Merck Manual

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Oxyuris equi Infection in Horses


Martin K. Nielsen

, DVM, PhD, DACVM, DEVPC, Maxwell H. Gluck Equine Research Center, Department of Veterinary Science, College of Agriculture, Food and Environment, University of Kentucky

Reviewed/Revised Dec 2019 | Modified Oct 2022
Topic Resources

The equine pinworm, Oxyuris equi, has a worldwide distribution. The parasite is reported to be far more common in young horses but can occur at any age. Although the larval stages cause inflammatory reactions in the mucosal walls of the cecum and ventral colon, no associated clinical manifestations have been described. Egg masses may cause local irritation and pruritus, which may progress to tail-rubbing. Cellophane tape tests or microscopy of perianal deposits may allow identification of the eggs. Treatments include washing of the perianal area along with oral administration benzimidazoles. Recently, O equi has been reported resistant to ivermectin on several different continents.

Equine pinworms are commonly found across the world. Two species exist, Oxyuris equi and Probstmayria vivipara, but the latter has not been found to be clinically relevant and is not discussed further.

For O equi, the infective stage is the embryonated egg. Upon ingestion, L3 larvae are released and invade mucosal walls,primarily in the ventral colon and to a lesser extent the cecum. They enter the intestinal lumen as L4 larvae and move to the dorsal colon, which is the predilection site for the adult stage. Gravid females migrate through the descending colon and rectum to deposit their eggs in packets onto the perianal skin (8,000–60,000 eggs in one deposit) and subsequently die in the environment. This process takes ~5 months to complete. Females can sometimes be observed in the feces, and due to their stout appearance (7–15 cm length), they can be mistaken for ascarids. However, their tapered posterior ends can be used to confirm identification.

Clinical Signs of Oxyuris equi Infection in Horses

Horses with patent O equi infection may present with tailrubbing and automutilation of variable degrees as primary manifestations. Clinical laboratory findings are typically unremarkable.

Diagnosis of Oxyuris equi Infection in Horses

Direct microscopy of egg mass material from the perianal area reveals numerous O equi eggs, which are morphologically distinguishable from other equine helminth egg types. This can be done by pressing the sticky side of a cellophane tape onto the perianal skin and then applying the tape to a glass slide for microscopy. Alternatively, a lubricated tongue depressor can be used to gently scrape the surface of the skin for subsequent microscopy. Fecal flotation and egg counts are not useful because the eggs are generally not released in the fecal matter. Female worms can sometimes be seen protruding from the anus as they deposit eggs or can be found in feces or attached to rectal sleeves after rectal examination.

Tail-rubbing can be caused by multiple factors, so due consideration of differential diagnoses is warranted.

Treatment of Oxyuris equi Infection in Horses

Treatment includes thorough washing of the perianal area and treatment of skin lesions and reactions as appropriate. Benzimidazoles are the recommended anthelmintics due to several documented cases of ivermectin resistance among O equi reported from different continents. Pyrantel efficacy has always been variable against O equi. Attempts to administer anthelmintic products by rectal lavage are not useful, given that the adult females are only transiently found in the rectum and that the predilection site of the parasite is the dorsal colon.

Prevention of Oxyuris equi Infection in Horses

Little is known about measures effective for prevention of O equi infection, but stable and stall hygiene could be considered, because eggs may be deposited onto various objects as a result of the tail-rubbing behavior.

Zoonotic Risk of Oxyuris equi Infection in Horses


Key Points

  • The equine pinworm, Oxyuris equi, is commonly encountered throughout the world.

  • Egg packets deposited onto the perianal skin can cause intense tail-rubbing.

  • Diagnosis is by microscopic identification of the eggs or by observation of adult females.

  • The anthelmintic treatment of choice is a benzimidazole administered orally.

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