The different tissues of the eye and associated structures can develop primary tumors or can be the site of spreading (metastatic) tumor cells. In horses, tumors of the skin, eye, and genital system are the most frequent, and about 80% of eye tumors are malignant (cancerous).
Tumors of the eyelids and conjunctivae are the most frequent eye tumors in horses. Most are either squamous cell carcinoma (a common type of cancer that usually develops in the outer layer of the skin and sometimes in mucous membranes) or sarcoid (a small, lumpy collection of fibrous tissue). Eye socket tumors are rare and are usually local extensions of eyelid, conjunctiva, or sinus tumors or generalized tumors (including lymphosarcoma). Tumors within the eyes are rare. Those that do occur are usually malignant melanomas.
Squamous Cell Carcinoma
Squamous cell carcinoma is a form of skin cancer that occurs most frequently in horses 8 to 10 years old. It may be more frequent in those with lightly-pigmented or nonpigmented eyelids. Appaloosas and draft breeds are affected most frequently. Ultraviolet radiation from excessive sun exposure may be important, because the incidence in North America is higher in southern and western areas and in areas of increased altitude or higher mean solar radiation. The eyelids, conjunctivae, third eyelid, and edges of the cornea can be affected with masses of slow-healing sores or multiplying cells forming a tumor. In most cases, only one eye is involved. Squamous cell carcinomas of the third eyelid are more likely to invade the eye socket than are those from other sites.
Anticancer drugs, surgery, radiation therapy, and exposure to high temperatures (hyperthermia) or low temperatures (cryotherapy)—alone or in various combinations—are available to control these tumors. Your veterinarian will recommend a medication program that is best for your horse. This may include referral to a veterinary specialist that routinely treats these types of tumors. It is unknown whether face shields or masks that block ultraviolet light are effective, but, if used, they should be started at a very young age.
Equine sarcoids are skin tumors that generally affect young horses (average 3.8 years old). They represent about 40% of all tumors in horses . When the eyes are affected, sarcoids appear as masses just below the skin in the eyelids or at the corners of the eyelids. They usually grow rapidly and may invade the skin, appearing as red, fleshy masses.
Treatment can be difficult because sarcoids are destructive and have a high recurrence rate after surgery. Treatment can include laser surgery or cryotherapy (freezing the tumor) to remove the sarcoid; chemotherapy; hyperthermia; radiation; or a combination of these therapies. After attempts to surgically remove the sarcoid, recurrence may be rapid and occur before the wound completely heals. Veterinarians can also use immunotherapy to strengthen the body’s immune system by giving a series of BCG (bacille Calmette-Guérin) preparation injections. This has about a 70% success rate. Other medications and radiation therapy may also be included in the treatment program. Because promising new treatments are or soon will be available, your veterinarian may recommend a consultation with a veterinary specialist that routinely treats these tumors.