The incidence of cancer of the eyelid in horses is directly related to color pattern. Specifically, the lack of pigment in the tissue at this sensitive junction between hair-covered epithelium and mucosal surface of the inner edge of the lid predisposes to a type of cancer (squamous cell carcinoma) when exposed to long-term sunlight.
This condition in the horse is somewhat analogous to the risk of skin cancer in people. The lighter complected people are the most susceptible to skin cancer if they spend a lot of time exposed to the sun’s rays. This condition in the horse is more analogous to "cancer eye" seen in Hereford or white-faced cattle. The lesions are rarely seen in young horses, so it can be assumed that it takes years of exposure to sunlight for the cancer to evolve.
There are, of course, preventative measures that can be taken to cut down on the risk of this disease in susceptible horses. I’m sure you’re all aware of the advice dermatologists give people on lessoning the exposure to direct sunlight. Applying a sunscreen lotion or cream on the eyelids of a horse is not a viable alternative.
In my opinion, the most practical approach in prevention is to keep a fly mask on these horses. As time goes on, I’m more in favor of the use of fly masks in horses for multiple reasons. There are some horses, of any color pattern, that develop inflammation of the lining of the eyes during the summer and a fly mask seems effective in controlling these problems. Another option is to tattoo the unpigmented eyelids. However, because of cost factors and a tendency for the tattoo to fade in time, its efficacy is somewhat questionable to me.
Treatment of this condition once a lesion evolves is like any other cancerous problem. The earlier the intervention, the better the results. Removing these lesions surgically can be tricky, because one doesn’t want to disturb the function of the eyelids. Eyelids are very important in protecting the eyeball physically, and keeping it bathed with tears. This consideration is also why it’s important to take seriously any traumatic wound to an eyelid in order to maintain integrity and function.
Besides surgery, there are other options in treating these lesions, such as anti-cancer-cell drugs, laser treatment or freezing with liquid nitrogen. This condition is only inheritable based on the genetics of color patterns. With sensible precautions, I believe the threat can be functionally dealt with.