Worms are a normal occurrence in horses and are usually present in most horses at varying levels of infestation. How a horse acquires worms depends on the lifecycle of the worm; in most cases, eggs are ingested from an infected pasture and develop inside the horse’s gut or lungs where they have the potential to cause disease. Eggs produced by the adult worm are shed in the feces to increase existing worm burdens on the pasture and to potentially infect new horses.1

Worms cause minimal problems when they are present in low numbers, however, when they proliferate, they can severely affect a horse’s physical constitution and can result in poor body condition, colic and general ill health.1 In serious cases of infestation, they can also damage a horse’s intestines and other internal organs, sometimes causing irreversible harm and potentially fatal consequences.

Worms Affecting Horses

The most common species of worms affecting horses include:

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

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. 

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 and 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 has a wide margin of safety and is excellent against all common worm parasites except bots. Pyrantel pamoate 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 is FDA approved in horses for the removal of large strongyles (Stronglylus vulgaris, S. edentatus, S. equinus), small strongyles (Trichonema spp and Triodontophorus spp.), pinworms (Oxyuris equi, Probstmayria vivipara), large roundworms and tape worms.3,4 Only worms in the horse’s intestine are vulnerable to pyrantel pamoate, meaning that deworming must be repeated minimally 2-4 weeks after the first deworming.

Pyrantel at twice the recommended dose is active against ileocecal tapeworms (e.g., Anoplocephala perfoliata); however, resistance has been reported.4

Where to buy Pyrantel Pamoate

Pyrantel Pamoate is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. PYRANTEL PAMOATE 360 MG/ML, ORAL SUSPENSION 250ML by NexGen Pharmaceuticals is indicated for the treatment and control of infections of large strongyles, small strongyles, pinworms and large roundworms in horses.

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.