Skin Tumors in Horses: Melanomas and Sarcoids

Melanomas are skin tumors that can affect horses of any color but are most often seen on gray horses. Some estimates predict that up to 80% of gray horses over the age of 15 will develop melanomas.

These tumors are most commonly located under the tail, in the genital and perineal areas, and sometimes on the lips or eyelids. They are black, somewhat shiny, and rounded in form.

Melanomas tend to grow slowly, and most are not considered a serious problem as long as they don’t interfere with the horse’s comfort. In many horses, the tumors are stable for several years and then begin to enlarge rapidly, sometimes breaking open and forming bleeding, ulcerated sores. Large masses around the anus may begin to block the passage of manure.

Treatment of melanomas is difficult. Small tumors are usually left alone because they rarely cause problems for the horse. After the tumors begin to grow, owners may not seek treatment until the tissue masses are quite large. Surgical removal of these larger tumors is not routinely successful, and the tumors tend to return, possibly because malignant cells have infiltrated the surrounding tissue. Cryosurgery (freezing) followed by surgical removal of the accessible part of the tumor also has a low long-term success rate. Treatment with cisplatin, either directly injected into the tumor or released by cisplatin-containing biodegradable beads implanted in the tissue, is somewhat successful, but no treatment is guaranteed to eliminate the tumors and prevent regrowth.

Sarcoids are the most common skins tumors in horses. Thought to be caused by a bovine papilloma virus, these growths are notoriously hard to get rid of, frequently recurring after surgical removal. As with melanomas, many therapies have been tried with variable success against sarcoids.

Electrochemotherapy is a tool that has shown promising results in eliminating sarcoids and preventing their regrowth. Electrochemotherapy combines chemotherapy and pulses of electricity in an effort to drive the anti-cancer drug cisplatin into the tumor tissue at a higher concentration than can be achieved with injections.

In trials at Toulouse Veterinary School in France, records were reviewed for 48 horses that presented with 194 sarcoids. Almost 40% of the tumors had been treated in some other way and had either recurred or had not responded to treatment.

The horses were placed under general anesthesia before treatment. In some horses, as much tumor tissue as possible was surgically removed and then the area was treated with electrochemotherapy. In other horses, electrochemotherapy alone was used on the tumors.

All horses tolerated the treatment well. Localized swelling for several days was the most common side effect. One to seven electrochemotherapy treatments were required for different horses, and tumors that were surgically removed required fewer treatments. For all treated horses, the nonrecurrence rate after four years was 97.9%.

Though some sarcoids disappear without treatment, others are in locations where they interfere with tack or cause other problems. For these horses, electrochemotherapy may be the best choice to remove the tumors and prevent their recurrence.