The most common skin tumor in horses is the sarcoid, and although the tumor does not spread to other organs, it can invade surrounding tissues. There are six different types of sarcoids. Regardless of the type, all are reported to be related to the bovine papillomavirus as DNA of this virus has been found in 100% of sarcoids. This is the same virus that causes warts to develop on cattle, although all horses exposed to the virus do not develop sarcoids.
To diagnose a sarcoid, a biopsy is required although in many cases, the diagnosis can be made by the appearance of the mass. Many different treatments for sarcoids are available depending on the location and size of the tumor, as well as the veterinarian's experience. Surgical excision can be used but it is important to remove a large area around the tumor to prevent recurrence. In many cases, this is not possible and the tumor that recurs can be larger than the initial tumor so surgical removal should be reserved for only certain cases. Cryotherapy or freezing of the tumor has been successful for sarcoids and seems to be effective, especially if the tumor's size is reduced by surgery before the freezing procedure. Several types of chemotherapy are used locally to treat sarcoids. Cisplatin can be injected into the tumor or can be formed into beads and the beads implanted into the tumor, as has fluorouracil (5-FU) when injected into tumors. Treatments such as BCG have been injected into the tumors to stimulate immunity and have been effective in some cases. There are also multiple topical medications that have been used to treat sarcoids successfully.