A lump or bump on your horse’s skin is most likely a sarcoid. These are the most common type of skin tumors found in horses and other equids like donkeys and mules. Some studies have estimated that up to 12 percent of horses will develop sarcoids in their lifetime.
Their appearance can range from rough, flaky areas of skin with less hair than normal to firm nodules in the skin. The most common sites are the skin of the head, belly and legs, although sarcoids can develop anywhere. They can occur as single or multiple tumors on the same horse.
Although these tumors are technically benign, they can be locally aggressive and invasive. This makes removing the entire tumor really challenging and can pose a major problem if they occur on an essential location, such as the eyelid, lips or girth because these can limit the horse’s ability to be ridden or have normal function, such as properly closing eyelids .
They often arise in areas where the skin suffered a previous injury. In terms of other risk factors, there is no consensus on whether certain ages or sexes are more likely to have sarcoids.
Sarcoids seem more common in certain breeds such as Quarter Horses, suggesting a genetic component.
A growing consensus supports that sarcoids are caused by infection with bovine papillomavirus. Modern molecular tests have detected the virus in sarcoid tumor tissue but not in other types of skin tumors such as papillomas, melanoma, and squamous cell carcinoma.
There are even studies that experimentally inject the virus into horses, which then go on to develop sarcoids.
In cattle, bovine papillomavirus causes warts that do not progress to tumors. Cattle warts regress when the animals’ immune system efficiently eliminates the virus.
Confusingly, it seems as though many horses are infected with this cattle virus, but not all develop sarcoids. The reason why some horses can’t clear the virus isn’t understood. One theory is that horses have different immune responses to infection. Some horses robustly clear the virus while others muster only a half-hearted immune reaction, which may allow the virus to persist and develop into a sarcoid.
Experimental immunization of horses against the bovine papillomavirus generates antibodies that seem to protect against sarcoid development. In horses with existing tumours, the tumors may resolve or stop their invasion following immunization. It is possible that in future, we will have a vaccine as treatment.
Within sarcoids, bundles of misbehaving tumor cells under the influence of a nefarious virus proliferate, divide and disrespect normal boundaries. They produce chemicals that stimulate new blood vessel formation and allow it to spread into nearby tissues. They produce a protein that stops normal cell death so the tumor cells become immortal. They also have unstable DNA, so mutations accumulate that allow it to invade and proliferate unchecked. Finally, they release factors that help evade the immune system.
Because sarcoids are so invasive, they are prone to regrowth following surgical removal. Other treatment techniques include immune therapy and freezing the tumour. If some tumor cells are left, the tumor can regrow, often more aggressively than before. For instance, one study found that nearly half of sarcoids had reoccurred within three years of treatment.
Other types of trauma or injury to the tumor itself can stimulate them to grow rapidly and become more invasive. Although these tumors invade the local tissue, they do not spread throughout the body. In some horses, they can regress on their own without treatment, likely as a result of an effective immune response.
How the bovine virus spreads is not well understood. Some studies have found the virus in flies in the areas where horses with sarcoids live, which suggests that flies could spread the virus from cattle to horses and also from infected horses to non-infected horses.
Horses that are kept near other horses with sarcoids and/or cattle are more likely to carry the papilloma virus. Horses may also be able to pass the virus from mare to foals during gestation.
Although sarcoids affect horses frequently, many mysteries remain about how they develop, why certain horses are affected and how best to treat them. Better understanding of sarcoids would greatly benefit the welfare of horses.
Dr. Jamie Rothenburger, DVM, MVetSc,PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine. Twitter: @JRothenburger