A pain in the posterior

How concerned should you be about pinworms and what else may cause similar signs? Kieran O'Brien MRCVS explains

- Kieran O'Brien

IT is becoming increasingly common for owners to contact their vets to report that they have seen one or more worms in their horse’s droppings. The worm is usually described as off-white, about 4cm long and one end is “long and pointy”. Sometimes, but not always, the horse has a history of tail rubbing. Very often the horse has been wormed only a few weeks previously and consequently the owner expresses surprise that the worms have now appeared.

These are almost always adult female pinworms (Oxyuris equi), so-called because of the long tapering egg-laying “tail” of female worms. These have migrated from the horse’s large intestine to the rectum to lay eggs, and while there have been inadvertently expelled when the horse passes faeces. They are sometimes alive and moving when first expelled, but very soon become immobile.

THE LIFE CYCLE

PINWORM eggs are oval and contain a coiled worm embryo that develops into a larva. The eggs are taken in by the horse during feeding. Once hatched in the horse’s intestine the larvae live in the horse’s right dorsal colon, where they attach to the intestinal wall. Although they do this, the current view is that they cause little pathology (disease or injury) even when present in large numbers.

Adult worms (of which females outnumber males by a factor of 10:1 and are much bigger) do not have mouthparts that permit attachment, so they cause no pathology.

To lay eggs, adult female worms migrate from the colon to the horse’s rectum and protrude from the anus by several centimetres, extending their elongated egg-laying “tail” downwards. They then secrete a yellow viscous “streak” of mucus containing between 8,000 and 60,000 pinworm eggs in tiny packets.