Melanomas in Horses: Early Excision Encouraged

Many believe that the quality of a horse’s coat is a mirror of its overall health. Supporting a shiny, healthy coat starts with a complete and well-balanced diet in combination with regular, thorough grooming.

“To support a healthy coat, your horse may benefit from supplementation with essential fatty acids, vitamins such as biotin, and minerals such as iodine and zinc, which also promote optimal skin health. If your horse is experiencing dry, itchy skin or a dull coat consider adding a dual-action hoof and coat supplement, such as Bio-Bloom PS (Bio-Bloom HF in Australia) to bump up the bloom,” advised Catherine Whitehouse, M.S., a Kentucky Equine Research nutrition advisor.

Despite our greatest efforts to maintain a shiny coat, healthy hooves, and thick mane and tail, or maybe even some dappling, unsightly masses or lesions can develop. Common lesions include sarcoids, melanomas, and squamous cell carcinomas.

Here are some fast facts about equine melanomas:

• They are the third most common skin tumor in horses;
• Melanomas occur in horses of any age, breed, or coat color;
• Most melanomas develop in horses around 6 years of age;
• Gray horses have a higher risk of melanomas, as the genetic trait for graying is also responsible for melanoma growth; and
• Approximately 80% of gray horses over 15 years of age have a high risk of developing melanomas.

“Melanomas in horses differ from those in other species, such as humans, where they grow aggressively. In horses, melanomas tend to grow slowly. The main concern with melanomas is their space-occupying nature at anatomically important structures near the eyes, nose, vulva, penis, or rectum,” added Whitehouse.

Because of their slow growth, veterinarians are not often asked to examine melanomas until they are at an advanced stage when owners become more worried. Further, according to veterinary researchers, some individuals believe early intervention of melanomas can worsen the animal’s condition.^*

“Our knowledge about this disease is very limited,” wrote the researchers. “The idea that it is normal for gray horses to have melanomas ignores long-term animal welfare. It is therefore important to assess what changes occur in these tumors in the long term, and what difference exists between tumors excised sooner and later.”

To learn more about these melanomas, the researchers reviewed data from 42 melanomas affecting 34 horses. Twenty-nine horses had multiple tumors, and five horses had a single tumor. Excision interval was approximately six years, with 27 tumors having an excision interval of greater than six years, and 15 tumors being excised less than six years after diagnosis. Five tumors had been present for more than 10 years.

This study found tumors excised later in life were significantly larger, as one might expect. Thus, removing these tumors early will minimize the chances of a tumor causing space-occupying issues or having compressive effects on nearby structures. Resecting larger tumors may require a more extensive, invasive surgery that could have been avoided by removing the tumor while it was still small.

According to the researchers, “Removing these tumors as soon as possible is the best clinical approach” and avoids the need for “advanced surgical techniques, which are not always accessible to general practitioners and may not be easily accepted by the owners due to economic constraints.”

While the authors concluded that early surgical removal is indicated when the tumors are smaller and the surgery easier, every horse should be treated as an individual. Some practitioners may deem early excision “aggressive,”  especially those that tend to take a conservative, “wait-and-see” approach. Before a management plan can be established, some facts and limitations should be considered: horse’s use, tumor classification, pathological behavior of the tumor, previous treatment results, and availability of special equipment for some treatment.

^*Pimenta, J., J. Prada, I. Pires, and M. Cotovio. 2024. The impact of excision interval on equine melanoma progression: Time matters? Animals (Basel) 14(8):1244.