The entire life cycle, from egg to adult, takes place in the gastrointestinal tract of a single horse. The life cycle begins with eggs being ingested (the eggs hatch in the first part of the small intestine). The pinworm larvae grow and migrate through the small intestine towards the colon. During this migration they become adult females and males.
The male and female pinworms mate in the intestines. Soon after, the male pinworms usually die and are passed out with faeces. The female pinworms settle in the large intestines where they attach themselves to the inner walls and ingest colonic contents. Soon, the female pinworms migrate towards the rectum and emerge from the anus. It is thought they do this to obtain the oxygen necessary for the maturation of the eggs.
While moving on the skin near the anus, the female pinworm deposits its eggs and dies. The eggs in the skin around the horse’s anus are often rubbed off onto the ground where they are then eaten by a horse and the life cycle repeats. Because of this bizarre life-cycle, pinworm eggs are not often seen within the routine fecal sample although a simple ‘sticky tape’ test can be done free of charge if returned with your faecal sample. Please contact for more details.