The statistics are a little daunting:

1. Up to 80% of gray horses older than 15 years will develop at least one melanoma, a type of cancerous tumor.


2. Approximately 30% of equine melanoma cases seen by at least one referral hospital had developed into extremely large, advanced, infiltrative, multinodular, metastasizing (spreading) or multicentric (having multiple centers of origin) lesions.


3. All melanomas are malignant (even the tiny ones) and possess the propensity to have further and unpredictable patterns of growth.

Yet many veterinarians and horse owners are fairly dismissive of melanomas, considering them to be benign lesions that merit neither biopsy nor treatment–unless or until the tumor becomes threatening.

Some veterinarians think that’s a potentially fatal attitude. “It’s important to realize that melanomas grow into serious problems in many horses,” warns melanoma researcher John L. Robertson, VMD, MS, PhD, Director of the Center for Comparative Oncology (CeCO) at the Virginia Maryland Regional College of Veterinary Medicine in Blacksburg, Va. “I routinely see horses that have the advanced stage of the disease. Most of those advanced cases will die from the disease, as we have very few treatments for it.”

Various studies have found melanoma incidence ranging from 3-15%, with 30% of melanomas occurring in horses that aren’t white or gray.

Melanomas 101

Most melanomas occur externally. “The common sites for primary melanomas include the undersurface of the tail, perineum (the area of skin under the tail and around the anus), and the external genitalia,” says Robertson, who is also a professor of pathology in the Department of Biomedical Sciences & Pathobiology at Virginia Tech.

According to Robertson, primary melanoma can also occur internally, extending into and around the anus, in lymph nodes, and within the parotid salivary glands (in the throatlatch region). He adds, “Other internal sites include the mammary gland, paralumbar and neck musculature, feet and legs, eyes, mouth, and vertebral column, although some of these may, in fact, be metastatic.

“There is some controversy about metastatic melanoma,” he said. “Tumors might simultaneously develop in separate locations (as opposed to migration); they don’t necessarily have to spread from a single common site. Metastasizing lesions can be found throughout the body, including lymph nodes, body cavities, liver, kidney, heart, and GI (gastrointestinal) tract.”

External melanomas vary in appearance and size. They can be flat, raised, verrucous (resembling a wart), or pedunculated (attached by a stalk), and they can range from less than 1 cm to 20 cm in size.

“Nodules are usually firm, slow-growing, and nonpainful,” notes Emily A. Graves, VMD, Dipl. ACVIM, founder and principal of Equine Consulting of the Rockies, in Ft. Collins, Colo. “Larger melanomas, particularly those that become abraded, can bleed and become a nuisance (attract flies), an obstacle for defecation, or painful.”

Internal melanomas “can cause profuse sweating, neurologic deficits, colic, respiratory difficulty, or local inflammation that leads to other clinical signs or problems in the larynx, guttural pouches, nerve trunks, chest,” says Graves.

Internal tumors can also interfere with other body functions, says Robertson. “We have seen colic with melanomas in the gut, heart problems with melanomas on the heart, abnormal movement with tumors in the muscles, and eating difficulties with tumors in parotid or throatlatch.”

Melanomas are often grouped by type, as proposed in a 1995 retrospective study done by Beth A. Valentine, DVM, PhD, Dipl. ACVP, now of Oregon State University. “Superficial lesions are usually seen in horses under 6 years of age,” explains Graves, citing the study. “Deeper lesions into the dermis (dermal melanoma) usually occur in teenage patients, with 70% or more occurring in gray horses. External or internal anaplastic/malignant melanoma usually occurs in gray horses 20 years and older.”

The cause of melanomas is unknown. It’s uncertain if exposure to sunlight is a factor.

Diagnosis

Gross appearance and surgical biopsy are the mainstays of diagnosis. “I have seen several lesions incorrectly diagnosed as melanoma that, on biopsy, were actually sarcoids with superficial pigmentation or superficially pigmented fungal plaques,” notes Robertson.

Internal melanomas can be confirmed, says Graves, through imaging (ultrasound or endoscopy), biopsy, or aspiration of masses seen on ultrasound.

“The most important thing for veterinarians to consider is that they are dealing with a tumor that is a malignancy, even if it is not growing rapidly,” Robertson says. “Although many small melanomas may never get bigger than a pea, quite a few start out being pea-sized and end up being bigger than a grapefruit or spreading widely. So veterinarians need to get a diagnosis and institute appropriate treatment.”

Treatment

The quantity of tumors, their locations, size, any effects on daily functions/jobs, and biopsy results are used to determine treatment, says Graves. Additionally, some veterinarians prefer to treat more aggressively, encouraging prompt surgical removal of lesions, while other veterinarians favor just monitoring small lesions.

“Most melanomas are superficial and can usually be monitored for growth,” Graves states. “However, if small enough, excision can be done easily under appropriate sedation/local anesthesia or general anesthesia, as needed, and is most often curative. If larger or depending on the tumor location, laser surgical removal or surgery under general or epidural anesthesia may be needed.”

Some veterinarians, including Robertson, prefer taking surgical action early: “I am not a fan of doing nothing! I see too many aggressive, infiltrative melanomas that started as small bumps and become virtually untreatable. Larger tumors and those in difficult sites cannot be surgically removed. Studies indicate there is benefit to resecting small melanomas before they become big melanomas.”

As for concerns that surgery (or biopsy) causes increased tumor growth or spread, Robertson points out that incomplete removal of any type of tumor could cause potential stimulation of growth factors from infiltrating inflammatory cells, possibly changing the pattern of tumor growth.

“Most equine surgeons who feel comfortable resecting small nodules are fully capable of doing this,” he says. “Larger masses are very tricky–they are hard, infiltrative, bloody; an equine specialty practice is best for handling these complex lesions.”

Other therapies used to treat melanomas include cimetidine, cisplatin, frankincense oil, and melanoma vaccines.

Cimetidine Opinions are divided on cimetidine (an anti-ulcer/heartburn medication). “Cimetidine has a good reputation with most practitioners for shrinking tumor size,” Graves reports. “But owners should know that some horses become ‘resistant’ to the drug’s effects. If that occurs, some veterinarians suggest stopping the drug for two to three weeks, then restarting therapy. The second round may or may not be effective.”

Robertson feels there is insufficient scientific data to support cimetidine use.

Cisplatin “Intralesional treatment with cisplatin (an antineoplastic drug, meaning it inhibits the growth and spread of malignant cells) in oil was shown in one study to be effective in shrinking tumor size,” says Graves. “This is not a common therapy, though.”

Frankincense oil This oil, injected or applied topically, has had some success in reducing tumor size, says Robertson, who is currently researching its efficacy in melanoma treatment. “It is much too early to tell if this oil kills all melanomas.”

Vaccines Melanoma vaccines or immunostimulants for treatment or prevention is without sufficient positive research data to support use, Robertson states. Nevertheless, Graves reports that, although she has had no personal experience using the vaccine to treat melanomas, some of her colleagues “have been happy with its efficacy in shrinking masses.”

Prognosis

“With timely, appropriate treatment, small, early-phase melanomas might be well-controlled with one or more types of therapy, including surgical methods,” says Graves. “Most gray horses will likely develop more as they age, but the superficial and dermal diseases are not necessarily life-threatening.

“The more aggressive, anaplastic (characterized by a loss of differentiation of cells and their orientation to one another and to their axial framework and blood vessels) melanoma can metastasize very quickly–within months to years–and involve many internal regions and organs,” Graves says.

Tumors that enlarge and interfere with general health or organ function carry a guarded to poor prognosis, Robertson says.

Take-Home Message: Be Proactive

Although there is no way to prevent melanoma occurrence, Robertson says increased surveillance, diagnosis, and treatment could reduce the number of horses that develop nontreatable lesions.

“No matter how rare cancer is in horses, a working hypothesis is that equine melanoma is a malignancy,” he explains. “The veterinary community and horse owners need to appreciate this and treat melanoma as such. We need to raise the bar so that equine practitioners and the general public understand that melanoma is not a benign thing, like a mole