What is Merkel cell cancer? Jimmy Buffett's cause of death, explained.

After battling skin cancer for years before his death, "Margaritaville" singer Jimmy Buffett died Friday at 76 from Merkel cell cancer, a statement on his website revealed.

Merkel cell carcinoma is rare but aggressive form of skin cancer, according to the Skin Cancer Foundation.

Merkel cell is 40 times rarer than melanoma, with an estimated one case per 130,000 people in the U.S., according to the foundation.

"Approximately 3,000 new cases are diagnosed annually in the U.S." the organization's website adds. "Experts expect that this will increase to 3,250 cases diagnosed annually by 2025."

What causes Merkel cell cancer?

Because of its rarity, it's not entirely clear what causes Merkel cell carcinoma — but cancer records have helped experts determine commonalities between cases.

"It's a skin cancer that typically affects individuals as they get older, so 70- to 80-year-old patients are our common demographic, and it typically more commonly affects men," Dr. Alexander Daoud, a Florida-based dermatologist, told CBS News Miami.

According to the American Academy of Dermatology, people most likely to get this form of cancer have all the following traits:

Male65 years of age or olderFair skin that may burn or freckle when exposed to the sunSeldom used sunscreen, clothing or shade to protect their skin from the sun

These risk factors helped scientists determine sunlight and a weakened immune system play key roles in causing this form of skin cancer.

"About 85% of the time, I'm pretty confident it's a skin cancer. About 15% of the time we get a diagnosis of Merkel cell carcinoma we don't know where it starts from, so it's complicated," Dr. David Miller, director of the Merkel cell carcinoma center at Mass General Cancer Center told CBS News Boston.

A virus called Merkel cell polyomavirus likely also plays a role in causing most cases of the cancer, researchers recently discovered.

"The virus lives on the skin and doesn't cause any signs or symptoms. Just how this virus causes Merkel cell carcinoma has yet to be determined," the Mayo Clinic explains. "Given that the virus is very common and Merkel cell carcinoma is very rare, it's likely that other risk factors play a role in the development of this cancer."

What are Merkel cell symptoms?

Typically, the first sign of Merkel cell carcinoma is a fast-growing, painless nodule or tumor on your skin, according to the Mayo Clinic. The nodule may be skin-colored or a shade of red, blue or purple.

"Most Merkel cell carcinomas appear on the face, head or neck, but they can develop anywhere on your body, even on areas not exposed to sunlight," the Mayo Clinic adds.

Daoud says to keep an eye out for a bump that grows very rapidly.

"The things that we look for — or should light off our suspicion — is something growing very quickly but doesn't have to hurt," Daoud says. "Those are the things you should call to the attention of your doctor."

The American Academy of Dermatology warns people often mistake it for an insect bite, sore, cyst, stye or pimple.

While Merkel cell carcinoma is often painless, the nodules can feel sore and tender, especially with growth, the organization adds.

Merkel cell cancer treatment

While the Skin Cancer Foundation estimates Merkel cell carcinoma is about three to five times more likely to be deadly than melanoma, it can be treated successfully with with early detection.

"Early stage disease can be treated with surgery and can be cured," Miller says. "Later stages, however, the disease can be quite aggresive and can be deadly."

Treatment options differ depending on the stage of cancer and other patient factors but can include also include radiation and immunotherapy.

Early detection and treatment is key as Merkel cell carcinoma can spread beyond the skin, according to the Mayo Clinic.

"Merkel cell carcinoma tends to travel first to nearby lymph nodes. Later it may spread to your brain, bones, liver or lungs, where it can interfere with the functioning of these organs," the clinic's website says. "Cancer that has metastasized is more difficult to treat and can be fatal."

Another important consideration is self-care after treatment and prevention.

"Protecting your skin from the sun and never tanning can help prevent a new MCC," the American Academy of Dermatology adds.

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