Pinworms (Oxyuris equi) are one cause of the "itchy-tail syndrome." The adult pinworm lives in the large intestine, but doesn't cause much disturbance there. Pinworms deposit their eggs around the anus, causing irritation, tail-rubbing, and hair loss. Eggs can fall to the ground wherever the horse rubs its rear end. Horses are infected with pinworms when they ingest larvae that hatch from eggs several days later. Your veterinarian will do a "scotch-tape prep" to diagnose pinworms, since there are other causes of tail-rubbing. A piece of scotch tape is pressed around the horse's anus, then pressed onto a microscope slide so the tiny pinworms eggs can be examined. Pinworms are easily treated with many dewormers, including the benzimidazole, ivermectin, and the organophosphates.
Anaplocephala perfoliata and Anaplocephala magna, the equine tapeworms, are less commonly diagnosed than the other parasites. Tapeworms have an interesting life cycle, which involves a stage inside a mite. The mite eats the worm egg in the manure, the horse eats the mite, and the immature tapeworm then emerges and develops into an adult. Adult tapeworms live in and near the horse's cecum, a large dead-end extension between the small and large intestine. Large numbers of tapeworm may cause blockage of the opening into the cecum, ulcers, or even rupture of the intestine. Tapeworms can be difficult to diagnose. A standard fecal egg count may or may not reveal the eggs, and the worms are seldom seen in the manure. Many of the dewormers in use today, including ivermectin, are not effective against tapeworms. Your veterinarian will help you choose the medication to eradicate this parasite.
There are many other worms that infest horse, but they probably won't be a problem for you.
Habronema larvae live in the stomach wall, where they form tumor like growths. These worms are also the cause of summer sores on the horse's outer body. While the stomach parasite is difficult to diagnose, it is killed by ivermectin; so you are probably eliminating any problem by using this drug in your program.
Dictyocaulus is a lung worm of horses and donkeys. Infection with lung worms will cause a chronic cough. Immature lung worm larvae are coughed up and swallowed, so they may be seen by microscopic examination of your horse's manure. Ivermectin kills these parasites.
Fasciola Hepatica, the liver fluke, is a parasite of cattle and sheep that occasionally infests the horse. Signs include weight loss, colic, and diarrhea. Fluke eggs may be found only with a specific type of fecal examination. Your vet will prescribe specific treatment for flukes since they are not killed with traditional deworming medications.
Thelazia or eye worms are a problem in a few areas. They cause conjunctivitis and irritation of the cornea, which can lead to blindness if untreated. Your veterinarian removes the worms from the conjunctival sac with forceps.