Equine pinworms, or Oxyuris equi, are common parasites that inhabit the horse’s colon. Female pinworms lay their eggs on the perianal skin (around the anus), which can cause intense itching and irritation for the horse. [1]

Pinworm infections are most common in younger horses, but horses of any age can be affected. These parasites are transmitted among horses through the inadvertent ingestion of eggs present in their environment. Eggs from infected horses are shed into the environment, contaminating forage and adhering to fence posts and stall surfaces where horses have been scratching. [2][3]

Although pinworms can be bothersome for horses and their owners, they are relatively harmless and can be effectively treated. If you notice signs of a pinworm infection in your horse, seek assistance from your veterinarian for proper diagnosis and to explore available treatment options.

You can help prevent pinworm infections by keeping your horse’s environment clean and establishing a deworming schedule. Consult with your veterinarian to determine the most appropriate deworming product and frequency based on the parasites prevalent in your area.

Pinworms in Horses

Pinworms are nematode intestinal parasites that belong to the family Oxyuridae. They are predominantly found in the colon and rectum of equids. Pinworm infections, referred to as oxyuriasis or enterobiasis, can cause mild to moderate health issues in affected horses. [4]

The adult female pinworm can grow to 15 cm in length and has a slender, thread-like body with a long, pointed tail that resembles a pinhead. The males are smaller in size, only reaching 1 cm in length.

Adult pinworms inhabit the large intestine of the horse, where they attach themselves to the intestinal mucosa and reproduce. [4]

Oxyuris equi are found throughout the world, especially among groups of horses kept in close proximity with poor cleaning practices.

Life Cycle

The pinworm life cycle begins when a horse ingests an egg from somewhere in its environment. The horse can come into direct contact with the eggs from another infected horse or by consuming feed into which the eggs have fallen. [3]

After being ingested, the pinworm eggs travel through the horse’s digestive system and hatch in the small intestine. The larvae, known as third-stage (L3) larvae, then penetrate the mucosal walls, or lining, of the ventral colon and cecum.

After 3-11 days, the pinworms emerge from the mucosal walls as fourth-stage larvae (L4) and attach themselves to the lining. Approximately 50 days later, they molt and develop into fifth-stage larvae (L5).

Finally, around 100 days later, they reach the adult stage, capable of reproduction. The adult pinworms live and mate within the horse’s dorsal colon.

The male pinworm dies after mating and the female pinworm migrates out of the rectum and anus. [3] The female pinworm deposits up to 60,000 eggs in a large sticky mass in the perianal region.

Over a period of 3-5 days, these eggs develop and become capable of infecting other horses. Subsequently, they detach from the skin and contaminate the surrounding environment or are transferred to surfaces through rubbing or contact.

The pinworm life cycle takes approximately 4.5 – 5 months to complete. However, research suggests that pinworms can reach adulthood sooner, allowing them to spread eggs at a faster rate. [3]

Signs of Infection

Pinworms generally have minimal detrimental effects on horses and rarely cause issues other than irritation. A horse with a pinworm infection may rub their rump on fences, stall doors or trees in their pasture to try and relieve the itch.

This can lead to the following signs of infection: [5]

  • Restless behaviour
  • Irritated hind-end
  • Inflamed skin around the tail head
  • Sores around the anus
  • Hairless patches on the rump

Horses affected by pinworms may have gelatinous streaks of yellow or white substances on their perianal skin, which represents sticky clumps of pinworm eggs.

In rare cases, pinworm infection can lead to a loss of body condition. [5]

In one study foals with pinworms were observed to have poor hair coat quality, loss of body condition and lymphocytosis (increase in white blood cells) indicating systemic disease. [3]

Risk Factors

Equine pinworm infections have a worldwide distribution and can be found wherever horses are present, spanning across different geographic regions.

Pinworm infections are seen most often in younger horses. Foals and yearlings tend to use their tongues and lips to explore their environment, which makes them prone to ingesting eggs. [6]

However, there is a growing trend of adult and