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Cancer In Horses

Although cancer (neoplasia) is not common in horses, they can occasionally develop a local or generalised form. Common cancers in horses include sarcoids, melanomas and squamous cell carcinomas.

Cancer is a condition where normal cells in a particular part of the body start to grow or reproduce uncontrollably and develop into a lump called a ‘tumour’. Cancer is not a single disease, there are many different types of cancer.

Tumours can be benign or malignant. Generally, benign tumours don’t cause a problem, but if they continue to grow they can put pressure on surrounding organs. The cells in a malignant tumour have the ability to spread to other areas of the body causing secondary tumours (metastasis).

Here is some useful terminology to help you understand more about cancer:

  • Benign – not cancerous
  • Malignant – cancerous
  • Tumour – a cancerous lump or growth
  • Carcinoma – a tumour involving epithelial tissue (tissue that lines the cavities and surfaces of structures throughout the body) or skin
  • Lymphoma/lymphosarcoma – when cancer cells affect the lymph nodes (important for the correct functioning of the immune system and filters cancer cells)
  • Melanoma – abnormal tissue growth involving pigmented cells
  • Metastasis – when cells from a tumour spread through the body forming new tumours
  • Neoplasia – abnormal cell growth, either malignant or benign
  • Papilloma – a small benign epithelial tumour
  • Sarcoma – a tumour involving connective tissue (tissue that supports, connects or separates different types of tissues and organs of the body)

Melanoma

These are common in grey horses because of their dark skin pigmentation and are usually malignant. These manifest as firm plaques or nodules ranging in size from small to very large. As they develop they can become ulcerated and discharge. As they grow, nodules situated close together can come together to form one very large nodule.

Melanomas are commonly found under the tail and around the anus; they are also seen around the lips and eyes, under the skin around the jaw line, within the guttural pouches and on/around the penis. They can spread to internal organs and can cause very severe problems such as colic.

Treatment for melanomas is very limited.

Squamous cell carcinoma (SCC)

SCC is common in areas of un-pigmented (pink) skin due to little protection against the harmful effects of ultraviolet sunlight, especially with little hair covering, e.g. the conjunctiva of the eye and on the penis. SCC on the eyelids is common in white faced horses and horses with light skin. Tumours around the eyeball are also common in sunny areas.

They do not tend to spread to internal organs but can cause problems if they become big or ulcerated.

The treatment for SCCs varies depending on where they are and how big they are.

Sarcoid

The most common skin tumour in the horse, sarcoids are localised tumours which form from cells that grow uncontrollably; they don’t spread internally and done metastasise. However, if they are not treated or removed in the early growth stages, tumours can grow so large that they become debilitating to the horse.

There are many different treatments for sarcoids, depending on what type they are, how big they are and where they are, but treatments include chemotherapy, topical gel/cream or surgical removal. However, if not all affected tissue is removed they can return.

Lymphoma/lymphosarcoma

The lymphatic system is the body’s defence system and is part of the circulatory system. It comprises of a system of lymphatic vessels that carry fluid (lymph) towards the heart, destroys infection and produces white cells.

There are four main types categorized on location:

  • Generalized: the most common form which is involves many tumours throughout the body’s lymph nodes. Common sites are in the throatlatch, superficial inguinal (in the groin), mesentery (in the abdomen) and pectoral (in the breast). Signs include large masses on the chest, base of neck, under the jaw and throatlatch, and under the belly. Weight loss, skin ulcers leading to crusty sores.
  • Intestinal: causes malabsorption issues of the intestines. Signs include weight loss, diarrhoea and colic.
  • Mediastinal: affects the lymph nodes in the chest. Signs include coughing, increased heart rate, fluid on or within the chest.
  • Cutaneous: the least deadly of all lymphomas as it does not metastasise. Signs include the formation of tumours under the skin.

Lymphoma is usually malignant and has a low survival rate.

Other cancers include mammary and ovarian cancer in mares, and prostate and testicular cancer in geldings/stallions, however these are very uncommon in horses.

Many types of tumours are internal, and due to the large size of the horse’s abdomen new growths may take a while before they are discovered.

The most noticeable sign is rapid weight loss; if your horse is being fed and managed appropriately, but is still losing weight you should call your vet who can perform various diagnostic tests to confirm if your horse has cancer or not.

External tumours such as sarcoids or melanomas are more noticeable, so if you notice any suspicious lumps or bumps you should call your vet so they can examine them to determine if they are cancerous or not.

You should evaluate your horse’s body condition and health frequently – it is important to be aware of any lumps and bumps that have appeared as well as any general changes in your horse, his routine or demeanour. Unusual lumps or sores that do not heal should be checked out by your vet immediately.