Internal equine parasites (typically known as “worms”) are very common in horses, and all breeds are at risk of infection. Horses have the potential to be affected by many different species of worms, although their life cycle and the symptoms they bring about may vary. The life cycle of most parasites includes stages that are transmitted to horses on the pasture (eggs and larvae); horses most often are infected by ingesting the eggs or larvae while grazing.1 After ingestion, the eggs or larvae develop into adults in the digestive tract or other organs of the horse and eventually produce eggs that can be shed through the horse’s feces.

Not all parasitic or worm infections cause clinical signs in horses. Many horses with a low worm burden often do not show any signs of illness, but some horses will display symptoms that can vary from mild (e.g., weight loss, mild to intermittent colic) to extremely severe (surgical colic, death).2

Worm Species Affecting Horses

The most common species of worms affecting horses include:

Small Redworms (Cyathostomes): These are the most common internal parasite in equines. The cyathostome larvae hibernate in the gut wall during winter. Their emergence in spring causes severe damage to the intestines.2 Infestations can cause weight loss, diarrhea and colic. Mass emergence of these worms in the spring can lead to fatal consequences. 

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

Roundworms (Ascarids): Adult roundworms can grow to 50cm (20 in) in length. These also burrow through the gut wall and migrate to the liver and lungs. The larvae are coughed up and swallowed by the horse, where they mature into adults within the gut; these in turn lay more eggs. Due to their size, roundworms are particularly dangerous to foals and young horses. They can cause respiratory symptoms (e.g., cough, 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):  Pinworms worms live in the horse’s rectum and only exit to lay their eggs around the perineum.1,2 This results in severe itching, thus the most common clinical sign of pinworms is a horse continuously rubbing its rear. Persistent scratching often results in hair loss and open sores around the tail head, which can become infected.

Threadworms (Onchocerca): This is an equine parasite that is more often found in tropical climates, but is still occasionally seen in other regions.1 Also called neck threadworms, these worms live in the large nuchal ligament that 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. This can lead to diarrhea and anemia in foals. 

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

Pyrantel Pamoate for Worms in Horses

Pyrantel pamoate is a carbamate, one of five classes of deworming agents designated by their chemical structure and mode of action. It is a depolarizing neuromuscular blocking agent which causes a sudden contraction in the worms, followed by paralysis, causing them to lose their grip on the intestinal wall.3 Pyrantel pamoate has a wide margin of safety and is excellent against all common worm parasites except bots. 

Where to buy Pyrantel Pamoate

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.