Know Your Enemy


    Intestinal parasites are a common problem in horses, but do you know what types of parasites commonly infect horses, what problems they cause, and how you treat them? Read on to find out more.

    Oxyuris equi is the equine pinworm. As adults, this parasite is long and thin and lives primarily in the cecum and large intestine of the horse. When ready to lay eggs, the female pinworm travels to the rectum and deposits large amounts of eggs in a sticky film around the horse’s anus. This film causes skin irritation, and the horse will scratch his rear end on fixtures such as fence posts and stall doors. The classic sign of pinworm infestation is a horse that repeatedly rubs his tail on the fence and has broken hairs at the base of the tail. These eggs are then deposited on fences and on the ground, where other horses ingest them and the life cycle beings again.

    Pinworms cannot be diagnosed by a fecal exam, like most other intestinal parasites. Instead, your veterinarian will perform a “scotch tape test” and apply some sticky tape to the horse’s back end. Under a microscope, the tape will show pinworm eggs. Fortunately, pinworms are easily treated with most of the major classes of dewormers, including ivermectin, moxidectin, fenbendazole, and pyrantel.

    There are two types of strongyles that infect horses: small and large strongyles. Of the two, large strongyles are the most dangerous. The most common large strongyle in North American horses is Strongylus vulgaris. This parasite is transmitted in horse feces where the eggs are shed and molt into third-stage larvae. When these larvae (called L3) are ingested by a grazing horse, they travel to the cecum and large intestine and then molt again to a fourth larval stage (L4). These L4 strongyles leave the intestinal tract by way of small arteries and migrate to the cranial mesenteric artery, a large and important blood vessel that feeds portions of the digestive tract. The L4 stay in this blood vessel for months where they continue to grow and cause inflammation and damage to the blood vessel. Then they are carried by blood flow back to the intestine, where they penetrate the intestinal wall and complete a last molt to a fifth larval stage (L5). At this point, these worms are mature and begin to reproduce. The eggs are passed into the manure to begin the cycle again.

    The migration of S. vulgaris through the blood vessels is what makes this parasite so dangerous; this migration can cause a condition called verminous arteritis, which is an inflammation of the arteries. In cases of severe infestation, the cranial mesenteric artery can become blocked by parasites and inflammation, cutting off blood supply to the intestine, resulting in colic. Even though S. vulgaris is a serious threat to horse health, it is susceptible to many different classes of dewormers. Ivermectin, moxidectin, fenbendazole, and pyrantel all treat S. vulgaris.

    Small strongyles (also called cyathostomes) aren’t as pathogenic as large strongyles, but they cause problems in a different way. These intestinal parasites have the ability to encyst themselves in the lining of the intestines, thereby protecting themselves from certain deworming medications. They do this during the winter, undergoing what is called “hypobiosis,” which is like hibernation. These worms will re-emerge from their protective cysts in the spring to release their eggs. This re-emergence can cause serious diarrhea in foals. Unlike large strongyles, small strongyles remain in the large intestine for their entire life cycle.

    Unfortunately, dewormer resistance is being seen in small strongyles. Fenbendazole does not work as well against this type of parasite as it did in the past. Ivermectin, moxidectin and pyrantel are likely to work better.

    Also known as an ascarid, the equine roundworm is scientifically named Parascaris equorum. This parasite can cause disease mainly in younger animals, causing rough hair coats, a pot-bellied appearance, unthriftiness and diarrhea. When the larvae of this worm are ingested in the field, they penetrate the wall of the intestine and migrate throughout the body, sometimes through the liver, lungs and trachea. After undergoing several moltings, these worms return to the intestine to lay eggs that are passed in the feces to re-infect the environment. Roundworm eggs are notoriously hardy and can remain infective in the soil for years. This parasite can grow very large in the intestinal tracts of young animals, and heavy infestations can even cause intestinal obstruction. Sometimes, a veterinarian will treat a young horse with mineral oil at the time of deworming to help prevent an intestinal blockage. Piperazine, fenbendazole, ivermectin, moxidectin and pyrantel are effective against roundworms.

    While the bot fly is not a true intestinal parasite, it does infect the gastrointestinal tract of horses during certain stages of its life cycle. Bot flies lay their small, yellowish eggs on the horse’s coat along his legs and belly. When the horse scratches with his teeth, he ingests these eggs and they pass to the stomach where they latch on to the lining and hatch into larvae. When further developed, the bot larvae pass into the feces and complete their life cycle by developing into flies. Ivermectin is the treatment of choice to control bots. Bot fly eggs can be easily identified on a horse’s coat and when seen, should be scraped off with a bot fly knife to decrease chance of ingestion.

    Horses become infected with tapeworms while grazing, as tapeworm larvae are carried in tiny mites that live in grass. When a horse ingests these mites, the larvae travel to the intestine and grow into segmented worms. Usually, horses aren’t infected with high enough numbers of tapeworms to become clinically ill, but occasionally the tapeworms will cause minor irritation of the lining of the intestine, resulting in mild diarrhea. Praziquantel is the dewormer of choice for this parasites. This usually comes in combination with another dewormer, such as ivermectin, so you can treat multiple types of parasites with one product. Pyrantel pamoate will also treat tapeworms.

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