Sarcoids are a common skin tumour in horses and donkeys. They can present in many ways, occur singularly or multiple and on any part of the body. There is evidense that horses can be genetically predisposed to developing sarcoids, but sarcoids have also been linked to the Bovine Papilloma Virus.
There are 6 types of sarcoids:
Occult type: These sarcoids are flat and grey and often look like ringworm.
Verrucose type: These sarcoids are most common and may resemble warts or scars. They have a dry and scaly surface. They usually only grow very slowly, but may end up covering a large area. Trauma to the verrucose sarcoid can sometimes cause it to change into the more aggressive fibroblast-type sarcoid.
Nodular type: These are well demarcated lumps, which may be covered by normal skin or may be ulcerated. In type A the skin that covers the sarcoid isn’t involved and can move independantly from the sarcoid. In type B the skin is also part of the sarcoid, is hairless and has a grey or wart-like surface. Trauma to the nodular sarcoid can sometimes cause it to change into the more aggressive fibroblast-type sarcoid.
Fibroblast type: This more aggressive and invasive type of sarcoid has many appearances, usually with a certain degree of redness and ulceration. The fibroblast sarcoid has many little blood vessels running through it, which makes it prone to bleeding. The fibroblast sarcoid may also hang from a fine stem.
Mixed type: This type has characteristics of verrucose, nodular and fibroblast sarcoids. It may represent a tansitional phase from one type to the other.
Malignant type: The name already says it, this type of sarcoid is more aggressive in the sense that it invades the surrounding tissues. It can spread to other parts of the body quite quickly, via the lymph system. Luckily, this type of sarcoid is very rare.
The exact cause of sarcoids is still unknown. However, some horses seem to be susceptible to it while others are not. The Bovine Papilloma Virus is thought to play a role, as the BPVirus can be found in the cells of equine sarcoids.
Sarcoids may appear in any area of the body, but most commonly they appear in areas sweat easily / become moist and that don’t have a lot of fur (around the eyes, the nose, the armpits, the thighs, the legs, etc).
Sarcoids are not infectious to other horses, but a horse that has a sarcoid may develop more sarcoids over time, which may all be of a different type.
Sarcoids usually don’t cause any issues at first. As long as the sarcoids are small, they usually don’t inhibit function, they usually don’t itch and they don’t cause pain or inflammation. When the sarcoids become bigger, they can get injured or become inflamed. This may cause pain. Sarcoids may be located in an inconveneint spot (around the eyes, in the girth area, etc.), which may annoy the horse or can make the sarcoid prone to trauma.
Sarcoids may resemble a fungal infection, warts or melanomas, and should therefore be diagnosed properly. Some sarcoids (mainly the verrucose and fibroblast ones) can be swabbed for the BPVirus, but others need to be biopsied. In case of the latter, it may be preferable to remove the entire lump and send it to the lab for diagnosis, because some sarcoids may become more aggressive after trauma.
Sarcoids can be difficult to treat, because not all sarcoids respond well to therapy and they can recur. There are different therapies available, and choice should be based on the type of sarcoid, the location, and other factors. Some options include various creams, surgery, cryosurgery and immunotherapy. If surgery is considered, keep in mind that the bigger the sarcoid, the more invasive and difficult surgery may be.
Without treatment, sarcoids may stay the same for a long time. However, they may start to grow or change types at any time (more so after injury). After therapy, sarcoids can recur, and it is not possible to predict whether or when this might happen. After treatment, the area should be monitored. A horse that has a sarcoid may develop sarcoids in other areas as well.