Scientific name: Oxyuris equi
Common name: Pinworms

Description: Pinworms occur in the large intestine, colon and rectum. Females are 3 to 6 in. (7.6 to 15.2 cm) in length. Males are much shorter, reaching a maximum length of 0.5 in. (13 mm). They are brown or dark-gray in color and have a pointed tail. Pinworm eggs are a dull-yellowish color and are sometimes found in clusters near the anus of an infected horse or in manure. Horses become infected after ingesting eggs through contaminated food or water. Pinworms occur in horses of all ages but are more common in younger horses.

Life cycle: Pinworms have a direct life cycle that begins when a horse ingests infective eggs through food or water contaminated with manure. Pinworm larvae mature in the large intestine in about 5 months. Mature females migrate to the rectal area and either lay eggs internally or crawl out the anal opening and deposit their eggs on surrounding skin. Pinworm eggs can survive unhatched for several months. When they do hatch, eggs develop infective larvae in a few days.

Symptoms: The principal effect of pinworm infection is irritation to the horse’s anal area. The severe itching and discomfort that accompanies pinworms often cause the horse to persistently rub their rump or tail on a solid object such as a fence post or stall door, creating a condition known as “rat-tail.” Constant rubbing may cause tail hair to break off and create sores that can become infected. The presence of pinworms can create a dull appearance, a decreased appetite and a general lack of interest in regular activities.

Click here to access detailed information in PDF format.