Worms are very common in horses, and all breeds of horse are at risk of infection. There are many species of worms that have the potential to affect horses, although the life cycle and symptoms they bring about vary depending upon the species of worm involved. The life cycle of most worms includes stages that are transmitted to horses on the pasture (eggs and larvae). Thus, most horses are infected by ingesting the worm eggs or larvae while grazing.1 From there, the eggs or larvae develop into adult worms in the digestive tract or other organs of the horse. They then produce eggs that are shed through the horse’s feces, continuing the cycle as these infect other horses.

Not all worm infections will cause clinical signs (disease) in horses; many horses with a low worm burden often will not show any signs of illness. Others will display symptoms that can vary from mild (e.g., weight loss, mild to intermittent colic) to extremely severe (severe colic, death).2

Worm Species That Affect Horses

Out of the worm species that typically affect horses, the most common include:

Large Redworms (Strongyles): Large redworms are one of the most dangerous internal parasites that infest horses. These worms burrow through the lining of the intestinal wall and travel through the blood vessels of the gut, causing bleeding and tissue damage. Heavy infestations can cause rapid weight loss, diarrhea and colic. Severe cases of infection have led to death in horses.1

Small Redworms (Cyathostomes): Small redworms are the most common internal parasite in horses. After ingestion, cyathostome larvae hibernate in the gut wall during winter, with their emergence in spring causing severe damage to the intestines.2 Infestations can cause weight loss, diarrhea and colic. Large mass emergence of these worms in the spring can lead to fatality. 

Roundworms (Ascarids): In the adult stage, roundworms can grow to 20 inches in length. These worms also burrow through the gut wall and migrate to the liver and lungs. Horses will then cough up and swallow the larvae, where they mature into adults within the gut and lay more eggs. Roundworms are particularly dangerous to foals and young horses due to their size. They can cause cough and nasal discharge as the larvae migrate through the lungs, and can cause gastrointestinal symptoms such as weight loss, diarrhea, or a pot-bellied appearance.2

Pinworms (Oxyuris):  The adult worms live in the rectum of the horse and emerge to lay their eggs around the perineum.1,2 This results in severe itching, making the most common sign of pinworms being a horse continuously rubbing its rear. Persistent scratching can result in hair loss and open sores around the tail head, which may also become infected.

Tapeworms (Cestodes): These worms can also grow to significant length. Tapeworms aggregate at the junction between the small and large intestines where they can cause digestive disturbances, loss of condition, colic and fatal blockages.1 Horses usually become exposed to tapeworms indirectly from eating infected forage mites found on grass.2

Threadworms (Onchocerca): Threadworms are most often found in tropical climates, but they are occasionally seen in other regions.1 Also called neck threadworms, these worms live in the nuchal ligament which runs from the poll to the withers.2 Threadworms often remain dormant in adult horses and transfer to newborn foals via the mare’s milk, leading to diarrhea and anemia in the foals. 

Pyrantel Pamoate for Worms in the Horse

Pyrantel pamoate is a carbamate, one of the five classes of deworming agents designated by their chemical structure and mode of action. It is a depolarizing neuromuscular blocking agent that causes a sudden contraction in the worms, followed by paralysis, causing them to lose their grip on the intestinal wall.3 

Pyrantel Pamoate is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. 

This product carries several potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1vetmed.illinois.edu.

2thehorse.com.

3equimed.com.

4Plumb’s Veterinary Drugs.