What Causes Warts on Horses?

Warts on horses are caused by a virus, and three types of warts are common. These are congenital papillomatosis, papillomatosis and aural plaques. Warts are very contagious and care should be taken to avoid spreading the virus between horses, though treatment often is not needed.

The papilloma virus is responsible for warts on horses. At least four variations of the virus have been identified as being able to infect horses, and the different variations cause the virus to present itself differently — such as appearing flat or raised — in the infected horses. Bovine papilloma virus 1 and 2 (BPV-1 and BPV-2) are kinds of papilloma virus that infect cows and spread to horses via flies or shared food and water troughs. The other two virus variations are both equine papilloma viruses (EPV). In humans, the papilloma virus is known as human papilloma virus (HPV), but humans and horses cannot infect one another.

Congenital papillomatosis is commonly known as baby warts. It occurs when a pregnant mare contracts EPV and the foal is infected while in the womb. The foal normally will develop a single small wart, but the wart can vary in size, up to 7.8 inches (20 centimeters). These common horse warts may be grayish pink or gray and they may be flat or look like flattened cauliflower. Treatment is normally required, because these warts will not go away on their own.

Pappillomatosis-derived warts also are known as grass warts. These kinds of warts on horses usually occur on young horses up to 1 year old but occasionally are found on horses in their twenties. The warts can be pink to gray, depending on the horse's skin color, and can be found around the muzzle, eyes, lips, genitals and lower legs. The warts may be in clumps and appear to be one large wart or could appear as a single wart. This type of wart can appear to look like a clump of cauliflower or may be smooth and round.

Aural plaques are commonly called ear fungus and occur in a horse's inner ear. A single wart or a cluster may form, with some being flat and others looking like cauliflower. They may be gray or have no color at all. These kinds of warts on horses often are the cause of problems such as head shaking or head shyness.

The EPV virus is contagious and infected horses should be kept away from the rest of the herd. Horses that share items such as feeding bowls can spread the virus, as can people who forget to sanitize their hands after handling an infected horse. Flies spread the virus from one horse to another. Disinfection is one of the most effective methods of preventing the spread of the virus and must be done correctly, because the virus can survive for around three weeks on infected items and even longer if the temperature is low.

Most warts on horses will go away on their own without treatment and are harmless, unless the warts are in an area that causes pain, such as the genitals. If treatment is required, the warts can be pinched off, crushed or frozen. Surgery also is an option. Chemical treatments also are available but may not be as effective.