Itchy bum?

There are a few causes of bottom scratching – pinworm and insect bite hypersensitivity (Queensland/sweet itch) are two of the highest on that list. Here I’ll discuss pinworm.

Cases of pinworm (Oxyuris equi) have been on the increase in the last couple of years. There are a few things regarding the lifecycle of the pinworm that make them difficult to treat.

The pinworm is different to the majority of other worms found in the horse’s gastrointestinal system, in a number of ways:

  • The life cycle is direct. This means there is no intermediate host and no migration through any other organ in the body other than the gut.
  • The life cycle is long. Worms can take up to 5 months to mature (in most other worms this takes only 3 weeks).
  • The adult worms live in the rectum, in contrast to other worms which spend their time in the intestines. Immature stages of the worm are less sensitive to wormers so may survive post worming. Most infestations take about 12 months to clear up.
  • Eggs are laid on the skin surrounding the horse’s bottom and not passed in the faeces like other worms, so will not show in a worm egg count.

Fortunately pinworms do not cause damage in the horse’s digestive system like other worms.  However, pinworms can cause severe irritation, sometimes so severe that horses will rub themselves raw around their tail head; this can lead to skin infections and further problems.



Adult pinworm protruding from the anus of a horse and laying a sticky mass of eggs

Treatment of pinworm needs to be tailored to its lengthy lifecycle. Contrary to some reports, oral wormers are effective against pinworm. However, there is no evidence that worming enemas work to treat pinworm.

  1. Treat regularly with an oral wormer that pinworm are sensitive to. Pyrantel based wormers such as Strongid-P or Strategy-T used at a double dose are most effective; this should be repeated every 6-8 weeks.
  2. Clean your horse’s bottom! The female worms crawl to the anus and lay their eggs during the night, this means the best time to wash your horse’s bottom will be first thing in the morning, twice daily if possible.
  3. Apply Vaseline to the skin surrounding your horse’s bottom after cleaning; this will help to reduce the number of eggs that stick to the skin.
  4. Be patient! The life cycle of the pinworm is long, immature stages of the worm are less sensitive to wormers so may survive after worming.  Most infestations take about 12 months to clear up.


Normal worming guidelines should be followed, such as weighing your horse to ensure the correct dosage is administered. Under-dosing your horse can lead to the worm becoming resistant to the wormer.  Good pasture management such as regular poo picking is also essential. Electrifying fences that your horse is rubbing on will also discourage this behaviour. Clean any favourite rubbing spots of eggs as this can be a source of reinfestation.

The take home message is be patient, persevere and don’t despair!

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