Alzheimer’s disease likely plays a much larger role in the deaths of older Americans than is reported, according to a new study that says the disease may be the third-leading cause of death in the United States.

The Centers for Disease Control and Prevention lists Alzheimer’s as the sixth-leading cause of death, far below heart disease and cancer. But the new report, published Wednesday in the medical journal of the American Academy of Neurology, suggests that the current system of relying on death certificates for causes misses the complexity of dying for many older people and underestimates the impact of Alzheimer’s.

While the CDC attributed about 84,000 deaths in 2010 to Alzheimer’s, the report estimated that number to be 503,400 among people 75 and older. That puts it in a close third place, behind heart disease and cancer, and well above chronic lung disease, stroke and accidents, which rank third, fourth and fifth.

Alzheimer’s is somewhat of a sleeping giant compared with other leading killers that have received more funding over the years. While deaths from these diseases have been going down thanks to better treatment and prevention, the number of people suffering from Alzheimer’s is quickly rising and the disease is always fatal.

More than 5 million people in the United States are estimated to have Alzheimer’s. With the aging of the baby-boom generation, this number is expected to nearly triple by 2050 if there are no significant medical breakthroughs, according to the Alzheimer’s Association.

The disease cost the nation $210 billion last year; that rate is expected to rise to $1.2 trillion by 2050.

“Scientists told us we need $2 billion a year over the coming 10 years” to see significant advancement in treatment and prevention, said Keith Fargo, director of Scientific Programs and Outreach at the Alzheimer’s Association. Funding by the National Institutes of Health for Alzheimer’s in 2012 was about $500 million, far below funding for heart disease and cancers. The estimated funding in 2013 was $484 million.

“We would like to see a response that is commensurate with the problem,” Fargo said. “Alzheimer’s disease is a serious disease and it needs to be taken seriously, and if we have the right kind of investment as a country, then we will be able to make strides similar to what we’ve made in heart disease, HIV and cancer.”

For the study, researchers at Rush Alzheimer’s Disease Center in Chicago followed 2,566 people 65 and older for an average of eight years, testing them annually for Alzheimer’s-type dementia and observing the risk of death in those who did and did not receive a clinical diagnosis of the disease.

But death certificates for many with Alzheimer’s often listed a more immediate reason for death, leading to a severe underreporting of the disease as an underlying cause, said Bryan James, the report’s lead author and an epidemiologist at the center. The study was funded by the National Institute on Aging and the Illinois Department of Public Health.

“Death certificates may not be the best way to measure how many people die from something that takes up to 10 years” to break down a person’s system, he said, adding that the disease leaves people more vulnerable to dying from infections and other problems. “We’re not saying they didn’t die of those things; we’re just saying, ‘Well, what put them in the hospital with that condition?’ ”

For example, if Alzheimer’s compromises one’s ability to swallow and results in food repeatedly going down the windpipe, that can leave a person more vulnerable to dying of pneumonia, he said.

A professor at George Mason University hopes a shoe with GPS tracking, developed for Alzheimer's patients can bring peace of mind to families and cut costs for local municipalities, who often carry the financial burden of searching for patients who wander off. (Marina Cracchiolo/The Washington Post)

“Trying to identify a single cause of death in elderly people is often not reflective of the real situation,” James said.

The idea that Alzheimer’s causes many more deaths is not new among experts, but the scope of the undercount is striking, said Dallas Anderson, science administrator for population studies of Alzheimer’s at the National Institute on Aging.

“Anybody who has somewhat of a knowledge of the disease registration system in the U.S. would not be surprised that there was an undercount; the surprising thing is how much of an undercount,” he said, adding that while earlier studies have had similar findings, this is the first peer-reviewed study to produce them.

“It’s shocking,” he said, “and it’ll be interesting to see how it plays out. . . . It would be nice if we could actually improve our death registration system.”

President Obama’s budget for fiscal 2015 included $100 million for an initiative to map the human brain to better understand diseases including Alzheimer’s.

But many in the field think Alzheimer’s should receive more funding.

“The disease is still very underfunded in comparison to other diseases,” James said. “Cancer has about 10 times the amount of funding, and only about three times as many people have cancer.”

Alzheimer’s is not always taken as seriously as other diseases, which may contribute to the historical lack of funding, Fargo said. “There are many people who still think of Alzheimer’s disease as just a memory problem — you forgot where you left your keys. But currently, Alzheimer’s is a universally fatal brain disease that has kind of fallen by the radar.”