Melanoma tumors are common in gray horses but that doesn’t mean they are harmless. A study from Portugal found that a delay removing these growths can have a serious impact on a horse’s life.

Unlike melanoma in people, which is less prevalent but often life-threatening, melanoma in horses occurs frequently—mainly on older gray horses—and is usually  benign. An estimated 80 percent of gray horses age 15 or older develop melanoma tumors. The distinct grape-like growths commonly form under the tail, in the perianal  area and on external genitalia.

Common but not harmless tumors

“A large proportion of owners consider it ‘normal’ for a gray horse to have melanomas, causing them no concern whatsoever,” says José da  Costa Pimenta, DVM, of the University of Trás-os-Montes and Alto Douro in Portugal. He adds that veterinarians may contribute to this misperception. “Many vets think that small melanomas don’t need to be removed since, in their opinion, removal could cause the tumor to become more aggressive. However, the scientific evidence shows the opposite. Leaving a tumor in place allows it to grow and multiply, as demonstrated  in our study. Plus, removing melanoma in no way increases its aggressiveness.”

For their study, Pimenta and his team reviewed the  records of 34 gray horses with a total of 42 melanoma tumors. The researchers  documented the size of the melanoma, the number of tumors and tissue analysis/histological classifications—benign or malignant. “Histological evaluation  allows us to assess specific characteristics of tumor cells that are compatible with malignancy, meaning they can spread to other sites of the body,” says Pimenta.

Benefits of prompt removal

From this collected data, the researchers determined that tumors excised six or more years after discovery tended  to be larger and were more likely to become malignant. Delay in melanoma removal was also associated with a higher number of tumors on the same horse.

Even if a tumor isn’t invasive or malignant, Pimenta believes it’s wise to excise it. “The main consequences of equine melanoma are due to the expansive growth of these tumors. This has the effect of occupying space and compressing adjacent structures,” he says. “Given the locations where these tumors typically form, they can compress the urethra and rectum, among  other structures, putting the horse’s life at risk, for example, by preventing it from defecating and causing colic.”

Furthermore, he says, “If a tumor develops in other locations, such as on the parotid gland or close to nerve structures, it can cause other problems such as respiratory difficulty or neurological signs, through the compression that the tumor mass exerts.”

Removing tumors early is also easier for the veterinarian, says Pimenta. “The larger the mass, the more difficult it is to remove and the greater the need for surgical skills. Most melanomas can be removed with the horse sedated, standing. If they are near the anus or vulva, an epidural can be performed. It is not a difficult procedure if the tumors are small.”

Better quality of life and prognosis

Pimenta advocates removal of all melanoma tumors, “ideally as soon as they emerge and are detected.” The sooner tumors are removed, the less likely they are to cause future complications, he says.

Pimenta adds that multiple melanoma removals may be necessary throughout a horse’s life. “All of them must be removed as early as possible. That way the procedure is easy and the impact on the horse’s life is practically zero,” he says.

Reference: “The impact of excision interval on equine melanoma progression: Time matters?” Animals, April 2024