Tail-rubbing is one of the top symptoms of pinworm (Oxyuris equi) infestation; it's because the female pinworm peeks out of the horse's rectum at nighttime, laying up to 60,000 microscopic eggs at a time in a gelatinous fluid that sticks to the skin around the horse's anus, causing intense itchiness. Over a period of days, the fluid dries and flakes off along with the eggs, leading to contamination of the barn or pasture environment and anything your horse has rubbed his tail against.

The good news is that pinworms are fairly innocuous compared to other worms such as strongyles; there is little proof of digestive upset or severe health issues caused by pinworms. A severe infection of late-stage O. equi larvae can result in mild colic signs, due to cecal and colonic mucosal inflammation, but a regular deworming program addressing other types of parasites will eliminate most, if not all, pinworms.
 
If you already have a deworming program in place and your horse is continuing to tail-rub, there are other causes to investigate, such as food allergies, mange, lice infestations, or hypersensitivity to insects. In some cases, tail rubbing can be a learned behavioral vice like cribbing. A little sleuthing, plus a visit from the veterinarian, could be in order. Check your horse's tail regularly, to be sure there aren't splinters or other objects embedded from the rubbing.
 
If one horse has pinworms, it's likely all horses in the environment might have them, especially if the itchy horse has been rubbing on communal feeding areas. Let us know if your horse is still rubbing his tail in spite of regular deworming.
 
A final note: Last month's WCNews guest expert, Dr. Martin K. Nielsen, DVM, PhD, had a response to a horse owner's inquiry about pinworms on TheHorse.com; Dr. Nielsen felt the owner's deworming program was reasonable and offered several methods for detecting pinworms, but advised her to also run fecal tests to gain information on overall parasite levels and drug efficacy for her current program. You can read his full response here:
Dr. Martin's response on pinworms at TheHorse.com