9 Most Common Internal Horse Parasites

Small Stronglyes

Small strongyles are the most common internal parasite in horses today. They are found worldwide, and any grazing horse is infected with some amount of these worms. Horses will only experience disease if the numbers of internal worms are quite high. Small strongyle eggs are passed in horse manure and hatch on pasture. The horse eats the larvae, which burrow into the horse’s intestinal wall, emerging seasonally to hatch and produce more eggs. The intestinal wall affected by lots of larval burrowing is less able to digest and absorb nutrients. Therefore, a poor-quality haircoat, pot-bellied appearance, poor muscling, diarrhea, and other signs of ill-thrift are signs of high cyathostomin burden.

It is important to realize that this worm has been quite good at developing resistance to deworming medications. To prevent a time when no medication effectively treats any horse with small strongyles, work with your veterinarian to develop a deworming strategy that considers season and how many worm eggs your horse is shedding in its manure at any given time (fecal egg count ) .

Large Strongyles

Large strongyles used to be a major cause of colic and death in horses. Also known as the equine bloodworm, these worms are ingested from pasture but migrate into the bloodstream of the large intestine as larvae before moving back into the large intestine as adults to release eggs. Arterial irritation by worm migration cause clots to form and migrate, while blockage of vessels can lead to loss of oxygen-carrying blood supply to part or all of the intestine, causing damage that often cannot be repaired even surgically.

Luckily this worm is still very sensitive to dewormers and effective against small strongyles, so your deworming strategy against cyathostomins should also be effective against large strongyles.

Tapeworms

Tapeworms are the third most clinically significant internal parasite of the adult horse. They have worldwide distribution but are less common in arid and pasture-poor environments. Horses inadvertently ingest a mite that carries the infective stage of the parasite. The parasite then matures into a worm and attaches to areas of the digestive system, specifically the end of the small intestines called the ileum and the start of the large intestines called the cecum . These attachment sites are clinically significant because a large tapeworm burden in this area can lead to gastrointestinal motility problems and life-threatening forms of colic. Once attached, the tapeworm sheds egg-filled segments to be excreted in manure.

Tapeworm burden cannot be diagnosed on fecal egg count, but the worm is very susceptible to a dewormer called praziquantel, which should be given at least once yearly, in the late fall or early winter.

Ascarids

Ascarids are a major cause of colic in foals and weanlings, but natural immunity is developed as a horse ages past about a year old. Ascarid eggs are remarkably hardy and remain viable in the environment. Once ingested by the foal, they hatch and release larvae. These larvae migrate through the liver, lungs, then trachea, where they are coughed up and re-ingested to mature in the small intestines before releasing eggs. Because of this circuitous lifecycle, foals most at risk of small intestinal obstruction by large amounts of maturing ascarids are usually around 5 months old. This obstruction often manifests as abdominal distension and colic behavior such as pawing and rolling.

A dewormer that causes the worms to die at once can lead to obstruction as well, so a careful deworming program specific to foals should be developed.

Pinworms

Pinworms are found worldwide, and any horse is susceptible. Horses ingest the egg, which then hatches and invades the intestinal wall, which doesn’t result in clinical disease. However, after maturation, worms migrate and lay their eggs on the perianal skin, which becomes immensely itchy for the horse. This results in tail-rubbing and loss of tail hair. Diagnosis can be made by looking for the worms on a piece of cellophane tape after it has been stuck to the tail head—the worms are thin and white.

Treatment consists of washing the tail and perianal area and a dewormer recommended by your veterinarian.

Bots

Bots are the larval stage of the equine botfly and are extremely common. The fly lays its eggs on the horse’s legs, which look like small yellow dots. These eggs can be difficult to remove and sometimes clipping the limbs or a specialized tool called a bot knife is required. If eggs are allowed to hatch, the horse will ingest the larva when self-grooming or grooming pasture mates. These larvae then migrate to the stomach where they may live for several months before being passed in feces to mature into flies.

Bots rarely cause disease aside from mild irritation and are extremely susceptible to ivermectin, a commonly used dewormer.

Equine Stomach Worm

While the stomach worm can cause irritation of the stomach wall, termed gastritis, the problem most associated with Habronema is the summer sore. In a normal life cycle, adult house or horse flies unknowingly transport stomach worm larvae from manure to the area around the horse’s mouth where the larvae are ingested, mature, and lay eggs to be passed with manure. However, if the fly lands on a wound, the larvae can’t be ingested and complete their lifecycle, and so they migrate within the wound. This causes inflammation and irritation, with non-healing of the wound compounded by the horse trying to relieve the itchiness associated with the sore.

These summer sores can be very difficult to treat and often require multiple approaches including deworming with ivermectin or moxidectin paste. Fly control is the best preventative. ¹

Lungworm

Lungworm eggs are passed in feces, usually passed from donkeys to horses. These eggs hatch and become larvae before being ingested. They then migrate from the intestine to the lungs where they mature, leading to bronchitis or even pneumonia. Affected animals often have a cough and unthriftiness. Infection with lungworm once leads to partial or total immunity from future infections.

Common dewormers such as moxidectin and ivermectin are effective.

Threadworm

The equine threadworm is transmitted in a mare’s milk to her offspring or by penetration of the foal's skin through bedding and is quite commonly found in foals one to two weeks of age. The primary medical problem for a foal with threadworm infection is diarrhea, which can lead to dehydration. Deworming the mare with an anthelmintic effective against strongyloides within 24 hours of foaling will reduce the transmission of this parasite to the newborn. However, clinical disease associated with the threadworm is rare and natural immunity is developed with age.