The occurrence of cancer is relatively rare in horses. Of those cancers that do occur, the equine sarcoid accounts for a third of all the reported skin tumours. This tumour is unique to the horse and is highly variable in appearance and behaviour. It is a benign type of cancer that does not spread to internal organs, yet can be locally aggressive, compromising sensitive tissues.
Although sarcoids are not fatal, their presence can affect a horse’s value and function.
The cause of sarcoids is multifaceted with both genetic and viral components having been shown to play a role in their appearance. This tumour “flies under the radar” of the horse’s immune system so all elements which influence the horse’s immunity must be considered when addressing sarcoid development and treatment. These include, but are not limited to, nutrition, environment, stress, and emotional makeup of a horse.
Sarcoids typically occur in horses, between the ages of three and six but can occur at any age. Although they can occur anywhere on the horse’s skin, they are more common on the face, especially around the eyes and mouth, the neck, groin, sheath and legs. They present themselves in many different ways.
The occult sarcoid is a circular or oval, flat area of hairless, thickened skin. This form commonly occurs on the side of the head or neck. Most often this superficial tumour remains quiescent for many years and is best left untouched. It can be mistaken for ringworm. If aggravated it has the potential to develop into one of the more serious types of sarcoids.
The verrucous form has a roughened grey surface, and often resembles a wart. They flake off easily and can transform into more dramatic forms of sarcoid if aggravated.
The nodular form of sarcoids are freely movable tumours of variable size under the surface of the skin. These are not troublesome until they begin to ulcerate. Common sites for the nodular or verrucous sarcoids are the head, sheath, or groin.
The “fleshy” or fibroblastic sarcoids look like exuberant granulation tissue, or proud flesh, and may grow slowly or rapidly. They are cauliflower-like fleshy masses that bleed easily, smell putrid, and often have ulcerated surfaces. They prefer the leg, eyelids, and sites of previous injury. It is the most aggressive form of tumour.
Sarcoids are unpredictable in all aspects of their development and treatment, and so each needs to be assessed on an individual basis. Their variable nature makes them a therapeutic challenge for both owners and veterinarians.
Time is often an initial approach strategy, particularly when the sarcoid is not bothersome to the horse. If given time some equine sarcoids can spontaneously resolve.
Under certain circumstances these tumours can change, becoming unsightly, an annoyance, or interfering with sensitive tissue such as an eyelid or a joint.
Whenever a treatment protocol is undertaken, sarcoid type, behaviour, and location are considered. Patient and owner compliance, budget, and clinical experience of the veterinarian and available modalities are all equally important to the treatment method selected.
Success rates are variable with all methods of treatment. Sarcoids may reappear for all treatment options, even after apparently successful treatment. They behave similar to a weed — as long as the roots remain within the skin and the horse’s immune system is compromised, the sarcoid continues to grow. As such surgical removal may only prune the sarcoid, with its rapid and often more aggressive return in 50 per cent of the cases. As a result surgical removal or debulking is generally combined with other therapeutic methods. Cryotherapy, thermotherapy topical and intralesional chemotherapy are considerations, each with their advantages and disadvantages.