Q: I bought my horse Dallas three years ago. At that time, I noticed a circular hairless area, about the size of a 50-cent piece, on the right side of his neck. My veterinarian thought it was ringworm, and we treated it accordingly, but the spot remained. 

From that point on it has been one failed treatment after another. We have tried fluoride washes, Betadine scrubs, medicated shampoos, various fungal creams, etc. We even kept the area shaded to see if it was being irritated by the sun. Nothing we tried seemed to affect it at all—in fact, it’s gotten bigger. Now the hairless area is about the size of my palm, and it has bumps that resemble minute cauliflower-like growths.

The area doesn’t seem to be causing Dallas any discomfort, but I am at my wit’s end. I would appreciate any insights you might be able to offer on what this skin condition might be and how to cure it.Linda Gilden; Fillmore, California

A: Based on your photo and description, I suspect that your horse has a “mixed” form of sarcoid. If so, you’re in good company. Sarcoids are the most common skin tumor in horses. Fortunately, they are usually benign, affecting only the local area of skin. They do not metastasize.

Sarcoids occur in horses of any age, with a higher incidence among those younger than 7 years, and of any breed, although they do appear to be more common in Quarter Horses, Appaloosas and Arabians. There is no predilection for the condition based on a horse’s gender, color, geographic region or season of the year. Sarcoids can occur anywhere on the body, but they are usually found on the legs, head, neck and the underside of the abdomen.

There are five types of sarcoids:

  • The occult form is characterized by circular areas of hair loss with a gray, scaly surface. It most commonly occurs on the inner thigh, sheath, neck and face.
  • The verrucous form has an ill-defined margin and a rough appearance with alternating, irregular, thickened areas and flat, scaly areas. It is most commonly seen in the hollow under the elbow or in the groin.
  • The nodular form appears as nodules under intact, normal-appearing skin. These are most common in the groin and around the eye, but they can occur elsewhere.
  • The fibroblastic type is the most aggressive and locally invasive form. The skin is often ulcerated, with the appearance of granulation tissue, and susceptible to trauma. These can occur anywhere on the body.
  • The mixed form is a combination of two or more of the previous types. These are most commonly on the face, in the hollow under the elbow or in the groin.

The cause of sarcoids is unknown. However, strong evidence suggests that the bovine papilloma virus is involved. While the exact route of transmission is not fully understood, direct contact, insects, contaminated brushes, tack and needles have been implicated.

Treatment options depend on the location, number and type of tumor. Some can be left alone and watched for change, provided they are not in a location susceptible to trauma, such as the girth region. Surgical excision is still the “gold standard” for sarcoids that are small enough to be removed completely without leaving too large a wound. For those that cannot be excised, treatment options include cryotherapy (freezing), radiation therapy, chemotherapy and immunotherapy. The use of papilloma virus vaccination or autogenous sarcoid vaccination is being investigated. Keep in mind that sarcoids are likely to recur or develop at other sites in horses who have multiple tumors. Rarely do they regress spontaneously.

Getting back to your horse, I would suggest that you have your veterinarian take biopsies of several areas within the hairless region—from both pigmented and nonpigmented skin, including the “wartlike” areas—to confirm the diagnosis and determine the tumor type or types. (The lesions in the pink-skinned area could be another type of tumor known as a squamous cell carcinoma. This is not a typical location for a squamous cell carcinoma, but it is a remote possibility.)

Once you know exactly what type of tumor you are dealing with, appropriate treatment can begin. Assuming that this is a sarcoid, and based on its appearance and the fact that it has been growing, I suspect topical chemotherapy may be warranted.

Steve Adair, DVM, DACVS
University of Tennessee, College of Veterinary Medicine
Knoxville, Tennessee