Pinworms occur in horses, sheep and goats and even humans but the equine pinworm fortunately does not transmit to other animals or humans as different species of worm are involved. And although many dog owners commonly believe pinworms may be causing their dogs to scoot on the ground, dogs do not get pinworms.
Pinworms are an unusual parasite as they are one of the only parasites in which the eggs cause disease. The adult female equine pinworm crawls out of the horse's rectum and lays eggs on the skin around the horse's tail. The eggs and surrounding fluid are irritating to that skin and causes the horse to itch, which is why horses with pinworms have hair loss around their tails. The eggs dry and then fall off into the environment, develop into larvae and are able to reinfect horses by being ingested. Diagnosis of pinworms is difficult as a routine fecal exam by your vet may or not find the parasite eggs because the eggs are not in the feces but are attached to the horse's skin. It is usually more effective to find the eggs by placing a piece of cellophane tape on the horse's skin around the tail and examining it under the microscope.
As far as treatment of pinworms, most of the routine deworming products are somewhat effective and I say somewhat as no dewormers are 100% effective against pinworms. Another helpful treatment is to bathe the horse's tail area every third day as this will remove any eggs before they become infectious and will decrease environmental contamination. So if you have a horse that is rubbing its tail area, pinworms could be involved. Check with your vet about treating these parasites and preventing infection.