What are melanomas?

Melanomas are one of the most common tumours in horses. They occur almost exclusively in grey horses, where they are usually benign. However, they are often associated with great discomfort for the horse and can even be life-threatening.

Etiology

Melanomas are tumours of melanocytes (pigment-producing cells). Normal melanocytes produce a pigment that is transferred to skin and hair cells, giving them colour. When these melanocytes degenerate into tumours, black bumps (melanomas) form. Up to one third of all tumours in horses are melanomas. They occur almost exclusively in grey horses, but occasionally in horses of other colours.

Grey horses

Melanomas occur in approximately 80% of grey horses over 15 years of age. The reason for the high incidence in these horses is probably related to the same genetic basis of the colour "grey", which is essentially a problem of the melanocytes. For example, Lipizzan horses that are homologous to a particular gene (i.e. that have inherited a gene from both their father and mother) have been found to turn white faster and more uniformly and to have more melanomas than other breeds of horses.

Typical locations

The horse's sex or exposure to sunlight do not seem to play a role in the development of these tumours. Apart from the base of the tail and around the anus, the salivary glands, genitals and eyelids are typical sites for melanomas. Less frequently, these tumours are also found inside the body, either as primary internal tumours or as metastases. For example, we have seen melanomas in the lungs, diaphragm, brain, spine and spleen. Melanomas can even appear in the eye! At the time of diagnosis, most melanomas are benign. However, it is described that up to 30% of all melanomas can become malignant, and thus spread to the rest of the body.

Diagnosis

Melanomas are usually diagnosed on the basis of the typical clinical picture (location, shape, colour). The tumours themselves can be single, multiple or clumped together. They can vary in size from a few millimetres to more than 20 centimetres. Their growth is usually slow. Sometimes there is ulceration of the tumours and black discharge can be seen. In case of doubt, a biopsy can be performed and the size of the melanoma can be assessed by ultrasound. Sometimes, in preparation for surgery, a CT scan is made to get an accurate picture of the tumours. It is important to distinguish melanoma from other tumours such as sarcoid (nodular), lymphoma, fibroma, etc.

Consequences

The consequences of melanomas depend on their size and location. Extensive melanomas around the anus may cause difficulty in defecating. They can also cause inflammation leading to a vicious circle of swelling, discomfort, pain, constipation, colic and even total inability to defecate if not treated in time, necessitating euthanasia. Some melanomas of the tail become so large that amputation of the tail is eventually necessary. In the neck region (usually of the salivary gland), melanomas can put pressure on the trachea and cause breathing difficulties.

Treatment

Not all melanomas need treatment, but if they can cause discomfort because of their location and size, we prefer to remove them surgically as soon as possible. We know from experience that the tumour is not very invasive, which is another reason why we prefer to remove them surgically as soon as possible.

New tumours

A single excision does not always cure a horse for life - new tumours often arise. Since tumours usually grow rather slowly, an excision can make a horse's life more pleasant for many years. Sometimes, complete excision is impossible and tumour tissue remains (for example, in the case of extensive tumours around the anus). Fortunately, activation and faster growth of melanomas after partial excision are much less common and much less pronounced than with other tumours. This is in contrast, for example, to sarcoids, which have a high risk of activation after surgery and therefore need to be removed with a greater margin than melanomas.

Delicate procedure

Due to the high impact on the horse, melanomas around the tail, anus, neck, lips, tongue, eyelid and salivary glands are mainly surgically removed. Unfortunately, due to their difficult location (around the spinal cord and brain), tumours causing neurological symptoms often cannot be treated surgically. Tumours around the anus and tail are usually removed standing up, under local anaesthetic. Tumours in other areas are usually removed under general anaesthesia. The removal of melanomas in the salivary gland area is an extremely delicate operation that requires a very experienced surgeon.

Non-surgical treatment

Other non-surgical treatments (oral cimetidine or autologous vaccines) are less effective. These therapies can be used in addition to surgery, as can local injections of cisplatin when a tumour cannot be completely removed.