Below is basic information on the most clinically important equine parasites in the north east. If you have a question pertaining to a specific parasite or treatment please consult your veterinarian.


Large strongyles and small strongyles (a.k.a cyathostomes) are probably the most important parasite of horses. Adults live in the large colon and eggs are passed in the horse manure where they develop on pasture to a non-feeding infective stage. The larvae climb to the top of blades of grass where they are eaten by horses while grazing. These larvae can withstand cold temperatures and desiccation so they can over winter in hay or on pasture.

Large Strongyles

Once ingested, large strongyles migrate in areas away from the gastrointestinal system before returning to the intestinal tract to finish their development. The three species that affect horses are Strongylus vulgarus, S. edentatus and S. equines (rarest of the strongyles in the US), with S. vulgaris being the most clinically relevant. S. vulgaris was the parasite most associated with having caused colic because the larvae would migrate and invade the arterial blood supply of the large colon, causing aneurysms and destruction of the blood vessels. Clots would break off causing blockage of blood to the intestinal wall leading to necrosis and colic. Treatment often required surgical intervention, but thankfully is now rare due to deworming practices using products such as avermectins or pyrantel.

Small Strongyles

There are 40 species of cyathostomes that have been described in horses and virtually all grazing horses are exposed to these parasites at some point in their lives. Unfortunately, many horses encounter these parasites throughout their life and since immunity to them is incomplete, deworming must be performed routinely. Small strongyles differ from the large strongyles because they either stay in the lumen of the large intestine or encyst (embed) within the wall. Horses can have thousands to tens of thousands of these worms in their cecum and colon. They can cause chronic weight loss, colic, diarrhea and protein loss if they encyst within the bowel wall. The disease known as larval cyanthostominosis is due to synchronous emergence of large numbers of larvae from the bowel wall. This tends to occur in late fall or early spring. In severe cases, the disease can be fatal in up to 50% of cases. The key to deworming these parasites is to kill the larvae, but since they can live in the bowel wall the dewormer may never touch them. Common dewormers that have been used to treat small strongyles have included benzamidazoles, macrolides and pyrantel. However, resistance to some of these dewormers is encroaching.


Parascaris equorum are roundworms that shed large numbers of eggs that contaminate the environment and are very resistant to environmental extremes. Horses consume them while grazing and the larvae migrate through the liver and lungs before reproducing in the small intestine. Ascarids can become large worms that cause disease by impaction, often occurring after deworming in weanlings and yearlings with heavy infection. Rarely, they can cause perforation of the bowel wall, or respiratory disease from migration through the lung. Deworming with pyrantel, benzamidazoles or avermectins typically kills these parasites although resistance is being reported in heavily populated farms with weanlings and yearlings.


Horses are hosts of three tapeworms of which Anoplocephala perfoliata is the only one thought to be of clinical importance. These are the smallest tapeworms, but can cause colic because they live in the ileo-cecal junction which is the junction between the small and large intestine. When the eggs are passed in the feces, a mite consumes them and the larvae develop within the mite. Horses become infected by eating these mites while grazing. Because these parasites have different nervous systems from the other parasites, yearly deworming with a dewormer containing the drug praziquantel is recommended.


These worms tend not to be too pathogenic to horses, but can cause tail rubbing. There is a fecal-oral transmission of these eggs. The tail rubbing is a reaction to the excretions of the female when the eggs are released. While the eggs are fairly resistant to desiccation these parasites are sensitive to most dewormers.


There are several species of gastrophilus (bots) but all are carried by bot flies which lay the eggs on horse hairs. The eggs hatch and get eaten by the horse while grooming. The larvae live in the mouth before migrating to the stomach where they attach to the non-glandular part. While horses can have large numbers of these parasites they are not thought to cause disease. Never the less, these parasites are extremely susceptible to avermectins.