Prevalent Parasites: Common Types of Equine Internal Parasites

There are many species of internal parasites that affect horses. Knowing common types of internal parasites and the symptoms caused by infestation is important when creating a deworming plan and strategy for parasite reduction.

Many horse owners know that deworming is important, but they may not be aware of all the specific types of parasites that dewormers (also known as anthelmintics) treat. Deworming, along with other management practices, helps to keep a horse’s internal parasite load at an acceptable level. Internal parasites are small organisms that live out part of their life cycle within the horse’s body, with the potential to cause illness. They inhabit specific tissues, organs, and systems of the horse, deriving nutrients from the horse so they can continue to grow and develop. While there are many species of internal parasites that infect horses, some of the most common are strongyles, ascarids, tapeworms, pinworms, bots, and threadworms.

This article will describe how parasites infect horses, why we are concerned about internal parasites, and the basic life cycles and symptoms of 6 common equine parasites.

How Do Horses Get Infected With Internal Parasites?

To start with, a parasite is defined as an organism that lives on or in a host (in this situation, the horse is a host). The life cycle of a parasite includes stages within the horse and stages outside the horse. The stage of life when the parasite can infect the horse is referred to as the infective state. Eggs and/or larvae are found in the horse’s environment (most commonly pastures). While grazing, the horse accidentally ingests the parasitic larvae or eggs. Once consumed, the parasite will migrate to the desired area of the body where it matures into an adult and reproduces. They can cause damage while migrating through and feeding on these organs. A lot of parasites find this new home within the horse’s digestive tract, but some migrate to other tissues and organs. For example, ascarids (roundworms) migrate from the gut to the heart, liver, and lungs, and then back to the gut. How long a parasite resides inside the horse varies by the type of internal parasite. Some parasites, such as ascarids, will stay in the body for weeks, while others, such as small strongyles, can remain in the gut for over two years.

Why Are We Concerned About Internal Parasites?

Heavy internal parasite loads can cause undesirable effects on your horse’s health.  Externally, you may notice weight loss, poor growth, coarse and dull coats, potbellies and/or excessive tail scratching. Various types of colic can also occur from internal parasite infestations. In young horses infected with ascarids, diarrhea is also a common symptom. While all symptoms are undesirable, internal damage is more concerning for horses with parasite overload.

Internal parasites can cause gastrointestinal lesions, oral lesions, liver damage, stomach and intestinal lining issues, telescoping of the intestines, impactions, and colic. These clinical signs and symptoms are typically found in horses with large, overwhelming parasite loads. It is expected that all horses will have some level of parasite burden. The goal as horse owners then is not to completely eliminate internal parasites, but to keep parasite loads at a level that does not cause illness. Parasite management should incorporate multiple methods to reduce populations, and not solely rely on dewormers.

Types of Internal Parasites

Large strongyles, small strongyles, ascarids, tapeworms, pinworms, bots, and threadworms are common internal parasites that infect horses. In adult horses, small strongyles and tapeworms are the largest concerns. Each type of parasite impacts the horse differently and has unique infective states (time in the parasite’s life cycle when it can infect the horse).  The age of a horse will also impact which species of parasites they are more susceptible to; for example, ascarids are more likely to be found in foals and weanlings than mature horses. A large concern with internal parasites is resistance to dewormers (known as anthelmintic resistance). Some parasite species have become very resilient, adapting to survive dewormer treatments. This is of particular concern with ascarids and small strongyles.

Large Strongyles             

Three species of large strongyles affect horses: Strongylus vulgaris, Strongylus edentates, and Strongylus equinus. Large strongyle larvae are found on pastures and are ingested while the horse grazes – the larvae “swim” up blades of grass in water droplets to increase their chances of being eaten. Once ingested, S. vulgaris larvae migrate to the large intestine, and can then migrate to the blood vessels and burrow through the walls of arteries and disrupt blood flow. Because of this, S. vulgaris are commonly known as “bloodworms” and are considered the most pathogenic and damaging of the large strongyles. The other two species of large strongyles will migrate from the digestive tract into the liver and then back to the large intestines, but these are not as damaging since they do not affect the blood stream to the same extent. Currently, large strongyles are not a large concern for domestic horses, as treatment strategies from decades ago have substantially reduced their presence.

Small Strongyles

Similar to large strongyles, there are a number of different species of small strongyles found in the horse. Small strongyles (Cyathostomes) have a similar life cycle to large strongyles in that they are found on pasture grasses, ingested by the horse as larvae, and then move through the digestive system. However, once the small strongyle larvae reach the large intestines they do not migrate into other organs or tissues. Small strongyles are unique in that they often burrow into the walls of the large colon and cecum, becoming what we call “encysted.” This makes management of small strongyles difficult, because while encysted, these parasites can remain in the body for up to two years and are not affected by most dewormer treatments. They will wait until conditions within the intestines are ideal for maturation, and then emerge from the intestinal wall to mature and produce eggs. Small strongyles are prolific and have become resistant to most dewormers currently available. Small strongyle resistance to deworming is one of the primary reasons recommended deworming protocols have changed within the past decade. It is assumed that all grazing adult horses are infected with small strongyles, but clinical symptoms are rare.


Ascarids (Parascaris equorum), or roundworms, typically affect younger foals and weanlings, as most horses develop immunity to them by the time they are yearlings. Foals ingest ascarids as eggs, consuming them from many different places in their environments. Unlike many other equine parasites, ascarids are not just found on pasture grass; they can be found on many surfaces such as buckets, troughs, and even the udder of the mare. Once ingested, ascarid eggs develop into larvae in the small intestine. Hatched larvae then migrate through a series of tissues; they move from the small intestine to the liver and then to the lungs. Once in the lungs, the parasites are coughed up, swallowed, and then migrate back to the small intestine for maturation. Similar to small strongyles, they are developing resistance to certain types of dewormers, which could lead to problems with dewormers remaining effective against ascarids in the future.


There are three species of tapeworms found in horses, but the most common one is Anoplocephala perfoliata. Tapeworms are unique in that they have two hosts in their life cycle; the first is the oribatid mite, and the second is the horse. Mites consume tapeworm eggs, which then develop into larvae inside of the mite. Horses then ingest the mites while grazing, and the mite moves to the small intestine of the horse where it releases the tapeworms. Adult tapeworms then continue to move through the small intestine until they arrive at the ileocecal junction. This is the junction between the ilium (the last part of the small intestine) and the cecum. Here, the tapeworms attach to the gut wall and may cause inflammation of the intestinal tract. Colic has been reported to be caused by tapeworms. Additionally, obstruction, ulceration, or thickening of the intestines can occur because of heavy tapeworm loads.


Pinworms (Oxyruris equi) are ingested by the horse as eggs and can be found in the horse’s environment. Once ingested, pinworms mature in the horse’s intestines. Adults then emerge out of the anus and lay eggs along the horse’s skin there, which makes the horse itch and rub its tail. Unlike other parasites mentioned, pinworms do not cause much harm to the horse internally, but instead cause unattractive hair loss from the horse scratching its tail. They are not a large concern from a health perspective, but treatment for this species of parasite is still recommended to reduce irritation.


Bots, often referred to as stomach bots, are unique because they are both internal and external parasites. The most common species of bot fly found in horses is Gasterophilius intestinalis. The adult bot fly, an external parasite, is a flying insect resembling a bee that lays yellow-oval shaped eggs on the horse’s coat. Eggs are found primarily on the lower forelimbs (and to a lesser extent the neck and mane). The eggs will remain there until stimulated to hatch by the horse’s saliva as it bites or licks the affected area. The newly hatched larvae burrow into the gums of the horse where they will stay for about four weeks. The parasites then move to the lining of the stomach where they stay for nine months before being passed in manure. Possible minor oral lesions and damage to the stomach lining can occur from stomach bots, but no serious health problems are generally caused by bots.


Threadworms (Strongyloides westeri) are internal parasites that primarily infect foals. Though they can be seen in older horses, they are rarely found in horses over 8-10 months due to developed immunity. Foals become infected in three different ways: by ingesting larvae through the mare’s milk, from the larvae penetrating the foal’s skin, or ingestion of the larvae from the environment. Larvae migrate to the lungs and end up in the small intestine. Diarrhea is often associated with threadworm infestation and can cause dehydration. Due to current dewormer treatments, threadworms today are quite rare.

All horses are expected to have some level of internal parasite burden. Each species of internal parasite infects and impacts horses differently. Understanding how each of these parasites function can help you better understand which management practices, such as manure removal from pastures, can help reduce populations. Knowing signs and symptoms of infestation for each parasite can also help you recognize when your horse may be dealing with a heavy parasitic load, and that treatment and consultation with a veterinarian may be necessary. Conducting routine fecal egg count tests can help determine what the parasite load of your horse is. If you suspect your horse has a heavy parasite burden, please contact your veterinarian immediately. Not all dewormers are effective against all parasites, so identifying the problem and the correct treatment is essential.

Courtesy of PennState Extension