Sarcoids are the most common skin tumour in horses and ponies and, although they may look like warts, they are locally destructive and are therefore considered by many vets as a form of skin cancer.
They are most often found on the abdomen, inside the back legs, around the sheath, on the chest and around the eyes and ears. They also often appear at the site of old scars, particularly on the legs. These are sites where flies typically congregate and insect transmission is thought to be involved in the development of the condition.
Young to middle-aged horses are most commonly affected and there may be a genetic predisposition. It is a condition that is unique to horses.
Sarcoids can appear singly as tiny lumps or in clusters. As they enlarge, the skin may ulcerate and become infected. In summer, they attract flies and can end up as open sores which will not heal.
When buying a horse look out for any skin lumps, even if you are told they are ‘just a wart’. If you have any concerns, discuss them with your vet. Sarcoids are always a concern at a pre-purchase examination and the veterinary advice will depend on a number of different factors.
If you buy a horse with sarcoids, they will be excluded by insurance companies as a pre-existing condition and treatment can be costly.
The appearance of sarcoids can vary considerably. In the early stages they may appear innocuous and are sometimes missed completely if they are concealed within the hair coat.
Some sarcoids may look like smooth, nodular skin lumps, especially in the early stages, while others are irregular and roughened form the start. The lumps frequently become larger, irregular in shape and cauliflower-like in appearance. Some will ulcerate and become aggressive at which stage they are described as fibroblastic or malevolent sarcoids.
Sarcoids can also appear as flat, slightly bumpy areas of skin with a dry, scaly appearance. This verrucose form of sarcoid is sometimes mistaken for ringworm, but it never clears up. Such plaques are often found on the neck and inner thigh. In time, they may develop into other forms of the tumour.
Horses do not die of sarcoids, but some are destroyed because the sarcoids prevent them from either working or enjoying a good quality of life.
Types of sarcoid
Occult: a flat patch of hair loss with a grey, scaly surface, which can be confused with ringworm, as they are often circular. Common on the face, neck and between the back legs
Verrucose: wart-like, grey and scaly but extends deeper than the occult sarcoid. More irregular in outline; multiple lesions often appear
Nodular: lumps under thin and shiny skin. These vary in size, some being more than 5cm in diameter, and occur commonly around the groin and eyelids
Fibroblastic: aggressive fleshy masses They can begin as a complication of a skin wound and sometimes grow rapidly, often ulcerated and “hanging” on a stalk (pedunculated) or extremely invasive into the surrounding skin
Mixed: a variable combination of two or more types of sarcoid, often of different ages, forming a “colony”
Malevolent: A term used to describe the most aggressive type of sarcoid. These spread through the skin and even along lymph vessels, with cords of tumour tissue interspersed with nodules and secondary ulcerative lesions. They can become large and difficult to manage
Treatment for sarcoids
The current recommendation is that even a single sarcoid must be taken seriously, as there is a risk of others growing elsewhere on the horse’s body. Early treatment is usually most effective, before they become large and more difficult to remove.
Although several treatments are available, there is no magic cure and there is always a high risk of recurrence. Treatment will depend on the position, size and number of sarcoids. Overall the porognosis is poor because of their tendency to recur despite treatment.
Treatment options include:
Surgery: The disadvantage of removing the tumour surgically is that there is a high failure rate – the wound often heals poorly and the sarcoid frequently recurs. If this option is chosen a wide surgical margin is important
Ligation: This involves applying a tight band around the base of the tumour. While it will work for some horses, there is a high risk of leaving tumour cells behind to grow back and it can be painful for the horse, especially if the sarcoid is in a delicate area
Cryotherapy: The tumour can be frozen to destroy it, but it often requires repeated lengthy treatments and often general anaesthesia for treatment to be carried out safely
Immune therapy: This method involves injecting the horse with substances, such as BCG, to stimulate the immune system to eliminate the tumour. It can work well for sarcoids around the eye, but several treatments are needed, often under heavy sedation. There is a reported risk of reaction to this treatment, so premedication is routinely given to reduce this risk
Topical treatment: This involves special creams. In the UK a heavy metal preparation is widely used from at the University of Liverpool and results show that the creams work well in some cases, particularly the smaller superficial lesions
Radiation therapy: This has been shown to be effective. Unfortunately, the danger of radiation makes the treatment expensive and, again, general anaesthesia is required. Radiation therapy is only available at certain specialised centres because of the technical difficulties involved
Laser removal: A relatively new development, a surgical laser is used to remove the tumour using standing sedation with local anaesthetic or under general anaesthetic
Whatever treatment option is selected it is essential that treatment is continued until there is an effective response. If treatment is stopped before the sarcoid has been eliminated, there is a strong risk of recurrence, sometimes with a worse lesion than was originally present.
Research is being done, which indicates that bovine papillomavirus may involved in the development of equine sarcoids. It is possible that a vaccine will be made available in the future.