When most people hear the term “melanoma,” they think of skin cancer caused by excessive sun exposure. In horses, however, melanomas are associated with coat color, specifically the gray coat color. Up to 80 percent of gray horses over the age of 15 will have at least one visible melanoma.
The gene that is responsible for the gray color is a dominant gene that causes progressive depigmentation of the hair. These horses can be born any color—black, bay, chestnut, even pinto, but the colored hairs are replaced with gray hairs as the horse ages. The first signs of gray hairs are found around the eyes or muzzle in foals and can be seen the day they are born. Young horses with mixed gray and colored hair may be confused with roans. A true roan maintains the original base color on the head and legs and will not continue to lighten as he ages. An aged gray horse might be called white, but true white horses have pink skin. A gray horse will have black skin and dark eyes.
How does this relate to melanomas? Melanin is a dark pigment produced in the skin by cells called melanocytes. It is thought that a disturbance in melanin metabolism associated with progressive graying of the coat results in an overproduction and accumulation of pigment within the cells. In time, these cells undergo a neoplastic transformation and begin to form tumors.
These tumors typically present as firm, dome-shaped, black masses in the hairless areas of the body. They are commonly located on the underside of the tail, around the anus or vulva, in the sheath or on the udder. They can also be found in the corners of the lips, around the eyes or in the throatlatch region. While most melanomas cause no clinical signs, some can be problematic due to size or location. Tumors in the lip corners can make a horse resistant to the bit. Large masses in the throatlatch can make neck flexion difficult. Larger tumors can ulcerate and attract flies and some will discharge a black, tarry substance. Massive accumulations of tumors around the anus can occur, which may interfere with defecation.
The majority of melanomas exhibit slow growth for years with no metastasis (spreading throughout the body). This is why melanomas are generally considered to be benign in horses. Some melanomas that have been static for years may suddenly grow and metastasize. A smaller percentage of melanomas will exhibit rapid growth and metastasis from the onset. Melanomas found in non-gray horses are generally considered to be highly malignant. Metastasis can occur in regional lymph nodes near the original tumor or in body cavities or internal organs (lungs, liver or spleen). Melanomas have also been found in the muscles, spine, heart and guttural pouches. Many horses show no clinical signs of internal metastasis, but lameness, colic or neurologic symptoms can occur.
Any dark mass on a gray horse is presumed to be a melanoma and most are diagnosed by appearance alone. A biopsy will confirm the diagnosis and may indicate how malignant that particular tumor is, although it is not necessarily predictive of tumor behavior or metastasis. Internal tumors may be found with ultrasound or rectal palpation.
While there is currently nothing we can do to prevent melanoma development, there are several treatment options available. Traditionally, melanomas have been left alone due to their slow growth and lack of pain or clinical signs, but early treatment may slow or stop progression.
Small tumors are easily removed surgically. Depending on the location of the mass and the temperament of the horse, they can often be removed with standing sedation rather than general anesthesia. Surgical lasers have been very successful at removing melanomas, even when the tumors are a bit larger. Complete excision will be curative for an individual tumor, but new tumors may arise later. Very large tumors or those in difficult locations may require extensive surgery and often cannot be completely excised.
The antihistamine cimetidine has immunomodulatory effects that have been reported to reduce the size and number of tumors. Long-term therapy (at least three months) is required and clinical efficacy has not been repeatable in every study. Intratumoral injection of chemotherapeutic agents, such as cisplatin, can be effective for small masses, but there are health and safety concerns for the horse, owner and treating veterinarian when using these drugs.
Recently, a vaccine was developed for treatment of melanoma in dogs. Early studies are showing promise for its use in horses. This vaccine creates an immune response against the melanoma cells, causing the body to fight the tumors. The vaccine is administered four times at two-week intervals. If the horse responds, the vaccine is then given every six months.
While it is a safe bet that if you have a gray horse you will deal with melanomas, the good news is that most are not life-threatening. Proactive early intervention may slow the progression of the disease and new treatments are on the horizon.
Jody Hallstrom, DVM, received her doctorate of veterinary medicine from the University of California Davis in 2000. She worked in private practice as an ambulatory equine veterinarian for 12 years before joining the team at Pioneer Equine Hospital, a large referral center in Oakdale, California. She is a general practitioner, but has a special interest in dentistry, reproduction and foal care. When she can get away from work, she enjoys riding dressage on her rescued Thoroughbred gelding.