What is melanoma?
In horses, melanoma is a type of skin tumour that is usually associated with coat colour, with up to 80% of grey horses developing some form of visible melanoma by 15 years of age. These tumours are characteristic in their appearance, generally presenting as rounded, raised, black nodules of varying sizes, commonly found under the tail, in the perineal and genital region, salivary glands and on the lips. Whilst usually considered to be benign (non-cancerous), some will be malignant and may even spread throughout the body. Many of the benign lesions become large and obstructive and may also ulcerate and fistulate; forming a hole that oozes a black discharge with secondary infections, pain and functional problems occurring. In non-grey horses, melanomas are almost always malignant.
Unfortunately there is currently nothing that can be done to stop horses from developing melanomas. Unlike in people, sun exposure is not thought to be linked to the development of these tumours. Horses with ulcerated lesions will require excellent fly control in the summer to reduce the risk of secondary infection and subsequent pain.
How can we treat equine melanoma?
Traditionally equine melanomas have often been ‘left well alone’ rather than actively treated, because they are usually slow growing and do not tend to cause pain or other problems in the early stages of the disease. However, the chance of eventual malignancy or clinical problems is high, and we know that early treatment is likely to be beneficial to slow down or even stop the progression of the disease.
Surgical removal is often the preferred approach for early disease, but can be difficult depending on the location and size of the tumours, so is not always a feasible option. Some melanomas can also be treated with locally injected chemotherapy agents, but this can be difficult and results are highly variable and confined to the individually treated lesion, as well as bringing up many health and safety concerns for the horse, the treating vet and the owner.
A promising new treatment for equine melanoma is a vaccine called Oncept®, licenced in the USA for treatment of melanoma in dogs, which is an option we provide to at risk horses at the AHT. This vaccine works by causing an immune response against the melanoma cells, thus causing the body to fight the tumours. This means that all melanomas will be affected, including those that are not suitable for other treatment approaches. A study has shown that horses vaccinated 4 times at fortnightly intervals develop an immune response against the melanoma. If the horse shows a positive response, the vaccine is then given at 6 monthly intervals thereafter.
There is currently a large clinical trial underway in North America to further investigate this treatment, but initial clinical impressions are good, with a number of horses showing a reduction in growth or even regression of the tumours. This means that specialist vets in the UK, including our AHT equine oncologists, can now give owners a new hope for this common and sometimes very debilitating disease.