Equine sarcoids are the most common skin tumors seen in horses, mules and donkeys worldwide. The incidence ranges from 15
percent to 60 percent of skin tumors, typically seen on the extremities, head and ventral abdomen. In one study, 35 percent
of 153 tumors occurred on the head, neck, eye and shoulder; 28 percent appeared on the hind legs; 24 percent manifested on
the fore legs, and 12 percent appeared on the trunk. It has been noted that sarcoids can occur at sites of earlier trauma
in some horses. Multiple lesions are common, but metastasis rare. Recurrence is common following surgical resection, occurring
in about 30 percent to 50 percent of cases treated only with surgery.
Sarcoids occur in young horses, at less than 7 years of age; 70 percent occur in horses less than 4 years of age. They can
occur in foals as young as 6 months. There is no sex or coat color predisposition. In one study, Quarter horses were found
to be at twice the risk as Thoroughbreds, and Standardbreds were at half the risk of Thoroughbreds, but this suggested breed
predisposition requires further study.
The most common cutaneous tumors include squamous cell carcinomas of the penis and the third eyelid. These conditions often
require aggressive surgical debulking such as penile amputation and complete eye removal.
Sarcoids are of variable appearance, being flat, raised, sessile, pedunculated or verrucous, and they vary from 1 cm to more
than 20 cm. They are firm, adherent to surrounding skin and underlying tissue and can be ulcerated. The rate of growth is
highly variable, from very slow to very rapid. Lesions that ulcerate may form granulation tissue at the margins, increasing
the size of the lesion and making it more likely to bleed when traumatized.
Despite their recognition for more than 90 years, the etiology of sarcoids is unknown. A virus has been suggested as the cause,
but this has not been definitively proven.
The incidence of melanoma is 3 percent to 15 percent, based on biopsy and necropsy studies. The frequency of melanoma increases
after age 6, and it is considered a somewhat inevitable occurrence in older gray and dilute-colored horses (approximately
80 percent of gray horses more 15 years old are susceptible), particularly Arabians and Percherons.
Early and aggressive debulking followed by either topical or intra-lesional chemotherapy is critical for complete resolution
of clinical signs of squamous cell carcinomas.
The association of occurrence with dilute coat color is significant. In a study of 67 melanomas, only 30 percent were found
in non-white/gray horses. Though associated with pale coat, that doesn't mean that those affected horses have a pale pigmentation
in their skin.
"We've looked at that very extensively," Robertson says. "People have asked for a long time whether there is any relationship
to exposure to sunlight, but we just don't know. We have started a project to try to figure that out by gathering statistical
data from equine centers and agricultural and veterinary institutions around the world to find out in two breeds (Arabians
and Thoroughbreds) whether, in fact, there is a difference based on ambient sunlight. We expect this study to take several
years because the data that may help us is hard to pull together."