By Will Dunham

WASHINGTON, Nov 14 (Reuters) - U.S. Latinos and blacks live longer after being diagnosed with Alzheimer's disease than whites even as autopsies show that the severity of the disease is similar among them, researchers said on Wednesday.

Latinos lived approximately 40 percent longer than whites after diagnosis with the disease, and blacks lived about 15 percent longer than whites, the researchers reported in the journal Neurology.

The researchers tracked 30,916 people ages 65 and older with Alzheimer's disease from across the United States.

"The explanations may be complex. It's not entirely clear why Latinos and African Americans have a survival advantage," Kala Mehta of the University of California, San Francisco, one of the researchers, said in a telephone interview.

Mehta said factors could include cultural issues like support from extended family. Differing levels of health and illness beyond Alzheimer's also might be a factor, Mehta said.

Alzheimer's is a degenerative brain disease that is the most common form of dementia among the elderly.

Of the people in the study, 81 percent were white, 12 percent black and 4 percent Latino, the researchers said.

On average, the people in the study lived for 4.8 years after diagnosis. Autopsies were conducted on 3,000 patients, showing that the severity of the disease was similar among the various groups.

"These findings were not explained by patient age, sex, educational level, marital status, living situation, and cognitive test score measured at their first evaluation" for the disease, the researchers wrote.

"Determining the underlying factors behind survival differences may lead to longer survival for (Alzheimer's disease) patients of all race/ethnic backgrounds," the researchers wrote.

(Editing by Maggie Fox and Eric Beech)

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