Sarcoids in Horses: Types, Causes, Prevention & Treatment

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

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]

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]

Sarcoid Treatment

Several different sarcoid treatments exist, but horse owners should understand that none are effective in 100% of cases. Consult with your veterinarian to determine the best treatment for your horse.

The following are some of the various sarcoid treatment options available:

Immunotherapy

The Bacillus Calmette Guerin (BCG) vaccine is the most common form of immunotherapy used with sarcoids. ^[16]

This vaccine was originally designed to prevent tuberculosis but has had successful results in treating sarcoids. It is usually injected directly into the tumor, but multiple injections are needed. ^[4]

The BCG vaccine triggers an immune reaction from the horse’s body to destroy or reject sarcoid tissue. It is most often used with eyelid tumors because this vaccine will not damage the eyelid or the eye itself. ^[4]

The BCG vaccine may cause initial swelling, and there may be skin damage. In extremely rare cases, death from anaphylactic shock has occurred. Horses treated with the BCV vaccine should receive anti-inflammatory drugs before each treatment. ^[4]

Response to immunotherapy appears to depend on the anatomic location of the sarcoid, its size, the number of sarcoids, and possibly, the tumor type. Sarcoids around the eye had an 83% remission rate in one study, while those in other locations only had a 49% remission rate. ^[3]

Complete resolution of sarcoids with the BCG vaccine may take anywhere from six weeks to one year or possibly longer. ^[3] Treatment is not usually successful with multiple or large tumors. De-bulking larger sarcoids is recommended before the vaccine. ^[3]

In recent years, veterinarians have used other types of immunotherapy for sarcoid treatment, though studies are limited. One of these therapies uses autologous polymerized tumor tissue combined with tuberculin purified protein derivative. ^[3]

Chemotherapy

Chemotherapy drugs can also be used for equine sarcoids. The injectable drugs cisplatin and Mitomycin C work by interfering with DNA replication in tumor cells.

Of these, cisplatin appears to be the most promising. AW4-LUDES, also known as Liverpool cream, is a topical chemotherapy cream. ^[2][3]

One drawback to chemotherapy medications is that they can damage healthy tissue. They must be used carefully, especially over bony areas, blood vessels, or nerves. Creams and injected chemotherapy drugs may cause inflammation and scarring. ^[4]

Chemotherapy is sometimes used with other sarcoid treatments, such as surgical de-bulking, excision or sesame oil injections. Sesame oil prolongs the drug’s activity and enhances its diffusion. ^[3]

Surgical Excision

Surgical removal is usually possible with small tumors and if there is only one or a small number of sarcoids. Once the sarcoid is removed, the skin may be stitched or left open to heal.

However, half of sarcoids treated this way re-grow. ^[2][4] According to one study, most tumor regrowth occurs within six months of the surgery.

If there is regrowth, using cryotherapy, immunotherapy, brachytherapy or laser surgery is recommended as the second course of treatment. ^[3]

Cryosurgery

Cryosurgery involves putting a horse under general anesthesia and using liquid nitrogen or another freezing agent to kill sarcoid tissue.

Large sarcoids can be de-bulked beforehand for better results. The freezing agent is often injected beneath the tumor to affect the entire sarcoid. ^[3][4]

Studies show that around 70% of horses treated with cryosurgery do not have regrowth. However, swelling, increased blood flow to the area, hemorrhage, and local edema may occur after treatment and healing may take 1-3 months. Hair regrowth may be white around the tumor site. ^[3]

With multiple sarcoids or those greater than 3 cm in diameter, it is often impossible to freeze the entire tumor during one period of general anesthesia. However, when only a few sarcoids are treated, spontaneous regression of untreated tumors is sometimes seen. ^[3]

Complications of cryosurgery can occur, including damage to nearby tissues or vital structures. Facial nerve paralysis, septic arthritis, loss of upper eyelid, and eye loss have all been documented. ^[3]

Laser Surgery

This sarcoid treatment uses a C0₂ laser to cut and evaporate tumor tissue with reportedly good results.

There is minimal bleeding and usually no swelling with laser surgery. The resulting wound can either be closed or left to heal. However, healing may be slow, and scars will likely form. ^[3]

The equipment needed to perform laser surgery is expensive and requires specialized training. Because of this, laser surgery is often limited to university veterinary hospitals and a few specialized private practices. ^[3]

Electrochemotherapy

Electrochemotherapy involves injecting cisplatin into the sarcoid, followed by application of high-voltage electric pulses to increase drug concentration in the cells.

Due to the electric shock, this procedure is done under short-term general anesthesia. ^[2]

Photodynamic Treatment

This treatment involves applying a chemical to the surface of the tumor and then exposing it to a specific type of light. The light activates the chemical and kills tumor cells. ^[2]

There is limited research on photodynamic treatment for equine sarcoids, but it has emerged as a successful treatment in people with locally invasive tumors.

Banding

This treatment may work with sarcoids that have a short stalk or neck. Banding involves applying a tight band around the base of the tumor to cut off the blood supply.

This causes the sarcoid to die and fall off within 10 – 14 days.

This treatment method is helpful for short-term control of large sarcoids on the limbs or abdomen but does not usually resolve the problem. ^[4]

Radiation Therapy

Radiation therapy is used at some veterinary teaching hospitals or specialized equine clinics. This treatment involves trimming the sarcoid to skin level and delivering a beam of radiation to the area while the horse is anesthetized. ^[10]

Radiation treatments are usually performed every week for four weeks as an outpatient procedure. It does not cause discomfort for the horse, but white hair often develops at the radiation site. ^[10]

Interstitial Brachytherapy

Implanting permanent isotope seeds of radon or gold or removable isotope needles of radium, cobalt, or iridium is another sarcoid treatment.

This technique delivers a high dose of radiation to the tumor while sparing nearby healthy tissue. It is preferable to BCG immunotherapy for tumors around the eyes as it carries few side effects. ^[3]

Interstitial brachytherapy should be considered for aggressive or surgically inaccessible sarcoids. It can also be combined with surgical de-bulking or hyperthermia for a better outcome.

This treatment may not be effective on large tumors and comes with risks such as local necrosis or infection. ^[3][11]

Interstitial brachytherapy also carries risk for the surgeon as it can result in radiation exposure. Special equipment is needed, making this a costly treatment. ^[3]

Hyperthermia

Radio frequency current-induced hyperthermia is another treatment option that has shown some success when repeated at intervals of one to several weeks.

However, it is only used on a limited basis. This treatment heats sarcoid tissue, burning it to kill off tumor cells. ^[3]

Topical Creams

In one study, a topical sarcoid treatment known as acyclovir led to sarcoid clearance in 68% of horses and significant regression of sarcoids in 32% of horses. This medication appears to work better on smaller lesions. ^[12]

Bloodroot extract comes from a flowering plant called Sanguinaria and exhibits cytotoxic and immune-modulating effects. It has been used to treat skin growths since the mid-19th century and has also been used in cancer treatment in human medicine. ^[13]

One study showed that the bloodroot extract marketed under the name XXTERRA had a 93% success rate in treating sarcoids. Side effects included hair loss with lesions on the head and neck and soreness with sarcoids on the trunk and legs. ^[13]

Another study on a different bloodroot extract product (Newmarket) revealed that 89% of horse owners saw improvement in sarcoids while 66% saw the complete resolution of lesions. Bloodroot extract products also seem to work better on smaller sarcoids. ^[13]

Application of another topical product (imiquimod) resulted in 80% of sarcoids having a 75% or greater reduction of tumor size by the end of a trial. The most common adverse effect with this topical treatment included oozing, reddening, depigmentation, and hair loss around the tumor and adjacent areas. ^[14]

Natural Remedies For Sarcoids

Many horse owners have used natural remedies to address sarcoids, though there are no formal studies on most of the purported remedies.

Some of these remedies include:

• Homeopathic treatments such as Thuja, Causticum, and Sulphur are given both internally or used topically. ^[15]
• Turmeric is a traditional Indian spice that contains curcumin, which has been shown to support the immune system. It is fed to horses with sarcoids and used topically in a paste.
• Chinese Herbal Formulas containing various adaptogens are used to stimulate the immune system in horses with sarcoids. ^[15]
• Mistletoe (Iscador P) extract has been safely used in Germany as a human cancer treatment and has also been shown to reduce or eliminate sarcoids 41% of the time. It is administered as an injection into the lesion several times a week. ^[15]
• Mushroom Matrix formulas created from a blend of medicinal mushrooms are a newer and lesser-known treatment for sarcoids. Mushroom extracts are purported to work by boosting the immune system.

Recurrence and Spontaneous Resolution of Sarcoids

As noted above, many horses successfully treated for sarcoids have a recurrence of the condition. Anywhere from 49-64% of equine sarcoids recur within three years of surgical removal. ^[6]

This is because the virus that causes sarcoids becomes incorporated into the DNA of infected skin cells, causing them to develop into tumor cells. This also happens with the papillomavirus that causes cervical cancer in women. ^[2]

After treatment, new sarcoids may regrow more rapidly than the initial tumor. Less severe sarcoids can also change into more aggressive types. ^[6]

Sarcoids can also undergo spontaneous regression in which they resolve without medical intervention.

A Swedish study showed that one third (33%) of affected horses became free of sarcoids without any treatment. Occult sarcoids were the most likely to undergo spontaneous regression. ^[18]

Summary

Sarcoids are one of the most common forms of skin cancer in horses, but these tumors are mostly benign.

Consult with your veterinarian to determine the best treatment plan for your horse. The recommended treatment will depend on the location, the extent of the lesions, your budget and the horse’s temperament. ^[2]

Sarcoids occurring around the eye can only be safely treated with certain methods. Sarcoids around the knees or hocks may be challenging to treat so as not to damage nearby vascular, bony, or synovial structures. ^[3]

Whichever sarcoid treatment you elect to use, it may take months to resolve lesions and sarcoids may return after treatment. If they do recur, a new treatment is likely needed. ^[4]

Make sure your veterinarian is aware of your intended use for your horse so this can be factored into your sarcoid treatment plan.