Melanomas are cancerous growths most commonly associated with mature grey horses, the majority of affected animals will enjoy long and successful careers with the masses having little direct effect on their quality of life. The tumours are usually benign and grow slowly, however in a small number of horses they can become locally aggressive and may migrate to multiple sites within the body. Grey horses who are more than five or six years old are the typical candidates for melanomas and approximately 80% of greys older than 15 years will develop a growth.
Melanomas develop when cells that contain the dark pigment melanin (melanocytes) proliferate and multiply. The majority of equine melanomas grow in the skin and are readily visible. They are most often found as clusters of firm, dark nodules or as solitary black bumps.
The most common locations include the under surface of the tail; the perianal region, and the external genitalia. They also frequently occur in the jaw region, the neck, the eyes, and the ears. Less commonly the eyes and limbs can be affected.
Dermal melanomas (those that are located in the skin) can be flat or raised, smooth or warty, smaller than a pinhead or can develop to be bigger than a grapefruit. Less often, melanomas may develop internally in areas such as the abdomen, the lungs and the guttural pouches (air-filled sacs located at the back of the pharynx). These internal tumours are not typically detected unless they cause systemic problems such as colic or breathing difficulties.
We know that exposure to ultraviolet light increases the risk of human melanoma and other skin cancers. It was once thought that grey horses must be exceptionally sensitive to UV light. However the risk to these horses comes not so much from sunlight as from their genes (the coded instructions within body cells that control how every living creature appears and functions). Researchers have tracked both grey coat color and high melanoma risk to the same mutation on a single gene, which explains why these tumours appear so often in grey horses but so rarely in bay or chestnut Coloured horses.
Appearance and location of suspect skin lumps are the keys to diagnosing and evaluating most grey-horse melanomas. Thorough physical examination taking particular care to assess the skin in the commonly affected areas of the body of a grey horse should be part of every regular general health check. A biopsy of a suspect lesion can confirm that a lump is melanoma and not something else, for example a sarcoid with dark pigmentation. Most often cells from the tumour will be sampled with a needle and sent off to the lab for evaluation (a fine needle aspiration).
Quite often treatment will be initiated simply on the physical findings. While histopathology (examination of tissue samples/aspirates) can identify an uncommon but highly aggressive form of the disease called anaplastic malignant melanoma, it cannot tell us whether a seemingly innocent skin melanoma will become more aggressive/malignant in the future.
Researchers are currently searching for biological markers that are associated with malignancy. For now, however, changes such as a rapid period of growth or the appearance of new melanoma clusters serve as a warning. Historically the veterinary advice given to owners of horses with melanomas would be to monitor their size and progression however because of many of the factors highlighted above it is now common place to advise prompt treatment to reduce the chance of a seemingly small inconspicuous nodule becoming a large space occupying lump or even worse metastasising to other parts of the body.
Current treatment options include -
Removal of melanomas by traditional scalpel blade excision or using small punch biopsy instruments is currently the most popular and common treatment used by veterinary surgeons. This can be done with the horse standing under sedation using a local anesthetic, or if the lesions are more extensive a short general anesthetic may be appropriate. Multiple melanomas can be removed from the lips, tail and perianal area at one time. Stiches or stapled are used to close any wounds left. The key in this technique is identification and removal when they are small minimizing the degree of tissue loss and trauma. This option is less useful for larger melanomas or for situations in which multiple melanomas have joined together. Post removal the melanomas may be sent to a lab to check that the surgical margins are acceptable.
This technique is also gaining popularity as it allows the veterinary surgeon to cut and cauterize in one procedure, leaving the horse with a small burn/scab rather than the stiches or staples. Lasers are very precise but require an operator who has training and experience in using them. They are often used for larger melanoma removal or for the ‘de-bulking’ of multiple clusters of tumors.
Chemotherapy drugs such as Cisplatin or 5-flurouracil can be injected directly into melanomas to shrink them. Chemotherapy can also be utilised along with surgery. This treatment (because it is being used locally) does not cause the debilitating side effects that people undergoing systemic chemotherapy often suffer.
A therapeutic vaccine (ONCEPT from Merial/Boehringer Ingelheim) trains the horse’s immune system to target an enzyme concentrated in melanoma cells so the system will attack and kill those cells. The vaccine was originally developed for use in dogs with melanoma, but some horses have responded well to the treatment. The horse gets an initial series of four vaccinations, followed by boosters every six months. Unfortunately this is a costly procedure and only veterinary internal-medicine specialists can obtain the vaccine. Use in horses is ‘off-licence’ for now although a version of the vaccine specifically for horses may be available at some point in the future.
High-frequency Irreversible Electroporation (H-FIRE)
Electroporation therapy may also be a useful therapy. Research is currently being conducted on this treatment. This treatment essentially destroys tumor cells with intense bursts of electricity. Short bursts last only 100 microseconds and are delivered directly into the tumor through tiny electrode needles. The microbursts destroy tumour cells by creating microscopic holes in their cell membranes, causing them to lose their contents and die. This novel treatment may be used along with chemotherapy +/- surgical excision. It is done with the horse standing and sedated, using a local anesthetic, and is said to be no more painful than an insect bite.
Understanding of the factors involved in the development and progression of equine melanoma is relatively limited, especially in contrast to the knowledge base associated with human melanoma. Further study is necessary if we are to understand the genetics and pathophysiology of this disease and ultimately produce successful therapeutic and preventative options.
In our opinion the key to addressing equine melanoma is the early detection (most affective through regular examination of at risk animals) and prompt treatment of small lumps before they have the chance to multiply/grow.