Tim Mair FRCVS looks at the most prevalent types of cancer in horses and how they are treated…
1. Cancer in horses: Sarcoids
What is it? Sarcoids in horses, although a benign cancer, can be aggressive and spread locally. There are various different forms of sarcoid, each having a characteristic appearance, and individual sarcoids can change from one form to another. Bovine papillomavirus is believed to be involved with the development of sarcoids, although its precise role is unknown. The number and type of sarcoids that individual horses develop varies considerably, but in the majority of cases they tend to get larger and more numerous with time.
How is it treated? There are many different treatments available for sarcoids. The choice of treatment will vary depending on the type of sarcoid, its size and location.
Sarcoids in certain locations, such as around the eyes, can be more difficult to treat. Treatment may include chemotherapy drugs (often applied topically), radiotherapy and surgery (including laser surgery and cryotherapy). A recently trialled treatment that can be useful in some cases is electrochemotherapy, which involves injecting a chemotherapy drug into the sarcoid and then applying an electric current that drives the drug into the sarcoid cells. This type of treatment must be done under general anaesthesia and often needs to be repeated at a later date.
Laser surgery can also be an effective form of treatment, and can sometimes be undertaken in standing, sedated horses using local anaesthesia. Its advantages over conventional sharp surgery include less bleeding, less swelling and less pain.
Don’t wait and watch. The bigger the sarcoids become the more difficult they are to treat. Some chronic and extensive sarcoids may be impossible to treat. Unfortunately, there is always a risk of recurrence of sarcoids following treatment; once a horse has developed a sarcoid it will be prone to getting more in the future, and repeated treatments may be necessary.
2. Squamous cell carcinoma
What is it? This cancer usually affects the eyes and eyelids, and the penis and prepuce of males. It is an invasive form of cancer in horses that is often slow-growing but later can become more malignant and spread internally.
How is it treated? Effective management relies on early diagnosis and treatment, which may include surgery, chemotherapy or radiation therapy.
The penile form of this cancer is most common in older geldings. Regular sheath inspection, if necessary using sedation, is important to identify and treat early lesions before they become extensive.
3. Melanomas
What are they? Melanomas are skin tumours which are more prominent in grey and cremello horses. They become more common with age and it is estimated that at least 80% of grey horses over the age of 12 years will have melanomas. They most commonly involve the tail and perineum (around the anus and vulva in mares), the sheath of males, the eyelids and the mouth.
How are they treated? In young horses, most melanomas behave as benign cancers but with age they are more likely to become malignant and can then spread internally. For this reason, removal (with laser surgery) is advised when the melanomas are small. There is a vaccine that has recently been launched to treat horses with melanomas, but currently there is limited information available about its effectiveness.
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