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(Bloomberg) — Alzheimer’s may be six times more deadly to U.S. residents than the most commonly quoted statistics suggest, according to a new study.

Only about 83,000 U.S. death certificates list Alzheimer’s as a cause of death each year, but the condition may actually kill about half a million U.S. residents per year, researchers report.

The condition is best known for causing memory loss and confusion, but it also damages the parts of the brain that control basic functions like breathing and swallowing.

Heart disease and cancer remain the top two U.S. killers.

Alzheimer’s would replace respiratory disease as the third most common cause of death, based on the study results, up from sixth.

Caring for patients with Alzheimer’s already costs more than $200 billion a year.

The new study justifies calls for the country to spend more on Alzheimer’s research, according to Bryan James, the lead author and an epidemiologist at Rush University Medical Center in Chicago.

“We think Alzheimer’s disease has been getting short shrift because it hasn’t been considered a major killer, when it’s really one of the top three in the country,” James said in a telephone interview.

The findings, published in the journal Neurology, also underscore the difficulty of identifying the ailments that cause people to die based on death certificates, which typically list only the immediate reason for the death, such as the heart stopping, James said.

James and his colleagues tracked two groups of people who enrolled in long-term studies and agreed to donate their brains after death.

All of the 2,566 volunteers were ages 65 and older. None had dementia at the start.

The researchers noted which volunteers developed Alzheimer’s and which didn’t, and then compared the death rates for the two groups.

After an average of eight years, 559 patients developed Alzheimer’s and 1,090 died. Patients with dementia were three to four times more likely to die than those who weren’t diagnosed with Alzheimer’s, and the median time from diagnosis to death was less than four years.

The researchers calculated the number of deaths that could be ascribed to Alzheimer’s by comparing the number of expected deaths based on those without the disease to the number of people who actually did die after an Alzheimer’s diagnosis.

“Deaths from all of the other major causes of death are going down over time and the only one going up is Alzheimer’s disease,” James said. “That’s because we are coming up with effective treatments and preventions for other diseases and we have yet to do that for Alzheimer’s disease.”

–Editors: Bruce Rule, Reg Gale.

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