Tumours in horses commonly affect the skin. In addition, however, there are some important internal tumours to be aware of that cause skin changes. These can either appear in the skin, such as melanoma or lymphoma, or as a paraneoplastic cutaneous condition, which can include pruritus or paraneoplastic pemphigus.
Skin tumours that are commonly seen in horses are listed below:
- Sarcoid. A cutaneous tumour which has a variety of appearances and can be difficult to treat effectively.
- Melanoma. The majority of melanomas are benign and have only space-occupying and cosmetic effects. However, some can be highly malignant.
- Squamous cell carcinoma (SSC). The most aggressive form is in the stomach, but there are highly dangerous variants of the cutaneous form that occur in the mouth, nasal passages and sinuses, bladder, and preputial and vulvar regions.
- Mast cell tumour. These tumours are mostly found in the skin or conjunctiva, but are also located in the skin of the face/head and distal limb regions.
- Lymphoma. Cutaneous forms are recognised (and these may be related to granulosa cell tumours, directly or indirectly); intestinal, mediastinal and generalised forms are encountered occasionally.
- Granulosa (theca) cell tumour. This is the most common reproductive tumour, it has well recognised clinical and endocrinological manifestations.
- Lipoma. These are rare in horses but mesenteric lipomas are common in older horses, with ponies probably being over-represented. They are invariably benign, but do have serious consequences with a high proportion of strangulating intestinal obstructions due to lipoma in older horses.
These tumour types make up over 98% of equine tumours in the horse.
There are also several less common skin tumours such as keratoma, keratoacanthoma, sebaceous adenoma and giant cell tumour. Little evidence is available on most of these. Rarer tumours include mesothelioma, haemangioma, pheochromocytoma, intestinal adenocarcinoma, basal cell carcinoma, haemangiosarcoma, seminoma, dysgerminoma.
In this section, we will discuss how to diagnose and manage the most commonly encountered tumours in horses.