Skin Cancer in Horses

Thankfully cancer in horses is much less common than in small animals but most tumours that do occur, around 80%, affect the skin. The most common of these are squamous cell carcinomas (SCCs), melanomas and sarcoids. 

Sarcoids are the most common skin tumour in horses. They can affect any part of the body and are very variable in appearance. Whilst rarely life-threatening they can be problematic if they grow large and interfere with tack or occur in areas prone to abrasion. They can be challenging to treat and the variety of treatment modalities reflects the lack of a silver bullet! The verrucous sarcoid shown in the images on the right responded well to immunotherapy – injection of a compound which stimulates the immune system to recognise the sarcoid as foreign. Photos show the initial lesion, after three injections with Equimmune and the same area several years later. Surgical removal is recommended in some cases but care must be taken to remove the entire lesion as there is a chance of aggravating it and making it more aggressive. 

Melanomas are most commonly seen in grey horses (affecting around 80% of geriatric greys) but they can occur in other coat colours and in fact tend to be more aggressive in non-greys. They often begin as small nodular lumps within or under the skin and usually grow slowly to begin with. Most commonly found on the face, prepuce and around the tail and anus (see the image below), over time they have the potential to spread to other sites in the body including internal organs. For this reason removal of small lesions is often recommended. Drugs such as cimetidine have been used to reduce the size of masses which are not amenable to surgical removal and off label use of a canine melanoma vaccine has shown promise in horses as well. 

Squamous cell carcinomas occur most often around and within the eyes (like the one bottom left affecting the third eyelid) and on the prepuce. They are locally invasive and usually slow to spread. Exposure to sunlight is a risk factor and SCCs occur more commonly on areas of non-pigmented skin. Sunlight protection in at risk horses is very important by providing shade, using face masks and application of sunscreen. Again, surgical excision is usually preferable but a variety of topical therapies are sometimes used in areas where this would be difficult, including cytotoxic drugs such as mitomycin C. Close monitoring is necessary as there is a higher risk of recurrence with these tumours. 

In summary, most of these cancers are treatable, either through surgical removal or medical therapy, so prompt diagnosis and treatment offers the best outcome. Carefully monitor any lumps or bumps and get your vet to check them as soon as possible.