Pinworms (Oxyuris equi) are the number one parasite that vets get questions about but they are usually harmless. They can be a nuisance if they cause the horse to itch or rub and they may cause alarm if they are seen in droppings.
Infective larvae are ingested from the environment and travel to the large intestine where they remain. The female worms travel to the rectum, where they deposit their eggs around the anus.
By placing sticky tape around the anus and then placing the tape under a microscope, pinworm eggs can be identified. Pinworm cannot be detected by egg counts in faeces (FECs).
Pinworm eggs are laid in a sticky substance that causes some horses to itch the dock and under the tail. Some horses can occasionally become distressed, while others show no sign of infection. If your horse is itching, it is important you work with your vet to rule out other causes of tail-rubbing prior to worming your horse (Figure 1).
Once pinworm infection has been confirmed by your veterinarian, treatment is two-fold and involves the judicious use of wormers and cleaning of the environment.
Benzimidazoles appear to be the most effective wormers for treating pinworms (eg. Strategy T) but no wormers are consistently effective.
You should not apply wormers around or inside the rectum as this will not be effective.
Clean and disinfect any area your horse has come into contact with (stables, fences, feed-bins etc).
Wash rugs regularly.
Pick up manure as often as you can, preferably twice daily.
Wash under and around the base of the tail daily or twice daily. Adult female worms are most active at night so cleaning first thing in the morning is recommended.
Figure 1. Tail-rubbing may be due to a number of underlying reasons. It is important to work with your vet to obtain a diagnosis prior to worming your horse.