Intestinal Parasites of Horses – Deworming

Various parasites infect the gastrointestinal tracts of horses. Despite the widespread availability and use of anthelmintic drugs, “worms” continue to compromise the health of many horses in all parts of the country. It appears that drug-resistant parasites have emerged over time, most likely as a consequence of the misuse of deworming agents. Intestinal parasites may be responsible for sub-acute to life threatening disease in horses and should not be underestimated as a significant threat to your horse’s health.

Of common farm livestock, horses may possess the largest average worm burdens, or the number of parasites that infect the host at one time. Compared to sheep and cattle, horses can sustain two to three times as many live worms in their intestinal tract – two thousand times as many as would parasitize a dog or a cat. This equates to approximately 100,000 live worms per infected horse.

Intestinal parasites spend their “adult” lives inside of the host animal, feeding and reproducing at incredible rates. It is during this stage that they cause disease. However, they begin life outside of the animal as eggs and larvae shed in feces. In fact, worms that infect horses cannot complete their life-cycle entirely within the intestinal tract. Microscopic eggs and larvae that lay waiting outside of the host are ingested by horses during grazing. It is by this design that these parasites are easily able to infect other animals that graze in common pastures. It is also the reason that despite treatment, horses are continually re-infected. With some female worms able to lay up to 200,000 eggs per day, it may be impossible to eradicate all sources of infection.

It is very important to follow a veterinarian’s recommendations for deworming regimens. Under-dosing, sporadic dosing, inadequate period of treatment, and use of no longer effective medications contributes to increasing drug-resistance in parasites. Every horse on the farm should have a strictly followed deworming schedule as established by the veterinarian.

While many parasites are capable of infecting horses, the most common types of intestinal parasites responsible for causing disease are bots, large strongyles, small strongyles, ascarids, pinworms, tapeworms, and threadworms. Below is a description of these parasites’ life-cycles, routes of infection, and mechanisms of disease. Deworming drugs and protocols are ever-changing in a response to resistance, so they are not described here. Consult the veterinarian for up-to-date strategies that are most effective in your geographical region and for your herd’s exposure risk.


Bots are the larvae of botflies. While in the adult, flying stage of their life cycle, these arthropods do not bite horses or feed at all. Bots only develop into botflies for the purpose of finding a mate and reproducing. They lay their eggs in the hairs of the horse’s forelimbs, face, and neck, appearing as tiny pale yellow flecks. One female fly can lay as many as a thousand eggs during its short adulthood, which are accidentally ingested when the horse grooms itself. The larvae hatch immediately and burrow into the mucosal layers of the mouth tissues, where they remain for 3 to 4 weeks. After this period of time, they migrate down through the esophagus, finally attaching and feeding upon the stomach wall with tiny but very sharp mouth parts. Their attack on the delicate tissues causes significant inflammation, ulceration, and bleeding, and their numbers can be so great as to interfere with normal digestion or even obstruct the opening into the small intestine. The larvae will continue to mature for 8 to 10 months, then detach from the stomach wall in order to pass through the intestines in fecal material unharmed. Once outside of their host, larvae burrow into the ground for a period of time before emerging as adult botflies seeking a host to rear their offspring for them.

Large Stongyles:

Large strongyles are parasitic nematodes. Commonly called bloodworms, red worms, and palisade worms, the three species that most often infect equines are Stronglyus vulgaris, S. equines, and S. edentatus. They are spread effectively by fecal to oral transmission. When eggs of the parasites are shed in an infected horse’s stool, they hatch into infective larvae which are ingested during grazing. The infective stage larvae are protected from stomach acid and digestion by resistant outer capsules. Once in the small intestine, the larvae molt into burrowing juvenile worms that migrate through various tissues of the horse’s body before settling in the large intestine at full maturity. The development into adulthood takes at least 6 months before the worms are able to reproduce and deliver eggs that are detectable under the microscope during a fecal exam. The migrating larvae can cause arteritis (inflammatory disease of the arteries) and lesions throughout the liver, pancreas, and vasculature of the kidneys. As adults, the worms are voracious blood-suckers equipped with sharp, pincer-like mouthparts. In large numbers, adult large strongyles can cause anemia, weight-loss, weakness, and diarrhea that leads to dehydration, protein loss, and electrolyte deficits. The migrating larvae can significantly decrease blood supply to the horse’s intestines which may result in severe colic, perforation of the gut wall, and death to the animal.

Small Strongyles:

Small Strongyles are also parasitic nematodes; however, unlike their larger cousins, the larvae of the numerous species that infect horses do not migrate through tissues outside of the intestines. They also pass by fecal to oral transmission and are protected from digestion by a resistant capsule, but when they arrive in the large intestine, the parasites can form cysts that imbed deep within the intestinal wall. At this stage, the small strongyles are very resistant to deworming medications. After a sometimes long period of dormancy, the cysts finally emerge as young adult worms that feed on the mucosal layer and reproduce, delivering eggs that are detectable under the microscope during fecal examinations. Because of their unique life cycle and tendency to remain encysted for long periods of time however, the eggs are passed sporadically and in lower numbers than would be the case with large strongyles. A fecal exam is often negative despite the horse being infected by the worms. Small strongyles cause damage and bleeding in the intestinal wall while emerging from dormancy. Adult worms cause inflammation while feeding on the mucosal layer of tissue. In general, small strongyles cause milder gastrointestinal disease (and no symptoms due to larval migration) than large strongyles, but in very large numbers they may cause unexplained weight loss and severe diarrhea. Mild transient colic and poor coat condition may be the only symptoms seen in some cases.


Ascarids are large roundworms that reach between 6 to 12 inches long during their reproductive adult phase. Although strongyles are technically in the roundworm family, ascarids are the parasites in equines that are commonly called by that name. They are passed by fecal to oral transmission. Like large strongyles, the immature larvae of ascarids migrate throughout various tissues in the body before reaching full maturity in the intestines. One strange aspect of their lifecycle is that the developing larvae must burrow outside of the intestines, through the liver and then the heart, finally into the lungs to be coughed up and swallowed by the horse, in order to reach adulthood in the intestines again. This migration pattern, when followed by a large number of larvae, can cause severe damage to the lungs and lead to pneumonia. In addition to coughing, ascarids cause colic and diarrhea, and may even block the intestinal tract when the adult worm burden is extremely heavy (also seen when all worms die at one time with deworming). Roundworm infection is especially common in foals. Female worms can deliver as many as 200,000 eggs per day.


Pinworms are also spread by fecal to oral transmission. Even though they are an intestinal parasite, pinworms are more likely to cause external symptoms than gastrointestinal disease in horses. The adult female worms travel to the opening of the anus to deposit their eggs in a sticky substrate that adheres to the horse’s skin. As the material dries and flakes off, it causes irritation and itching. Infected horses may rub incessantly on any convenient object to find relief. This may lead to hair-loss on and around the tail, and occasionally broken skin.


Tapeworms are so named because of their flat, ribbon-like appearance as adults. They require an intermediate host to cause infection in horses. Mites in the pasture feed upon tapeworm egg-sacs and become infected. Horses inadvertently consume the mites while grazing. While the mite will be digested by the animal, the tapeworm larvae remains unharmed and matures to adulthood within the stomach and small intestines. Tapeworms cause disease when they attach firmly to the gut wall with sharp mouth parts, causing ulceration that is susceptible to infection, telescoping of the small intestine into the cecum (causing colic), and rarely perforation of the gut wall. They are not associated with emaciation in horses. The adult worms are segmented, and each segment at the tail end of the female worm contains egg “baskets”. The segments detach one by one and find their way to the horse’s anus. Once in the environment, the segments desiccate and are consumed again by mites.


Threadworms (Strongyloides westeri) are a problem for foals, which ingest the parasite larvae present in a mare’s milk while nursing. Larvae in the foal’s bedding can also penetrate the skin and migrate to the intestinal tract. The threadworm usually causes little harm to an adult horse. Infected foals show signs of colic, diarrhea, and overall failure to thrive. Horses develop immunity to threadworms after several months of age, but very young foals can acquire significant parasite infestations that may lead to dehydration, electrolyte imbalance, protein loss, and other consequences of severe diarrhea.

In addition to following a recommended deworming protocol established by the veterinarian, it is important to institute environmental control measures in the areas a horse owner is able. Keeping stables and paddocks sanitary by removing manure regularly and providing good water drainage will help to reduce the incidental ingestion of parasites. Use elevated feeders for hay. Over population is also a factor that leads to parasite infestations (as well as other contagious disease) in the herd and should be avoided. Establish an effective fly control program to reduce botfly numbers. These efforts will all help to reduce the frequency and severity of intestinal parasite outbreaks.

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