A sarcoid is a slow growing tumor that occurs in the skin of a horse most often on the legs, in the flanks, on the abdomen, or anywhere on the head and neck. They are of an uncertain origin but are thought to be linked to a virus similar to the bovine papilloma virus. Sarcoids may also form in the location of a previous wound, but the relevance is not understood. Breed predilection might exist in Appaloosas, Quarter Horses, and Arabians, but this too is unconfirmed.
Sarcoids take on several outward appearances which may lead to a presumptive diagnosis. They can be wart-like (verrucose) – poorly defined margins with raised bumpy surfaces that may or may not be ulcerative; fibroblastic – cauliflower shaped, pedunculated (attached by a stem), and ulcerative; flat (occult) – hairless, smooth, or slightly scaly; or combination (transitional) types.
The physical appearance of the sarcoid may help to determine the level of urgency in taking action. In general, fibroplastic and transitional tumors pose more of a risk to the horse in terms of secondary infection (i.e. tetanus) and local invasion into surrounding tissues. They also attract the attention of flying insects which can cause greater damage and transmit pathogens (possibly even Sarcoids to other horses). An ulcerative tumor will be addressed with greater haste than a smooth, unremarkable mass.
Histopathological examination of the tumor cells and structure is the only definitive way to confirm a sarcoid. The tissue is fixed in formalin and delivered to the laboratory for microscopic evaluation by a pathologist, a veterinary specialist trained in the cellular appearance of disease. Many veterinarians believe from past experiences that a partial biopsy of the mass may cause it to become active; therefore, a complete excision (removal) is planned or performed before initial confirmation in the lab. Differential diagnoses for Sarcoids include granulation tissue over a wound or skin cancer (melanoma or squamous cell carcinoma).
Excision can be performed under general or local anesthesia, depending on the size and location of the tumor. Because complete removal with clean tissue margins is recommended and often curative, general anesthesia is preferred. (The term “clean margins” refers to the ideal finding that no tumor cells are seen within a comfortable distance of the edges of the mass under the microscope.)
Other treatments that may be needed if the tumor is poorly differentiated include radiotherapy (focused x-ray exposure), chemotherapy (drugs that target abnormal mutations of cells), immunotherapy (drugs and supplements that boost or suppress certain functions of the immune response), and follow up surgery.
Sarcoids are mostly locally invasive. They can become very large and difficult to remove, or ulcerated and infected, which obviously increases the risk to the horse and the expense of treatment. There is also new data that shows that sarcoids may metastasize (spread) to other areas of the skin.
Every mass discovered on a horse should be evaluated by a veterinarian.