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

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]

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Types of Sarcoids in Horses

Verrucose

This type of sarcoid is slow-growing and appears as a flat, scaly tumor with an irregular surface. It may be mistaken for a scar, ringworm, or wart and is often seen on the face, trunk, or sheath area.

Verrucose sarcoids are the least aggressive but can develop into a more serious form known as fibroblastic sarcoids. [2][4][6]

Nodular

Nodular sarcoids form a noticeable lump, which may be covered by normal skin or ulcerated. This type of sarcoid is round with a narrow, stem-like base.

These sarcoids grow moderately quickly but can change over time. [2]

There are two sub-types of nodular sarcoids:

  • Type A, which lie entirely under the skin
  • Type B, which protrude from the skin

Nodular sarcoids range from 0.5 to 20 cm in width, and a horse may have one or hundreds of them. They are often found in the groin, sheath, or eyelid areas. [6]

Interfering with this type of sarcoid can lead to rapid growth and possible transformation to a fibroblastic type. [6]

Fibroblastic

Fibroblastic sarcoids are aggressive and fast-growing. They are locally invasive, possibly reaching the tissues beneath the skin. [2][6]

These types of sarcoids are irregular in shape, fleshy, raised, and firm lumps that often occur in clusters of different sizes and shapes. They are usually smooth and hairless, but smaller ones may be covered with normal-looking skin.

Fibroblastic sarcoids frequently become ulcerated and bleed as well. [3][6]

As with nodular sarcoids, there are two sub-types of fibroblastic sarcoids: [6]

  • Type 1 have a stalk with a slight base that can be felt in the skin
  • Type 2 is broader-based without a recognizable stalk structure, usually with an ill-defined margin

Fibroblastic sarcoids are often found around the groin, eyelid, lower limbs, coronary band, or at the site of a skin wound or trauma.

Despite their aggressive appearance, they do not metastasize. However, they can spread to local areas and often have a poor prognosis for treatment. [6]

Occult

This is the most stable form of sarcoid. Occult sarcoids occur around the mouth, eyes, neck, as well as other relatively hairless areas on the body such as the inside of the forelimbs, armpits, and thighs.

Occult sarcoids are usually small, flat, and slow-growing. [6][8] They may not change for years.

However, in some cases, they may transform into verrucose or fibroblastic types. [6]

Mixed

Mixed sarcoids are a combination of several types of sarcoids and are quite common.

Mixed sarcoids likely represent a progressive or changing state between verrucose, occult, nodular, and fibroblastic types. [2][6][8]

Malignant

Malignant sarcoids are the most severe form but are also very rare. They can infiltrate the lymph vessels, resulting in nodules and cords of palpable and visible tumors. [6]

Malignant sarcoids often form on the jaw, face, elbow, or medial thigh area and can rapidly spread over the body. They may develop after trauma or failed sarcoid treatment. They may also ulcerate and bleed. [6]

Diagnosing Equine Sarcoids

If you suspect your horse has sarcoids, getting an accurate diagnosis from your veterinarian is important. Your vet will ensure that your horse is not affected by a different type of tumor, habronemiasis (summer sores), or another skin disorder.

Veterinarians sometimes disagree on how to diagnose sarcoids. A skin biopsy can provide a definitive diagnosis, but it can also cause trauma to the area, which may stimulate the growth or spread of sarcoids. [3][9]

Some researchers recommend that a diagnosis be made on clinical appearance alone, and a biopsy only performed if the diagnosis is uncertain due to atypical appearance or the location of the lesion. [9]