and occasionally rupture of the stomach.

Ascarids, which are quite common in foals, can grow up to 12 inches in length within 4 weeks and block the small intestines.

Pinworms are a more common parasite of stabled horses. They live in the rectum; the female crawls out of the anus, irritating the area and laying eggs around the perineum.

Tapeworms are an emerging parasite threat. They can grow many feet long and are a cause of colic by intussusceptions.

LYME Disease

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Evergreen Equine
of Vermont
Dr. Heather Hoyns

Mailing Address:
P.O. Box 126
Reading, Vermont 05062

(802) 484-9100
(802) 484-9104

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