Mast Cell Tumours are rare; making up 4% of equine tumours in the United Kingdom and 3% in North America.

Mast cell tumours usually present as a solitary mass in the skin.

Male horses (stallions and geldings) are thought to be at increased risk

Arabian and Cob breeds of horse may be at increased risk.

The disease is sometimes called mastocytosis or mastocytoma.

Cutaneous nodular

These mast cell tumours are relatively common but are benign, slow growing or stable masses. However sudden rapid growth is possible. They occur around the head (particularly the ears and in the skin around the eyes), the trunk (body) and on the limbs.

Malignant

Malignant mast cell tumours are rare but are locally invasive and can spread to distant organs (metastasis).  They occur on the limbs (particularly around the knee and hock joints)

Congenital (new born foals)

Congential mast cell tumours are very rare. They are benign and tend to regress spontaneously  in around 30 days. They can occur on the trunk (body) and on the hind limbs.

Mast cell tumours are usually solitary however rarely they may present as multiple masses and sometimes occur in clusters.

Mast cell tumours are usually found within the skin or tissue just below the skin (subcutaneous tissue) and are rarely found to involve muscle or bone.

Mast cell tumours can vary widely in size (0.5-20cm) and are usually well demarcated. The skin over the mass may be normal, hairless or ulcerated.

The Mast cell tumour may be firm or fluctant (feeling fluidy).

When sampled mast cell tumours may have a “gritty” feeling due to granular mineralisation

On the legs mast cell tumours are usually firm and fixed in position and are usually found around joints (knee, hock, fetlock, pastern) without affecting the joint itself.

Some Mast Cell Tumours are “itchy”. This might be due more to the aggregations of eosinophils (a white cell that is involved in allergic reactions) in the tumours.

In horses, manipulation of mast cell tumours has not been associated with a massive histamine release which can look like an allergic reaction. This is however seen in dogs with the disease.

Mast cell tumours can be readily diagnosed after removal via histopathology (examination of the tissue under a microscope).

The preferred option is to examine the tumour under a microscope after surgical removal. Special stains are sometimes required to characterise the type and grade of tumour present.

The diagnosis can also be made by fine needle aspirates, impression smears and biopsy. Aspirates and impression smears will not highlight the grade and likely behaviour of the tumour, whilst a biopsy or complete removal with histopathology will.

The pathologist will also rule out other diseases that can look similar.

Surgical removal with wide margins around the tumour is the best treatment option. Recurrence after removal is rare.
In some patients, chemotherapy or radiation can be useful adjunctive measures.
Injection of certain drugs into the mast cell tumour may be effective in some cases.
The prognosis for the common benign form of mast cell tumour is good.
Malignant forms of mast cell tumour have a much more guarded prognosis.