Pinworms – an unwanted irritation

Pinworms (Oxyuris equi) are small, white-gray, roundworms that live in the large intestine of the horse, attached to the intestinal wall, where they feed off the mucosal lining.
The male worms are small (less than 1 cm long), but the female worms can grow up to about 15 cm in length and have a long tail that tapers to a point, hence the name pinworm.

Pinworm eggs are ingested by the horse, which are usually found on pasture (from faeces), contaminated water, feed, fences and walls.
The eggs hatch releasing L3 stage larvae into the small intestines, these then travel to the large intestine and develop into L4 stage larvae which then emerge and mature into adult pinworms. This lifecycle takes approximately 5 months. The female adults then migrate from the large intestine to the anus where they partially emerge to lay their sticky eggs on the skin around the anus.
The eggs hatch outside of the horse’s body and become infective within a few days, but can survive for several months unhatched!

If your horse has pinworms they will cause irritation around the anus, so you will probably notice that your horse is particularly itchy around the tail. Your horse may also try to bite and lick the hindquarters. In general, your horse will show very similar signs to a horse suffering from sweet itch.
You may also notice the eggs, which will appear as a yellow, gelatinous mass around the anus.
Your vet can perform an adhesive test where they will apply a transparent piece of adhesive tape over the anus and then examine it under the microscope to identify pinworm eggs.

Pinworms are not considered harmful to horses, they are however a nuisance and an irritant.
Be warned that faecal egg count monitoring will not show up a pinworm infestation because they do not lay their eggs internally.

Pinworms are susceptible to the same drugs used during routine worming, such as moxidectin, ivermectin, fenbendazole and pyrantel. So, by worming your horse on a regular basis, your horse should be adequately covered.
You should also wash your horse’s perineal area (anus) on a regular basis to ensure the removal of any eggs that may have been deposited there by existing worms.

You should pay particular attention to stable hygiene, including regularly and thoroughly cleaning out feed and water buckets, as well as haynets and rugs; a heavy duty disinfectant can be used for this. It is also wise to avoid using grooming equipment on more than one horse to avoid cross-contamination.
Regular worming with an appropriate anthelmintic (wormer) will also prevent your horse from getting an infestation.