The condition we call cancer is not really a single disease entity but a broad range of diseases. The thing they all have in common is that normal mechanisms of cell growth and death are disturbed. With the increased longevity of our horses, more old age problems, including cancers, are becoming apparent.

Cancer develops when cells start to proliferate in a disorganised and unchecked manner. This leads to visible “lumps” if the cancer tissue is near the surface of the body.

If the cancer is deep within the body, for instance, in the abdomen or chest, the lumps can press on vital organs, preventing them from working properly. Cancers growing in the head region of the horse rapidly cause distortion of the face due to local invasion.

Another feature of cancers of some types is the ability of the abnormal cells to spread to other sites in the body. A cancer that begins as a clump of abnormal cells in the spleen of a horse can spread around the body via the blood stream and other routes to establish small clusters of abnormal cells elsewhere. These clusters or “metastases” will grow and soon become further cancerous lumps.

Cancerous cells not only display abnormal growth, but the function of the new cells can also be abnormal and very harmful to the horse. Some cancer tissue can produce inappropriate hormones that disturb the normal hormonal balance of the animal.

An example of this situation is the excessive production of testosterone by mares with a type of tumour of the ovary known as an equine granulosa cell tumour.

This male hormone, normally present in very low levels in mares, leads to stallion-like behaviour and aggression towards other horses. Fortunately, this particular cancer type is usually amenable to surgical removal and the mare returns to normal after the cancerous ovary has been removed.

Cancers of the intestine and abdominal organs may only become apparent once the disease is in a very advanced state. Masses of cancerous cells can grow within the abdomen without any outward signs of a problem.

It is only when the cancer begins to obstruct intestines, or when it occupies so much of an organ that there is little normal tissue left, that the horse starts to show signs of disease.

In the case of abdominal cancers, the first sign of a problem is often recurrent bouts of colic, with the intervals between episodes becoming shorter.

Alternatively, horses with massive cancer infiltration of their intestines will slowly become less able to absorb nutrients from their food. Such horses will show slow but progressive weight loss, despite eating as much extra feed as you can provide.

Fortunately, some of the most common types of cancer in the horse are benign; they are non-aggressive and do not spread to other parts of the body.

Grey horses frequently have nodular masses or “melanomas” about their body. These are most frequently found under the tail and around the dock, but can also occur behind the jaw and in the eye. With the aid of instruments such as laparoscopes and endoscopes to look inside the horse, vets can often observe small melanomas within the abdomen of
greys and in the guttural pouches (throat area).

In many cases, melanomas remain small and harmless to the horse. This is contrary to the situation in humans, where a large number of melanoma cases are very serious.

However, on rare occasions something that starts as a small, innocuous black lump under the tail or below the ear will spread throughout the body and transform into an invasive, destructive cancer.

It is for this reason that vets often note the presence of melanomas in grey horses presented for pre-purchase examination and they may be a cause for concern. It can be almost impossible to predict how they will progress on one clinical examination alone.

It has been estimated that 80% of grey horses, of either gender, aged more than 15 develop melanoma, some of which will behave aggressively and not be amenable to treatment.

Many of the cancers that occur in humans are now amenable to treatment through surgery, chemotherapy and radiotherapy, and these techniques have been applied to equine cancer with considerable success.

The treatment of particular cancers depends upon the type involved. Some types of cancer cell are very effectively killed by radiation, while others are relatively unaffected.

It also depends upon the location of the cancer. A cancer that is confined to the skin can have creams applied; those deep within the chest or the abdomen are relatively inaccessible.

  • This veterinary feature was first published in Horse & Hound (26 August 04)

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