Melanomas in horses (equine malignant melanomas) – more than just an unsightly lump

• This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.

Melanomas in horses are a type of tumour made up of the cells that produce black pigment. They are most commonly found in grey horses over five years old, although they can occur in horses of any colour. They are typically a black circular growth that forms part of the skin, or is located just below the skin, but can be found elsewhere.

Equine malignant melanomas: Signs | Are they serious? | Diagnosis | Treatment | Prognosis

While melanomas used to be considered not a major cause for concern, in more recent years it has been recognised that the commonly benign tumours have potential to transform into a malignant form, which can spread rapidly around the body with fatal consequences, hence the current veterinary advice is to remove them early, while they are small and not problematic.

Typical signs of melanomas in horses

Melanomas are fairly easy to recognise due to their typical raised, smooth, black, knobbly appearance when on areas of the horse’s skin that is without hair. In other areas they will be covered with hair. They can occur singularly or in groups.

They are most commonly found:

• On the underside of the tail
• Around the anus
• On the vulva of mares
• Around the sheath of male horses

Other possible locations include:

• On the ears
• On the iris of the eye
• On the lips
• Behind the large rounded cheek bone (parotid salivary gland area)
• On the neck
• On the limbs
• In lymphatic tissue, internal organs and the guttural pouch

Are they serious?

Many horses with melanomas have long and successful careers with the tumours having little effect on their quality of life beyond their unsightly appearance. But as horses age, the likelihood of a growth developing increases (studies have suggested approximately 80% of greys older than 15 years will have at least one melanoma) and most tumours will also increase in size over time.

As they grow in size or expand in number, they can become increasingly problematic depending on their location. Those in the area under the tail have the potential to interfere with the horse’s ability to pass droppings and there is always a concern that they may lead to impactions occurring. In reality the constant pressure for droppings to pass means such impactions are rare, however that can cause significant discomfort, especially as the tumour masses enlarge and ulcerate. Growths around the horse’s throat can prevent the horse from flexing its neck normally and may affect the ability to swallow comfortably.

When a cluster of melanomas occurs, or a single melanoma becomes very large, they can ulcerate and release thick black fluid. In these cases the potential for secondary infection is high, which can cause complications.

There is also the risk that the benign tumours can switch to a malignant form spreading to the horse’s internal organs, including the heart, lungs, spleen, kidneys, bone, brain and guttural pouch with fatal consequences, particularly in elderly horses.

Diagnosis of melanomas in horses

Horse owners commonly notice melanomas as a small nodule under the skin while grooming their horse or undertaking regular healthcare tasks. Any lumps should be brought to your vet’s attention. A vet will often diagnose based on the location and appearance of the lump, although a biopsy can be performed if the nature of the lump is unclear.

Treatment of melanomas in horses

While most vets used to advise leaving melanomas alone, it is now widely recommended that small growths should be surgically removed as early as possible when their location allows and before they enlarge sufficiently to cause problems. This prevents the problems associated with larger tumours and removes the risk of the tumours becoming malignant at a later date.

Removing a small single growth is typically far less complicated and results in better healing than when dealing with a larger cluster of melanomas. Modern surgical techniques mean many melanomas can be removed using laser technology, with standing sedation and local anaesthetic techniques, such as epidurals. Once removed, the growth can be sent to the lab for further analysis to confirm the diagnosis. There is a risk of recurrence after removal.

When removal is not an option, treatment becomes more complicated. There are a range of drugs being used to try to reduce the size of the tumours, including cimetidine and chemotherapy drugs, cisplatin and 5-fluorouracil, but treatment is expensive and the results are inconsistent.

A trial of a melanoma vaccine that is licensed for use in dogs in the United States has seen some promising results¹. The vaccine prompts an immune response against the melanoma cells throughout the body, enabling treatment of growths that cannot be seen externally as well as those immediately under the skin. Currently the vaccine is only available to vets in the UK with specialist qualifications (diploma holders). As the vaccine is currently not licensed for use in Europe, vets must obtain authorisation from the Veterinary Medicines Directorate — the organisation that regulates drug use in the UK — to import it.

If melanomas ulcerate, they need to be kept clean and the area protected from flies. If a secondary infection occurs, then antibiotics may be needed, but sadly are not going to solve the problem long term and is far better to consider local treatment in the early stages and be prepared to repeat it if the masses recur.

Prognosis for cases of melanomas in horses

Melanomas are relatively common² and some horses will function well despite their melanomas, but the tumours are a risk that needs to be considered. While some horses will carry on without suffering any obvious ill effects throughout their lives, the growths can lead to euthanasia on welfare grounds in others.

If you are buying a horse that has any lumps on the skin, mention this to the vet ahead of the pre-purchase examination so he can assess their nature. If you buy a horse that has melanomas at the time of purchase then any treatment they require will not be covered under your insurance as they are a pre-existing condition.

Useful references

• Dog vaccine on trial as cure for melanoma in horses
• A database survey of equine tumours in the United Kingdom

Also: Melanoma in horses: Current perspectives

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