Although pinworms live in the intestinal tract, that is not where they cause problems – female pinworms actually crawl out of the horse’s rectum and deposit a very sticky substance on the skin around the anus. This sticky substance contains the pinworm eggs. This sticky substance, not the eggs, causes the problem, because it can be very irritating.
Affected horses typically rub their tails against objects because of the itching and irritation of the anus caused by the sticky substance. This can lead to hair loss over the tailhead and a “rat-tailed” appearance.
Most dewormers are effective against adult pinworms, including ivermectin, moxidectin, fenbendazole and pyrantel pamoate, although resistance to ivermectin has been reported in some areas.
There is very little that can be done to treat the irritation of the horse’s
tailhead directly, but it will resolve on its own once the pinworm infection
has been treated.
Watch my video about the handling of the horse while inserting the rectal syringe.