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Parasites - The Pinworm

The pinworm, Oxyuris equi

Larvae hatch in large intestine and after five months grow to adults to commence egg laying. Adult worms are found in the colon and caecum.
Female worms move down and deposit eggs under the tail of the horse causing irritation. Infective larvae develop inside the eggs in three to five days.


Eggs containing larvae drop off, contaminating the litter. They are easily destroyed at this stage by dryness or cold. They do not hatch unless eaten by a horse.
Horse eats contaminated litter and eggs are 
transported into the large intestine.

The pinworm, Oxyuris equi, This worm has a long development period in the horse before it shows up as a problem, but can be seen in any horse over 6 months of age.


The pinworm lives in the large intestine of the horse where the females may reach 10cm in length. They are white in colour. The egg-laying procedure of the female causes intense itching to the tail region of the horse, and the most obvious sign is constant rubbing of the tail which results in damage to hairs of the tail.


The life cycle of the pinworm is illustrated above.

The female pinworm undergoes a long period of development in the large intestine. When it is ready to lay its eggs, it lays its eggs around and under the tail and then dies. It is the sticky material that the worm uses to stick the eggs to the horse's skin that causes the intense irritation that is characteristic of infection by this worm.

When the horse rubs its rump, the eggs fall off, onto whatever foodstuff may be lying below. Here the eggs undergo a 3-5 day period of development. As with the roundworm egg, a larva develops, but stays protected in the egg shell.

When taken in by the horse, the larvae hatch out in the small intestine and find their way to the large intestine. Here they undergo a five month period of development, feeding on faecal material, to become egg-laying adults.

Because the life cycle relies on the direct contamination of feedstuffs or water, pinworm is mainly a disease of stabled or closely confined horses.


A single treatment with an oral wormer (paste) will normally clear a pinworm infection.


Because of the long development period for this worm in the horse's intestine, the previously recommended six-weekly treatment programme for bloodworm and redworm will control this parasite.

Management control procedures should be directed at placing feed bins, waterers and haybags/nets so as to avoid their contamination when infected horses rub their rumps.

Treatment Chart Click here


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