Melanomas are very common tumours of older grey horses. They are tumours of the pigment-producing cells, therefore appear where there is black skin and are also very black on cross-section. The most common sites are under and on the tail, around the mammary glands, around the head and throat, and they also occur internally.
In most cases, these tumours grow slowly and do not cause any problems. At any time, however, they might start growing more rapidly and start causing interference with other structures, such as the anus or may become ulcerated and sore. In severe cases, the tumours can enlarge internally, causing a range of other problems, such as lameness, interference with gut function or respiratory problems.
Until recently, we did recommend surgery on many of these, but we do know that this can stimulate further spread. In general, if the melanomas are not causing any problems, we would not recommend treating them.
There has been some success reported from the use of Cimetidine, an anti-ulcer medication for growing melanomas, but results of treatment are highly variable and we are not really sure why. If your mare’s tumours start to grow, spread rapidly or start to cause other problems, it is recommended that treatment with Cimetidine is started.
In some cases, the effects are quite rapid and dramatic, and the tumours do not recur once treatment has ceased. In others, no results are seen at all and there is a range of possibilities in-between those two extremes. Some cases respond, but the tumours enlarge again as soon as treatment ceases.
One advantage of treatment, however, is that internal tumours respond in the same way as the external ones, so the effect on general health might, in some cases, also be dramatic.
Deidre Carson BVSc (Syd), MRCVS, is one of the team of Veterinary experts at Rossdale and Partners.