Oxyuris equi Infection in Horses

Last Updated on March 29, 2022 by Allison Price

The equine pistolary, Oxyuris Equi, is widespread. Although the parasite is more prevalent in young horses, it can also occur at any age. The larval stages can cause inflammation in the ventral colon and cecum mucosal walls, but no clinical manifestations have been reported. Local irritation and pruritus may occur from egg masses, which can lead to tail-rubbing. The identification of eggs may be possible through cellophane tape or microscopy of the perianal deposits. The treatment includes washing the perianal region and oral administration of benzimidazoles. Recent reports have shown that O Equi is resistant to ivermectin in several continents.

Pinworms of the equine species are common throughout the world. There are two species of Oxyuris and probstmayria vipara. However, the former has not been proven to be clinically useful and is not being discussed.

O Equi is the infective stage. It is the embryonated eggs. L3 larvae become infected when they are inhaled. They invade the mucosal walls primarily in the ventral colon, but also the cecum. They become L4 larvae in the intestinal lumen and then move to the dorsal colonies, which are the preferred site for the adult stage. Gravid females move through the rectum and descending colon to deposit their eggs on the perianal skin. The entire process takes approximately five months. Sometimes, females can be seen in their feces. Their stout appearance (7-15cm) can lead to them being mistaken for ascarids. Their tapered posterior ends can be used for identification.

Clinical Signs

Horses suffering from patent O Equi infection might present with tailrubbing or automutilation in various degrees. Most clinical laboratory findings are not remarkable.

Oxyuris equi Infection in Horses


Direct microscopy of egg material from the perianal region reveals many O Equi eggs that are easily distinguished from other equine-helminth egg types. You can do this by applying the sticky side of a cellophane to the perianal skin, and then using a glass slide to microscopy. A lubricated tongue pressor can also be used to lightly scrape the skin surface for microscopy. Egg counts and flotation are not recommended as the eggs are usually not released into the fecal matter. Sometimes, female worms can be seen protruding out of the anus to deposit eggs. They can also be found in feces after rectal examination.

Tail-rubbing can result from multiple factors so it is important to consider differential diagnoses.


The treatment includes thorough cleaning of the perianal region and treating any skin lesions or reactions. Benzimidazoles should be the preferred anthelmintic due to numerous cases of ivermectin reactivity among O equi from different continents. The effectiveness of Pyrantel against O Equi has been variable. Rectal lavage is not a good method to administer antihelmintic drugs, as the adult females only occasionally inhabit the rectum. The dorsal colon is also a predilection area for the parasite.


We don’t know much about the prevention of Oequi infections, but it is possible to improve stall hygiene and stable care, as eggs can be dropped onto many objects by tail-rubbing.

Zoonotic Risk


The Key Points

  • It is common to find the equine pinworm, Oxyuris Equi all over the globe.
  • The perianal skin may be irritated by egg packets.
  • Diagnostics can be made by microscopy identification of eggs or observation of mature females.
  • Orally administered benzimidazole is the best anthelmintic treatment.
Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!