Skin Tumors in Horses

Tumor perianal melanoma
An example of a large, multinodular advanced stage malignant melanoma, under the tail of a horse.  Note that the tumor is ulcerated and draining in several areas.  This is, unfortunately, a very common appearance of this tumor, with large, dark confluent nodules.

 (Photo courtesy of Dr. Ken Sullins, Marion DuPont Scott Equine Medical Center, Leesburg, VA)

Tumor large corneal SCC
This squamous cell carcinoma of a horse protrudes from the eyelid. This tumor has been found to be more common in horses with pink skin around the eyes, and can also be found on the prepuce/penis and vulva in affected horses. (Photo courtesy of Dr. Ken Sullins, Marion DuPont Scott Equine Medical Center, Leesburg, VA)

Tumor mb
Equine sarcoids, such as this tumor on the eyelid/face, can have several different appearances, including flat/wart-like, ulcerated, or protuberant.  They may have variable amounts of pigment and need to be biopsied to differentiate them from other skin tumors. (Photo courtesy of Dr. Ken Sullins, Marion DuPont Scott Equine Medical Center, Leesburg, VA)

Some general principles should guide owners and veterinarians when encountering horses with ?lumps and bumps? suspected of being tumors.

Any persistent and unexplained bump on the skin should be considered a neoplasm until proven otherwise! Areas on the skin that are ulcerated and persistently bleeding may be sites where tumors are growing. Owners need to call these bumps to the attention of their veterinarian, who can then examine the horse (the whole horse, of course) and the bump.

To definitively identify whether a bump is a neoplasm, the vet will need to biopsy it, by surgically removing a small portion of the bump, placing it in a fixative solution that will preserve it, and sending it to a laboratory for processing and diagnosis. Depending on the size and location of the suspect neoplasm, some practitioners may elect to remove the entire mass (called excisional biopsy) and trim a portion of it for evaluation by the pathologist, before placing it in fixative solution and submitting it for diagnosis.

Would you recognize skin tumors on your horse?

Here's help in understanding them, how they are diagnosed, and how some may be treated....

The terms tumor and neoplasm are interchangeable. Neoplasm literally translates as new cells.  All neoplasms share common characteristics, including uncontrolled and progressive growth if untreated.  All neoplasms arise when genes in cells are damaged (mutated), allowing cells to escape normal processes that limit cell growth and lifespan. 

Horses, as a species, develop fewer neoplasms than dogs, cats or people.  Common neoplasms affecting people, including breast and prostate tumors, and neoplasms of the stomach and intestinal tract, are so uncommon in horses that there is very little written about them in the veterinary medical literature.  Malignant lymphoma is often seen in dogs, cats and people, but rarely in horses.  Reasons have been debated, but not resolved.   One possibility, which needs to be more fully studied, is that horses may be less susceptible to changes in genes (mutations) than dogs, cats or people.  Horses may also be less susceptible to the effects of cancer-causing chemicals, or have less exposure to these harmful substances, than other animals.  Understanding why horses do or do not get tumors may be important for understanding how to detect and control tumors in all species; research in this area is called "comparative oncology."

Horses do get skin tumors and they get a lot of them. 
Common types:

  • equine sarcoid
  • malignant melanoma
  • squamous cell carcinoma

Equine sarcoid is considered to be a benign neoplasm.  Benign tumors tend to grow locally, although some of them may show a pattern of growth that involves sending "tendrils" of tumor tissue into the skin distant from the main body of the tumor (this is termed 'infiltrative growth').  Sarcoids are made up of a population of elongate and interwoven tumor cells, usually growing just under the superficial layer of the skin.

When seen by owners, sarcoids may be flat, firm thickenings in skin, or have either a wart-like or domed appearance.  Sarcoids may grow slowly or they may grow aggressively.  Some published studies have found that sarcoids may be more common in Quarter Horses than in either Thoroughbreds or Standardbreds.  Research done in our laboratories showed that sarcoids might be more common in greys, but we need to study this more fully.

Most vets prefer to use cryosurgery (freezing), hyperthermy (heating with electricity or laser), and/or application of cis-platinum to sarcoids rather than surgery to treat sarcoids.  Published studies show that up to 50% of sarcoids may recur following surgical therapy alone, since residual tumor cells can regrow.  These recurrent lesions can grow more aggressively than the original tumor and this causes concern to both the owner and the veterinarian.  There have been reports that topical application of some herbal preparations may reduce the growth of equine sarcoids; this needs further study to validate this information.

Some scientific studies indicate 70-80% of grey and white horses will develop at least one melanoma by the age of 15 years, with reports dating back to 1781.  Heated debate has been ongoing since the early 1900s about whether melanomas in horses are benign (like human moles) or malignant (having aggressive patterns of growth and the ability to spread to distant tissues).

Work in our laboratories now indicates that melanoma is a malignancy, growing slowly at first, but with the capacity to grow aggressively, infiltrate tissue and to spread to distant sites.  Many owners know that their horses may get small melanomas that never grow very much.  In our practice, we see 30-40% of our cases with large, infiltrative and multiple masses.  We think this may reflect changes in the pattern of growth of melanoma over time.

By the time melanomas reach large sizes, infiltrate soft tissue (especially around the tail and rectum), or are present at multiple sites, treatment is very difficult.  A study by Emma Rowe and Ken Sullins showed that surgical excision of small, solitary tumors gave good results, but this method is not applicable when tumors are large.  Some owners of horses with large melanomas have administered oral cimetidine to slow tumor growth; the results of this treatment have generally been reported to us as having limited value.

We have worked for the past two years with several botanical oils, which we have injected into large malignant melanomas.  Some promising results for slowing tumor growth have been seen in several horses.  Once again, more research is needed before we know if these oils will help in the treatment of malignant melanomas.

Squamous cell carcinoma is a tumor derived from skin cells.  It is most common in horses that have white patches of facial fur and pink skin (areas of skin lacking pigmentation).  These tumors may be seen around the eyes/eyelids and on either the external part of the vulva or penis/sheath.

Small tumors can be removed with surgery or laser therapy, cryosurgery or hyperthermy.  Some veterinarians also use topical and/or injectable chemotherapy agents like 5-fluorouracil or cis-platinum to treat difficult tumors.

Skin tumors are common in horses and require attention by owners and their veterinarians.  A number of new treatments for these tumors are being developed and they may help control or eliminate tumors in affected horses.  More research into the underlying causes of the tumors and how they grow is the best way to help eliminate them in our four-legged friends.  Without this much-needed research, we will not solve the problem of how these tumors develop or have effective, new therapies, to treat them.

John Robertson, VMD PhD, is Director of the Center for Comparative Oncology at Virginia Tech. With the help of his colleagues and team members at Virginia Tech and Wake Forest University, he hopes to understand why companion animals get cancer.  Armed with this knowledge, they seek to find new, effective ways to control and cure this dreaded disease.

Category: Horse Disease

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Opinions expressed herein are those of the experts consulted and do not necessarily represent the opinions of the editors and publishers. The information in this publication is for educational purposes only. It is not intended to medically prescribe or diagnose in any way. ~ Holistic Horse Inc.