Written by Dr. Kris Hiney
This month we will discuss other aspects of horse management that directly affect the nutritional status of your horse. While most horse owners are familiar with deworming their horses regularly, current recommendations from many equine practitioners are to be much more strategic with our deworming. There is a growing concern that parasite populations are developing resistance to almost all types of anthelmentics (drugs used to eliminate internal parasites). As no new anthelmentics will soon be offered to the public, this could represent a real risk to the health of our horses. In order to understand these issues, we will begin with a review of the major parasite classes in horses.
While there are many types of worms which infest horses, we will address the major classes that represent the most health risk to your horse; ascarids, strongyles, tapeworms, bots and pinworms. Ascarids, or Parascaris equorum, are a type of round worm which grow to a substantial size of 8-15 inches within the intestine. They are yellowish in color and may be occasionally seen in the feces. Despite their robust size, much of the damage created by these parasites involves their life cycle and migratory journey through the horse. Adult females pass eggs into the horse’s feces, where they spend 1-2 weeks in the environment before they are capable of infecting a new host. Horses ingest the infective eggs by grazing or eating in contaminated areas. Once inside, the larvae burrow through the intestinal lining and enter the bloodstream, where they travel to the liver. They then travel to the heart and then the lungs. Ultimately they enter the alveoli of the lungs where the horse coughs them into the oral cavity and then are swallowed back down into the stomach and intestines. The entire life cycle of the ascarid takes about three months and the journey these parasites take can cause significant damage and scarring of the tissues. A heavy parasite load of adult worms can even lead to blockage of intestines. Young horses are the most susceptible group of horses to acquire ascarids, as well as weak, or malnourished horses. Coughing and nasal discharge in young horses may actually be a sign of ascarid infection. Older horses eventually develop an immunity to these parasites, so ascarids are primarily an issue with horses under two years of age.
Strongyles exist as both large and small strongyles, with many sub-species. The three main species of the large strongyles are Strongylus vulgaris, Strongylus edentus, and Strongylus equinus. Small strongyles actually have about 50 different species. Strongyles are also the most damaging of the parasites that horses will encounter. Similar to the ascarid, the females lay eggs which are shed in the feces. Unlike ascarids, they hatch into infective larvae that the horse ingests. The larvae molts three times before it is ready to infect the horse. The larvae actually crawl up the blades of grass in the dew. The larvae can crawl up or down multiple times waiting for a host, or even burrow into the ground when the weather isn’t favorable. Unfortunately for the horse owner, these parasites are extremely hardy and can persist through the winter.
The characteristics of the large and small strogyles life cycle make them particularly damaging. Large stronglye’s life cycle involves two stages where they migrate through the arterioles and arteries which supply blood to the intestine. Unfortunately, wherever these larvae burrow through the intestinal wall to migrate, all of them will return to one single location, the cranial mesenteric artery. Here they congregate and can cause immense damage. They can cause hemorrhaging, blood clots, or even rupture. The blood clots themselves can break free and travel further down through the blood supply to where they block blood flow and create a thromboembolic colic and even death. Oddly, enough lameness can also result from blood clots traveling to the legs as well.
Small strongyles have an additional strategy to help them survive. As they pass through the horse’s intestinal wall, the horse’s immune system is also trying to wage war against the larvae. However, the larvae are too big and travel too fast to be eliminated. The final migration of the larvae and complete maturation is actually held in check by the presence of adult strongyles in the lumen of the intestine. Essentially the adults provide feedback to the larvae that there is no room at the inn. When the larvae get that message and slow their migration, they become encysted within the intestinal wall by the immune cells. Here they can lie in wait for several years to take their turn at being the adult worms in the intestine. The horrifying reality is that when the adults die of either natural causes or by our purge deworming of the horse, the encysted larvae “wake up” and emerge to replace the newly vacated intestine. Within 6-8 weeks they will have matured and begin laying their own eggs to begin the cycle anew. Again, it is the pattern of traveling through the tissue that can cause a great deal of damage to the horse.
Relative to those bad boys, the rest of the worms which typically invade horses are mild in nature. The other major parasite classes which trouble horse owners are pinworms, stomach bots and tapeworms. Pinworms have a very simple life style compared to ascarids and strongyles. Adult females have a rather interesting feature, however. Not content to just shed her eggs into the feces, she actually deposits the eggs on the horse’s anus. This causes irritation to the horse who then scratches on anything available in the environment, effectively dispersing them. The horse then incidentally ingests the eggs, which hatch in the intestine where the larvae mature. Thankfully, these worms do little damage to the horse because their life cycle does not involve migrating through sensitive tissues. However, they can cause great irritation to the horse and robust itching of the tail head.
Tapeworms in horses can also cause reduced nutrition and potential blockages due to the preferred location in the horse’s gastrointestinal tract. The main species of tapeworm which inhabits the horse fixes at the ileocecal junction, or where the terminus of the small intestine joins into the cecum. A heavy parasite load can result in blockages, thickening of the ileocecal valve or even intusussecption, when the intestine rolls over itself due to regular peristaltic action. The tapeworm also has a separate host for part of its life cycle. While the adult parasite resides in the horse, the eggs of the tapeworm are actually ingested by a type of mite, which the horse then later ingests while grazing. There does not appear to be any age related immunity to tapeworms, as they are found in all ages of horses.
Finally, stomach bots are frequently seen in horses as well. The stomach bot, or Gasterophilus, also has subspecies, which include the horse bot fly, the throat latch bot, and the nose bot fly. The adult fly form can actually fly for several miles in search of a suitable subject on which to lay its eggs. The female hovers near the horse and deposits single eggs on one hair at a time. The eggs actually hatch into larvae within 7-10 days of being deposited. They then wait to emerge until the horse licks or scratches at the eggs. The larvae then enter the mouth and bury themselves in the gums, tongue or lining of the mouth where they hang out for a month. As they mature to later stages of larvae, they move into the stomach where they attach to the non-glandular or upper part of the stomach. The larvae live in the horse’s stomach for 9-12 months, before they and pass out into the feces. This typically occurs in late winter to early spring. There the larvae pupate and remain in the feces for several months. The flies then emerge in late summer or early fall, find mates and renew their life cycle. The damage the bots cause to the horse can occur in the mouth where they cause great irritation and even form pus pockets or cause the teeth to loosen. Large numbers of larvae in the stomach can cause blockages and erosion of the stomach lining. They, like all internal parasites, can result in reduced nutrition being delivered to the horse. An important heads up to horsemen: when handling horses with bot fly eggs on their hair, use caution. While rare, the larvae are capable of burrowing into human skin, and if one rubs their eye after handling bot eggs, they larvae can actually invade the eye. I’m quite sure the last thing anyone wants is a bot larvae living in your eye!
Next month we will use what we know about these parasites to develop management strategies to reduce their ability to infect our horses. After that, we will discuss strategic methods in using anthelmentics in order to reduce our reliance on medications and reduce the spread of resistance in parasites which invade our horses.