Todayโs disease is ๐ฆ๐๐ฅ๐๐ง๐จ๐ฆ๐, also inspired by a recent case!
๐๐ก๐๐ญ ๐ข๐ฌ ๐ข๐ญ?
Melanomas are tumours that come from ๐ฆ๐๐ฅ๐๐ง๐จ๐๐ฒ๐ญ๐๐ฌ, which are a type of cell primarily found in the skin. Melanocytes produce ๐ฆ๐๐ฅ๐๐ง๐ข๐ง, a dark pigment, that helps protect the skin cells from UV light.
๐๐ก๐จ ๐ ๐๐ญ๐ฌ ๐ข๐ญ?
Pretty much any species can get it, however we most commonly see it in horses. In fact, it is one of the most common skin tumours seen in horses! Grey horses are predisposed to melanomas, with some studies showing up to 80% of older horses have at least one melanoma. They are most commonly found on the ๐ฉ๐๐ซ๐ข๐ง๐๐๐ฅ ๐ซ๐๐ ๐ข๐จ๐ง (around the anus) and the underside of the tail, however they can be found anywhere on the body.
๐๐จ๐ฐ ๐ข๐ฌ ๐ข๐ญ ๐๐ข๐๐ ๐ง๐จ๐ฌ๐๐?
A veterinarian can typically diagnose a melanoma based on the appearance of the tumour, and the location. If they are uncertain, they can take a ๐๐ข๐จ๐ฉ๐ฌ๐ฒ (a sample of tissue) or an ๐๐ฌ๐ฉ๐ข๐ซ๐๐ญ๐ (a sample of cells) and send it to a pathologist for analysis.
On biopsy or aspirate, melanoma cells are easily identified due to their characteristic dark pigment granules. These dark granules can sometimes cover up the nucleus of the cell though, which is where the pathologist will look to determine how aggressive the tumour is. To get around this, the pathologist might order a ๐๐ฅ๐๐๐๐ก๐๐ ๐ฌ๐ฅ๐ข๐๐, which is exactly what it sounds like, because the bleach will remove the dark pigmentation of the granules. Then we can see the nucleus!
Some melanomas may be poorly pigmented, and without those easy-to-see dark granules it can be hard to tell whatโs going on. So there are also special tests the pathologist can order to help confirm the diagnosis. One of those tests is ๐๐๐ฅ๐๐ง-๐, which is an ๐ข๐ฆ๐ฆ๐ฎ๐ง๐จ๐ก๐ข๐ฌ๐ญ๐จ๐๐ก๐๐ฆ๐ข๐ฌ๐ญ๐ซ๐ฒ ๐ญ๐๐ฌ๐ญ (๐๐๐). In this type of test, dyed antibodies against melanin pigment are applied to the section. When you look at the slide, anywhere that has melanin pigment shows up as coloured, allowing you to identify the pigment easily. Neat!
๐๐จ๐ฐ ๐ข๐ฌ ๐ข๐ญ ๐ญ๐ซ๐๐๐ญ๐๐?
These tumours are typically treated by surgical removal, and possibly chemotherapy. There has also been some work done to develop a โvaccineโ for the tumour, primarily in dogs, which works by stimulating the immune system against the tumour cells.
๐๐ก๐๐ญ ๐ข๐ฌ ๐ญ๐ก๐ ๐ฉ๐ซ๐จ๐ ๐ง๐จ๐ฌ๐ข๐ฌ?
Unlike some tumour types, these tumours are typically slow growing and slow to metastasize. Therefore, they have a relatively good prognosis, but if left untreated, they may become more aggressive and spread quite quickly. If this occurs, horses may show severe signs of disease due to the tumours interfering with the normal function of organ systems. It is important to check your horses, especially grey horses, for lumps and bumps frequently!
๐๐ก๐จ๐ญ๐จ๐ฌ
1) The classic appearance of melanomas on the underside of the tail of a grey horse.
2) An aspirate slide showing cells with characteristic dark pigment.
3) A biopsy slide showing the characteristic dark pigment. This slide happens to show the tumour cells within a blood vessel, on their way to metastasize elsewhere in the body.
4) Melanomas look black, even without a microscope! This photo shows what one of the nodules shown in the first photo might look like when cut.
5) Melanomas can metastasize to anywhere with a blood supplyโฆ even the brain!
๐๐จ๐ฎ๐ซ๐๐๐ฌ
Phillips, J.C., Lembcke, L.M. Equine melanocytic tumours. Veterinary Clinics of North America: Equine Practice. 2013.
Photo 1-5 courtesy of Noahโs Arkive.
