Skin disease is surprisingly frequent in horses. In the Blue Cross 2017 National Equine Health Survey, it was the most reported problem.
This means that skin disease is even more commonly described than lameness, possibly because any skin defect or damage is so obvious.
This is particularly true with equine juvenile warts, properly called viral papillomatosis. The warts are caused by a host-specific papilloma virus, which only affects horses — typically younger horses aged between six months and four years.
Immunologically susceptible horses can also be affected, however, as well as older animals that have not been exposed previously to the virus. Similarly, the condition can affect horses that are immunosuppressed, such as those suffering from Cushing’s disease.
These warts are most commonly found on the muzzle and lips, although they do occasionally pop up elsewhere, such as on the eyelids, lower limbs or in the groin. They look like typical warts, usually beginning as tiny 1mm-diameter raised, smooth and shiny grey-to-white lumps. Each wart tends to grow rapidly and an increased number of lesions will appear over a one- to two-month timeframe. It is not unusual for up to 100 juvenile warts to sprout on one young horse’s face.
Ranging from 0.2 to 2cm in diameter, they measure around 0.5cm in height and vary in colour from grey to pink or white. The nodules have an irregular, typically warty surface, like tiny bits of discoloured cauliflower on little stalks. It sometimes looks as if a horse has stuck his muzzle into a bowl of feed and ended up with crusty bits of bran stuck to his face.
The diagnosis can be confirmed by biopsy, but the appearance is so typical that this is rarely required.
One lump or two?
Although they look pretty unsightly, most cases of equine juvenile warts typically resolve spontaneously within three months with no treatment.
It is rare for a vet to advise to ignore a problem, but treatments are often not warranted. It usually makes sense to avoid treatment, as it may result in a loss of pigmentation and more of a scar. The exception is cases with older horses where lesions may persist.
Sometimes, the warts coalesce into larger lumps. These are more of a nuisance, particularly if they are somewhere irritating, such as an eyelid. If larger lesions need to be removed for aesthetic or health reasons, then surgical excision or cryosurgery (freezing) is effective. It has been anecdotally stated that surgical removal of larger lesions may encourage the others to regress. However, one controlled study designed to test this theory showed that the duration of other lesions was not decreased and, in fact, may have been increased.
There are also reports of using a variety of different topical antiviral or wart creams on the lesions, but they can make the sensitive, delicate skin on the muzzle sore and are generally best avoided. Usually, affected horses show no signs of pain or illness associated with these warts, although the lumps may bleed a little if they are knocked or rubbed by tack.
Juvenile warts are sometimes called grass warts, as they tend to appear when young horses are turned out together — the virus spreads by direct and indirect contact. Prevention of infection in young horses is difficult. There have been recommendations to rest specific pastures, where horses have been frequently infected, but this is rarely practical as the virus can survive for years in the environment.
Thankfully, the condition is a cosmetic and transient nuisance only. Immunity should develop with age, although previous infection with the juvenile wart papilloma virus does not reduce the horse’s risk of developing sarcoids or other skin conditions, such as aural plaques inside the ear.
Ref Horse & Hound; 23 November 2017