Page 69 - The Veterinary Care of the Horse

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Figure 2.19 Pinworm

Life cycle The mature adult worms live in the right dorsal colon of the horse. The females

migrate and protrude through the anus to deposit eggs in a sticky fluid on the skin surface. As
it dries the fluid becomes irritating to the skin. The eggs become infective in 3–5 days and
drop off into the environment or adhere to surfaces such as stable doors, walls, mangers,

gates and fences on which the horse may rub. If ingested by a horse they develop into larvae
in the small intestine and then move to the large intestine where they go through several more

larval stages to develop into adult worms. The life cycle takes 4–5 months but recent studies
indicate that this may occur in 3 months in young foals. Pinworm can also be passed between

horses by mutual grooming and the use of contaminated grooming kits and feed buckets.
Clinical signs The main clinical sign is skin irritation although it is possible for a horse to

have pinworm and show no clinical signs. Horses often swish their tails and can inflict mild
to severe self-trauma by rubbing their tails and buttocks on fence posts and stable walls

(Figure 2.20). Heavily infected animals may have a poor coat.