Worms are a common occurrence in horses and are present in most horses at different levels of infestation. Worm eggs are usually ingested by horses from infected pastures and develop inside the horse’s gut or lungs where they have the potential to cause disease.¹ Eggs produced by the adult worms are then shed in the feces. This increases existing worm burdens on pastures as well as the risk of potentially infecting other horses.

Most horse owners and managers understand the need to worm horses regularly. However, with the increasing resistance of worms to anthelmintic agents, new strategies have been developed to protect equines from infestation.

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.

Species of Worms

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.¹Horses typically become infected indirectly through eating infected forage mites found on grass.²

Pyrantel Pamoate and Praziquantel for Worms in Equines

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 used as a deworming agent in the treatment 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.

Praziquantel is an anticestodal anthelmintic with activity against a wide range of parasitic worms. Praziquantel is FDA-approved in the U.S. for the treatment of Dipylidium caninum, Taenia pisiformis, Echinococcus granulosus and E multilocularis in dogs and for the treatment of D caninum and T taeniaeformis in cats.³ Other uses are extra-label. 

Praziquantel paralyzes the worm’s sucker function and stimulates motility.³ At high concentrations it paralyzes the worm’s strobilla and causes irreversible focal vacuolization and cestodal disintegration. The parasite then becomes susceptible to digestion. In schistosomes and trematodes, praziquantel directly kills the parasite, allowing it to be phagocytized by the host.⁴

Where to buy Pyrantel Pamoate + Praziquantel

Pyrantel Pamoate + Praziquantel is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. PYRANTEL PAMOATE 360 MG/ML / PRAZIQUANTEL 140 MG/ML by NexGen Pharmaceuticals is indicated for the treatment and control of infections of large strongyles, small strongyles, pinworms and large roundworms in horses, including foals.

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.