NEW YORK -- One in 4 Alzheimer's deaths in the United States are now occurring at home -- a startling increase that marks a shift away from hospitals and nursing homes, according to a report released Thursday.

Alzheimer's deaths in hospitals and nursing homes or other long-term care facilities shrank from more than 80 per cent to 60 per cent over 15 years. Meanwhile, those dying at home rose from 14 per cent to 25 per cent, the Centers for Disease Control and Prevention found.

Alzheimer's disease is a progressive brain disorder that impairs memory, judgment and other mental abilities. It's most common in the elderly. In the final stages of the disease, people have trouble eating and carrying on a conversation, are vulnerable to pneumonia and infections, and often need around-the-clock care.

The Alzheimer's death rate has risen nearly 80 per cent since 1999 and the disease is the nation's sixth leading cause of death.

Deaths have been climbing for some time. A big reason is declines in other causes of death -- particularly heart disease and cancer -- are enabling more people to live long enough to die from Alzheimer's, experts say.

In the new report, the CDC analyzed death certificates from 1999 to 2014 and found a surprise -- a shift in where people with Alzheimer's are dying.

Experts said it's not clear why more people with Alzheimer's are dying at home than in hospitals or nursing homes.

One possible explanation is patient preference. For people losing their memories, it can be a comfort to remain in familiar surroundings, said Jeff Huber, president of Home Instead Senior Care, an Omaha-based company that provides home care to tens of thousands of clients with Alzheimer's and dementia.

But it can be difficult for families. Even if they have the insurance or money to get home-care workers to help, caring for an Alzheimer's patient can take an exhausting toll, experts said.

As baby boomers age, "the numbers dying from Alzheimer's are just going to get worse," said Keith Fargo, director of scientific programs for the Alzheimer's Association.

The CDC also released Alzheimer's death rates for counties.

The counties with the highest rates over 10 years were tiny Eddy County in east central North Dakota, and Hamlin County in eastern South Dakota. Several counties in the South were near the top of the list.

It's not clear why. Doctors in some places might be better at diagnosing Alzheimer's, or coroners and medical examiners might more commonly put Alzheimer's on death certificates instead of other causes, said Christopher Taylor, a CDC epidemiologist who was the report's lead author.