The cause of warts in horses is the same as in people, a virus infection in the skin (the virus name is the papilloma virus). They are most often found on the muzzle, but can occur elsewhere in the body as well.
The incubation period (the time frombeing infected by the virus, to when warts become visible) is about two months. Once the warts begin to appear, they reach maturity (full size) in one to two months. Typically, an infection will result in about a dozen warts but larger numbers (e.g. a hundred) can also occur.
There may be a single wart or a scattering of warts. They can also appear as clusters of warts (resembling a cauliflower in shape and general appearance, although of course much smaller). They are generally gray or pink in color.
Although unsightly, they seldom affect the horse's overall health and consequently are more a cosmetic issue. In addition, they will usually eventually disappear on their own, although this will take some months (up to a year) if untreated.
How Warts are Spread
Although not serious, horse warts are contagious and can spread from one horse to another by:
The virus can remain active on items which come in contact with warts (e.g. tack, fence posts, feeding bowls) for up to three weeks at room temperature (longer at lower temperatures). Consequently, to prevent the spread of the disease, physical separation of both horses and equipment, plus disinfection of common items, is important.
Unaffected horses should be separated from affected horses as much as possible, to prevent the spread of the illness. This includes not only physical separation, but avoidance of indirect contact (e.g. common equipment or handling of horses with first disinfecting hands and common tack. See above section (How Warts are Spread) for further information.
Wart areas are subject to injury as the warts stand out and have a rough surface. Consequently, the affected skin is subject to wear (e.g. from bridles or halters) or being torn open if rubbed against a hard surface. Damaged skin, like any open wound, may then be subject to secondary infections. During warm weather, open wounds can develop in sores and may also become infected by maggots. Consequently, wart areas should be checked on a regular basis and any open wounds disinfected to prevent secondary issues.
Due to the similarity of warts in horses and people, the potential treatments are similar in both method and success rates. However, due to the pain involved in the quicker methods (freezing or surgical removal), they are difficult to administer without an anesthetic, especially in the case of multiple warts. Due to the cost and risk associated with a general anesthetic, especially as treatment may need to be repeated, it is not uncommon for horse owners to wait for the warts to disappear naturally. Alternative treatments include chemical application, but these have lower success rates and care must be taken not to use any toxic substances which the horse may lick and ingest.
It has been observed that warts occur less frequently and disappear quicker in horses which are healthy and have solid nutrition, probably due to a better immune system. Consequently, improving the quantity and quality of feed for malnourished horses is advised.
Warts are found less often in older horses for a variety of reasons. Older horses has thicker and tougher skin, with more hair, so it is more difficult for the virus to enter the skin. Older horses also have better immune systems, especially if the horse has had warts before (previous exposure builds up immunity to the virus). Consequently, most affected horses are under 2 years of age and less than 10% of affected horses are over 3 years old.