Worms are common in horses and are usually present in the majority of horses at varying levels of infestation.¹ The life cycle of worms, like most parasites, includes stages (eggs and larvae). Worm eggs can be ingested by horses from infected pastures, which then develop inside the horse’s gut or lungs where they have the potential to cause disease. 

When present in low numbers, worms cause minimal problems. However, when present in moderate or larger numbers, they can severely affect a horse and can result in poor body condition, colic and general ill health. Some worms can also damage a horse’s intestines and other internal organs, sometimes causing irreversible harm with potentially fatal consequences. Symptoms of worm infestations typically range from mild (weight loss, mild to intermittent colic) to extremely severe (surgical colic, death).

It is essential that horses are treated with the right wormers at the right time of year, and this can be achieved through targeted worming programs. With an increasing risk of resistance to worming medications (anthelmintics), it is vital that horse owners and managers adopt a proper worming strategy to protect horses from the possibility of damage caused by worm infestations.

Worms Affecting Equines

Horses may be affected by numerous different species of worms, whose life cycles and symptoms vary. Species of worms that typically affect horses include:

Small Redworms (Cyathostomes)

Small redworms are the most common internal parasite of horses. These worms produce larvae that hibernate in the gut wall during the winter and emerge in large numbers as adults in the spring, where they can cause severe damage to the intestines.¹ Small redworms have a worldwide distribution; most horses are infected with these worms in some form or have been infected at some point in their life. Small redworms can cause weight loss, diarrhea and colic with potentially fatal consequences, particularly at the time of emergence.¹

Large Redworms (Strongyles)

Large redworms encompass several species and are among the most dangerous internal parasites of equines.¹ They can cause serious symptoms, and severe infection can be life-threatening. The life cycle of strongyles includes a larval stage during which they migrate in the blood vessels and can cause anemia and blood clots. These worms can eat through the lining of the gut wall and travel through the blood vessels of the gut, causing significant bleeding. This often results in rapid weight loss, diarrhea and surgical colic. 

Roundworms (Ascarids)

Parascaris equorum is the roundworm species that most commonly affects horses.¹ The adult worm is large, measuring up to 50 cm in length and 6 mm wide. Roundworms can be particularly serious in the case of young foals, because they can cause blockages of the intestine that may require surgery. Roundworm larvae can migrate through the gut wall to the liver and the lungs. The larvae are coughed up and swallowed, where they mature to egg-laying adults within the gut. Roundworms can cause respiratory signs (cough and/or nasal discharge) as the larvae migrate through the lungs, or they can cause gastrointestinal symptoms such as weight loss, diarrhea or a pot-bellied appearance.¹

Pinworms 

Pinworms (Oxyuris equi) can damage a horse’s bowel prior to laying eggs around the outside of the anus, causing intense itching and irritation.¹ Unlike many worms affecting horses, pinworms do not migrate through any organ tissue. Female pinworms crawl out of the horse's rectum, deposit eggs on the perianal region and crawl back into the rectum. Irritation often leads to persistent scratching, this being one of the chief signs of infestation.

Threadworms

Threadworms generally remain dormant in adult horses but can transfer to newborn foals through the mare’s milk.¹ Given the underdeveloped immune response of foals, this leaves them susceptible to diarrhea and anemia, and their growth may also be affected. It is common practice to worm foals against threadworms as early as 4 weeks old; worming the mare during pregnancy can aid in reducing the number of worms transferring to the foal.

Tapeworms (Cestodes)

Anoplocephala perfoliata, (the equine tapeworm) is present in most places where horses have pasture access,² and the majority of horses harboring tapeworms tolerate them without any clinical signs. The presence of tapeworms depends on climatic conditions favoring the orbibatid mite, an intermediate host.^1,2 In the horse, tapeworms typically grow to around 8cm and form into clusters at the junction between the small and large intestines. In some cases, they can cause digestive disturbances, loss of condition, colic and fatal blockages. 

Lungworms 

Lungworm disease is caused by the parasitic roundworm Dictyocaulus arnfieldi. Infection often results in bronchitis or pneumonia evidenced by severe coughing. As a result, lungworm disease can be difficult to distinguish from other respiratory ailments. Lungworms are ubiquitous in pastures frequented by donkeys, which are the lungworm’s natural host.¹ Donkeys typically show few signs of infection, but horses that share pastures with donkeys are likely to become infected. The severity of lungworm disease is generally related to the number of larvae ingested.

Where to buy Ivermectin

Ivermectin 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.

¹vcahospitals.com.

²aaep.org.

³Merck Veterinary Manual.