Of the many skin conditions horses can develop, sarcoids are the most common. The term “sarcoid” was first used in 1936 in South Africa as a way to distinguish this skin lesion from other tumors. [1]

Sarcoids are benign (nonmetastatic) skin cancer believed to be caused by papillomavirus infection. They affect up to 11.5% of all horses. [6]

Sarcoids usually appear as rough, raised, hairless patches or nodules on the skin that are not painful or itchy. Some sarcoids are protruding, moveable masses with overlying skin still intact. [2][3]

The tumors are cosmetically unappealing and, depending on their location, can interfere with the function of the horse. But in most cases, the prognosis for affected horses is very good and some sarcoids resolve without any intervention.

Effective treatments are available, including surgical removal, immunotherapy, cryotherapy, and laser surgery. However, sarcoids have a high rate of recurrence in horses.

Sarcoids in Horses

Types of Equine Sarcoids | Mad Barn Canada

Sarcoids are non-malignant fibroblastic, wart-like tumors that form on the horse’s skin. Horses usually develop lesions in multiple locations, but single lesions can also occur.

Equine sarcoids can grow up to 8-10 cm in diameter, but most are smaller than this. They may cause irritation, interfere with tack, or bleed if rubbed. They can also attract flies and become infected. [4]

Sarcoids are commonly found on the horse’s head (especially around the eyes), chest, ears, lower limbs, beneath the abdomen, or around the sheath area. Sarcoids on the head tend to be smaller, while those on the extremities are typically larger. [3][5]

Risk Factors

Sarcoids are most common in equines between 3-6 years of age, but they have been seen in horses as young as yearlings and senior horses in their thirties. Young male donkeys and stallions appear to be at increased risk for sarcoids. [3]

Quarter Horses appear to have almost twice the risk of being affected by sarcoids than Thoroughbreds. Standardbreds have a lower risk of the condition when compared to all other breeds. [3]  This suggests genetics may influence susceptibility to sarcoids.

Additionally, there appears to be a link between sarcoids and skin trauma. [6]

Cause of Equine Sarcoids

Sarcoids are associated with immune function, but environmental and genetic factors appear to play a role as well. [3]

Researchers believe the tumors are triggered by bovine papillomavirus (BPV), either type 1 or 2. Papillomaviruses are usually species-specific, but they can also cause cross-species infections. [2][7]

It is unclear how horses are infected by BPVs, whether by exposure through environmental contamination or biting flies. Studies trying to answer this question have not produced consistent results. [7]

Bovine Papillomavirus

Most equine sarcoids contain BPV1 or BPV2 DNA. [17] However, papillomavirus can also be detected within the skin of horses without sarcoids.

For this reason, researchers believe a horse must be genetically susceptible for the virus to produce sarcoids. [7]

Interestingly, the bovine papillomavirus DNA in sarcoids differs between countries. For example, one study showed that around 90% of sarcoids in New Zealand horses contain BPV2, and only 2% contain BPV1.

These percentages are similar to Canadian horses. In Europe, however, the majority of sarcoids contain BPV1 DNA. [7]

Some sarcoids do not contain either BPV1 or BPV2 DNA, suggesting they can also be caused by another type of papillomavirus. Identifying the exact cause of equine sarcoids is important for developing vaccines. [7]