Equine Sarcoids Explained

by Dr. Owen Schumacher

The most common type of equine skin tumor is called a sarcoid. They account for 15 to 65% of all neoplasms in horses and are far more prevalent than other common equine neoplasms, like squamous cell carcinoma, melanoma, and papilloma.

Sarcoids are classified on the basis of clinical appearance; this classification tends to correlate with the biological behavior of the individual tumor.

Sarcoids display a wide variety of clinical forms and also affect other equids, including donkeys, mules, and zebras.

6 Distinct Types of sarcoid Based on Gross Appearance & Behavior:

1. Occult
2. Verrucous
3. Nodular
4. Fibroblastic
5. Mixed
6. Malevolent/malignant

Occult (flat) sarcoids are roughly circular, non-raised areas of hair loss and hyperkeratosis (thickened skin). They most commonly arise around the mouth, eyes, neck, and other relatively hairless areas. They often start with subtle changes in hair color & quality. They can spontaneously regress, remain static for years, slowly enlarge, or progress to the verrucous form.

Verrucous (warty) sarcoids have a wart-like appearance and most often arise on the face, body, and sheath. They occur less commonly on the limbs, but are sometimes found on coronary bands. Though they are slow growing, they can become extensive and are often bordered by a perimeter of thickened skin.

Nodular sarcoids tend to affect a horse’s eyelid, sheath, or groin. They are firm, well-defined, movable subcutaneous nodules. The number may vary, and there can be as many as several hundred in one area.

Fibroblastic (nodular and ulcerated) sarcoids often look like exuberant granulation tissue (proud flesh). They can develop at the site of a wound or where another type of sarcoid – like the occult or verrucous variety – has been injured. They can develop anywhere but are most common in the groin, eyelid, and distal portion of the limb. These tend to be locally invasive, and are commonly complicated by flies and bacterial infections.

Mixed sarcoids are relatively common and contain any combination of occult, verrucous, nodular, & fibroblastic-type tumors. They may occur spontaneously or in association with wounds.

Malevolent (malignant) sarcoids are the last and most aggressive form of sarcoids. These are a rapidly growing, invasive mix of nodular and fibroblastic sarcoids. They infiltrate locally along fascial planes, blood vessels, and lymphatic vessels. They can invade local lymph nodes, but metastasis is rare. These have a high incidence of recurrence after treatment.

Etiology & Predispositions

The pathophysiology, or cause, of sarcoids is complex, multifactorial, and not fully understood. They are likely caused by a complex association between bovine papilloma virus (BPV), inheritable genetic traits of the horse, and environmental influences.

Quarter horses, Appaloosas, Arabians, and Thoroughbreds are at the highest risk of developing sarcoids, whereas Standardbreds are at a lower risk. Sarcoids have been diagnosed in yearlings, but the tumors are usually first identified when horses are 3 to 6 years old. The risk of occurrence declines when horses reach about 15 years of age. A familial predisposition has been described. In one report, all five descendants of a sarcoid-affected mare also developed sarcoids, whereas none of the unrelated horses on the premises developed these tumors.

Bovine papilloma viruses’ role in sarcoid development is not fully understood. The DNA of the papilloma virus has also been detected on the skin and blood of healthy horses. Experimental inoculation of horses with the papilloma virus DNA has resulted in sarcoid-like tumors, but the majority of these lesions regress spontaneously.

A vector, such as a fly, may explain the predisposition for tumors to develop around the eyes, the distal aspect of limbs, and the ventral aspect of the body. BPV DNA has been detected in flies collected near sarcoid-affected animals. Although there is no clear evidence of equid-to- equid transmission, a population of donkeys that had no sarcoids housed in close proximity to an unrelated population of sarcoid affected donkeys had an increased risk for developing sarcoids.

Overall, the pathogenesis of sarcoids is poorly understood, but given the genetic and breed predispositions, sarcoids most likely develop as a result of a combination of host, viral, and environmental factors.

Diagnosis

Sarcoids most frequently occur on the head, neck, the distal portion of the limbs, and the ventral aspect of the body. They may occur at sites of previous injury and progress spontaneously into a more aggressive form.

Each form of sarcoid has quite a few differentials based on gross appearance of the sarcoid, therefore, an accurate diagnosis of sarcoids cannot be based on clinical appearance alone. In one study of 345 equids, there was found to be a 31% error in presumed diagnosis of sarcoid. Since other tumors and inflammatory diseases can appear similar to sarcoids, biopsy for histological examination is required for confirmation. It is important to be aware that biopsies can exacerbate previously inactive sarcoids (especially the occult or verrucous forms).

Treatment

Horses are notoriously difficult to treat for sarcoids. The large number of available treatment options reflects the variable success, as well as the high cost of certain treatments. It is important to note that not all horses affected with sarcoids need to be treated. Some sarcoids may remain quiescent for years or even spontaneously regress. These treatment options are for the more severe forms.

1. Surgical excision without adjunctive therapy has a recurrence rate of 15 to 82%. The likely reason for this is that sarcoids project fronds of malignant cells into the surrounding normal tissue, leading to incomplete excision. If wide margins are obtained and other therapies are also employed, surgical excision can be a good option for some horses with sarcoids.

2. Cryotherapy (freezing) is historically one of the most used methods for treatment of horses for sarcoids and one of the most readily available options. In the short term, the success rate has been reported to be 60 to 100% with cryotherapy. More recent studies, with longer follow-up times, however, have shown a recurrence rate of 91% for tumors treated with cryotherapy.

3. Chemotherapeutics such as intralesional injections of cisplatin have resulted in resolution of up to 80% of sarcoids. 5-Fluorouracil can be applied topically or injected into the tumors as well. One study showed resolution of sarcoids in nine of 13 horses treated with intralesional 5-Fluorouracil.

4. Topical immune response modifiers such as Imiquimod 5% cream (Aldara) is a topical immune response modifier and has antiviral and antitumor activity. It has been successfully used to treat basal cell carcinoma and genital warts in people. In one study, imiquimod therapy resulted in over 75% size reduction in 12 of 15 sarcoids. Topical application of acyclovir is another treatment option. Acyclovir is an antiviral medication used to treat people for herpes virus infection. One study showed that it decreased tumor mass in 100% of sarcoids, and 68% of those sarcoids regressed completely.

5. Immunostimulation: because spontaneous regression of untreated tumors has been reported, immune recognition likely plays a role in regression. In one retrospective case series, the odds of failure of treatment for sarcoids was significantly lower if the patient had received conjunctive treatment with immunotherapy. A variety of immunostimulants have been used, and multiple intralesional injections are required.

6. Electrochemotherapy is performed at referral centers and involves the combined use of chemotherapeutic drugs and high voltage electrical pulses. These pulses increase the intracellular concentration of the chemotherapeutic agent, which improves the efficacy of the chemotherapeutic agent. In two studies, 92 to 100% four-year resolution rates were reported using electrochemotherapy, either alone or in combination with surgical debulking. This treatment requires that the horse be anesthetized because the electrical pulses are not tolerated by conscious horses.

Summary of Treatment

Sarcoids are known for their high rate of recurrence, and it's not uncommon for new tumors to appear at different sites even after treatment. The key to managing them effectively lies in early detection, consistent monitoring, and timely re-treatment when needed. If you suspect your horse may have a sarcoid and aren't sure what steps to take next, give us a call at Oakhurst Equine Veterinary Services. We're here to help guide you through the best options for your horse’s care.