Warts (Papillomatosis) in Horses

Papillomatosis usually affects young horses with undeveloped immune systems. Typically the symptoms disappear by the time the horse is 3 years old.

There are several types of papillomavirus, each of which are typically mild, and develop on different parts of the body.

Grass warts usually develop on the face and muzzle and on the prepuce in males. It is theorized that this type of papillomatosis is transmitted by black-fly.

Type 2 papillomas are typically found on the genitals, and may develop into squamous cell carcinomas. This is the most severe type of papilloma due to its link to cancer.

Pinnal acanthosis or aural plaques develop in the ears. The papillomas are variable in appearance with some appearing as small areas of raised pigment, and others like large clusters of white masses.

A fourth type of papilloma is found on the back of the pastern. These are often larger than the ones found on the muzzle. This form of papillomas are prone to infection by other viruses or bacteria because of the pastern’s exposure to waste and mud and its susceptibility to injury.

Papillomatosis is common in horses under the age of three. Papillomatosis is a mild condition that typically resolves on its own.

Veterinary attention is useful to differentiate papillomas from other skin diseases such as verrucous sarcoids (a type of tumor) or molluscum contagiosum (another viral infection), which require treatment.

Horses who are in contact with infected horses are at greater risk of contracting papillomatosis.

Papillomatosis is caused by several equine papillomaviruses. These viruses are common, and are transmitted through close contact with other infected horses, or with surfaces that have been in contact with infected horses. Biting insects may also spread the virus.

Older horses can be carriers of the virus without developing symptoms because their immune systems are sufficiently developed to fight the virus. Young horses have less developed immune systems, and therefore tend to develop papillomas.

In some cases, horses are born with congenital papillomatosis. The immune system never activates against the virus, since the papilloma is recognized as part of the horse by the immune system.

These warts are most common on the muzzle and sides of the face, genitals, ears, or pasterns.

Diagnosis of papillomatosis is straightforward, and based on physical examination. In rare cases, a biopsy is also necessary to confirm the bumps are papillomas.

Treatment is usually not required since papillomas resolve on their own once the immune system matures sufficiently. This usually happens within 1 to 6 months of the onset of symptoms, or by the time the horse is 3 years old.

In cases of congenital papillomas, or where the papilloma needs to be removed for the comfort of the horse, surgical removal, topical creams, or cryosurgery (freezing them off) are available.

The prognosis in uncomplicated cases is excellent. Papillomas that are surgically removed also have an excellent prognosis, and typically the papillomas do not return after removal.

The prognosis for genital papillomas that develop into squamous cell carcinomas is more guarded.

Prevention of papillomatosis depends on avoiding contact with the papillomaviruses. Strategies include:

• Avoiding contact with horses infected with papillomavirus
• Disinfecting the living environment and equipment on a regular basis
• Proper overall management to ensure excellent health
• Using fly repellant sprays to reduce insect bites

This is a common virus, but the development of symptoms is not common.

• Benign neglect
• Creams
• Cryosurgery