Tail rubbing is a term that hardly does justice to the devastation a horse can do to their tail hair and skin when they start violently trying to scratch the area. Insect irritation is one cause and Culicoides can be hard to spot. They also don’t mind regular fly sprays so try essential oils of mints and thymol in an ointment or petroleum jelly base. Another possible cause is a dirty sheath or build up of sweat, cells and dirt in the crease between the thigh and the sheath or udder. If none of these apply, think pinworms.
Adult Oxyuris equi – Pinworms
Pinworms (Oxyuris equi) live in the terminal portions of the intestine. The females exit the anus at night to lay their eggs on the perianal skin. Eggs are encased in a sticky fluid which helps them adhere to the skin. It also causes irritation, making the horse rub its tail and in the process deposit eggs on any surface it rubs against. Eggs also eventually drop off.
Cases where O. equi was resistant to treatment with moxidectin or ivermectin have been reported so this is one instance where you probably should stay away from those drugs. They are still sensitive to fenbendazole and the pyrantel family of dewormers.
There are also anecdotal reports of inability to clear pinworm infections with any dewormer. However, pinworms have a 5 month life cycle and the dewormers only get adults and very late stage larvae so the drugs may well be effective but they are seeing the younger larvae mature to egg laying after the drug is gone.
Putting the dewormer dose into the rectum instead of orally has been tried. This is safe but hasn’t been effective. Consider trying a combination of Strongid paste (pyrantel pamoate) or fenbendazole plus daily pyrantel tartrate (Strongid T or Equi Aid CW) using the daily pyrantel for 3 to 4 months. The daily pyrantel will kill younger forms before they mature to egg laying stages.
Fecals are usually negative but you can check for eggs on the skin by applying a strip of clear tape to the skin and examining this under a microscope. If you get some microscope slides you can put the tape sticky side down on the slide and take it to your vet’s office.
Also start daily gentle washing of the underside of the tail, anal area and surrounding skin with Dawn dish soap in warm water to help remove eggs without irritating the skin. Do this in the morning since they lay eggs at night. Another washing in the evening won’t hurt. Try coating the skin with baby oil, Vaseline, Desitin, etc. at night to reduce egg packets sticking to and irritating the skin. Skip that step if you are going to check for eggs on the skin the next morning. The oil may also help you identify areas where the horse is rubbing and depositing eggs in the environment. Eggs may also come off on bedding or ground where the horse lies down to sleep so don’t feed anything off the ground.
Trying to decontaminate the premises may be futile! When egg packets dry and fall to the ground they can land anywhere. Eggs survive for about 8 to 10 weeks so you should be clear by the time you finish the treatment protocol.
Eleanor Kellon, VMD