Sarcoids are raised, irregular-shaped skin growths, often with flat tops, that resemble large warts. Unlike true warts, which can spread from horse to horse, the horse is a dead-end host for sarcoids. He can’t infect other horses with it.
Sarcoids share features of both warts and tumorous, but not cancerous, growths. In their early stages, sarcoids grow slowly and don’t cause a problem unless they’re irritated.
Bovine papilloma virus (BPV) has been found in equine sarcoids, and infection with this virus, probably carried to the horse via flying insects, is believed to be the cause of sarcoids. This would also explain why sarcoids tend to occur most often in areas that are attractive to flies, such as at old wound sites, around the eyes, in the genital areas and around the girth.
Poor immune response to the virus was suspected to be a factor when it was found that simply injecting the virus into horses didn’t always produce sarcoids or, if it did, the lesions would spontaneously disappear. However, genetic studies subsequently identified genetic types particularly susceptible to sarcoids.
The so-called verrucous sarcoids are small and similar in appearance to warts. They don’t easily bleed and may even disappear spontaneously. Fibroblastic sarcoids grow more rapidly and bleed easily, frequently showing invasion of the surrounding normal-appearing skin so that attempts at removing them often leave abnormal cells behind.
Unfortunately, a cure isn’t likely, unless the sarcoid is treated early and when tiny. This is partially because the tumor tends to send finger-like projections into the surrounding tissue so that getting it all out is difficult and partially because horses that develop sarcoids often have that genetic profile that results in a poor immune response to the virus. However, regression lasting for months to a year or more is reasonable.
Sarcoids have a variety of treatment options. The success rate for routine surgical excision is low, except for tiny sarcoids. Better results are obtained with either laser excision or killing the tumor by freezing it (cryonecrosis). Both options are bloodless and heal extremely well, even when the skin defect created by removing the tumor is large.
In one Dutch study, all the horses treated by cryonecrosis alone had a complete regression. The 11 horses in the study with a tumor too large for freezing were treated cryoreduction (size decreased by freezing it) combined with immunotherapy. Eight of the 11 had complete regression; three didn’t respond.
Radiation therapy is effective, either alone or following surgery to reduce the bulk of the tumor. In a large study at the University of Liverpool, 66 sarcoids were treated by radiation with an almost complete regression response. The disadvantages are that multiple treatments are required and the horse must be put under general anesthesia for the treatments.
Herbal immune-stimulating approaches, such as echinacea, have been tried, but the immune response requires activation of multiple arms of the immune system so this may not be terribly effective.
Vaccines or homeopathic nosodes have resulted in partial regressions, but in at least one study the control group that was not treated by vaccination had an equally good regression rate.
On the other hand, injection of the immune-stimulant BCG (bacillus calmette guerin cell wall extract or live organisms) directly into the tumor is often successful. The University of Liverpool study reported a 69% response rate. In a Dutch study, a 70 to 80% complete regression rate was reported after just one BCG injection, while an Australian study reported 59% complete remission rate and 18% partial responders.
Another option is injection of the sarcoid with chemotherapy agents. A study at the University of California at Davis reported complete remission in 18 of 19 sarcoids treated by four injections of cisplatin, separated by two-week intervals. A recent American Association of Equine Practitioners (AAEP) panel discussion on treatments for sarcoids concurred that the most consistent response observed by AAEP members attempting intralesional injection of chemotherapy drugs was when cisplatin was used.
Regardless of treatment, prognosis is best when lesions are treated early. We have found the University of California at Davis veterinary school is the most active center in sarcoid research and treatment. Your veterinarian could contact the school on your behalf to explore different treatment options.