Humans were long believed to be the only animals to get Alzheimer's disease. But recent research suggests that one of our close relatives, the chimpanzee, may also develop very similar hallmarks of the illness.
Alzheimer's disease is a form of dementia that affects millions of Americans, usually starting after age 65. The illness gradually erodes a person's cognitive faculties, leaving them unable to remember information, carry on conversations, walk or swallow. Named after a German physician, the disease was first described around the turn of the 20th century, and today is estimated to affect more than five million Americans.
But despite its widespread impact, including notable victims like former President Ronald Reagan, scientists have yet to pinpoint a cause—or a treatment.
Along with dementia, Alzheimer's causes physical changes in the brain, reports Ryan Cross for Science, namely the buildup of sticky proteins called amyloid plaques and knots of tau proteins wrapped around each other called neurofibrillary tangles. It has been theorized that these physical changes somehow impair the functioning of the brain, although research has been inconclusive.
In a study published this week in the journal Neurobiology of Aging, researchers found evidence of these physical marks of Alzheimer's in the brains of 20 elderly chimpanzees that had been collected by a program aiming to increase research of great apes' neurology.
“Brain samples from great apes, particularly aged individuals, are incredibly scarce, so a study of this size is rare," co-author Mary Ann Raghanti says in a statement.
Among the 20 brains, which came from chimps ranging in age from 37 to 62, reports Cross, 13 were found to have amyloid plaques and four were found with neurofibrillary tangles. While these are good signs of the presence of Alzheimer's disease, reports Helen Thomson of New Scientist, no definitive diagnoses can be made.
“Our samples had been collected over decades, without any consistent or rigorous cognitive data accompanying them,” Raghanti tells Thomson. “So it wasn’t possible to say whether the chimps had devastating cognitive loss or not.”
However, bolstering the theory that the physical signs of Alzheimer's disease may be unrelated to its symptoms, Thomson reports, there have been no documented examples of the rapid mental decline into dementia characteristic of Alzheimer's among chimpanzees.
“I’m cautious to say that they don’t get this kind of devastating decline, but we haven’t seen it yet,” Raghanti told Thomson.
Alzheimer's disease has long been thought of as an illness unique to humanity, with some scientists speculating that its creation was driven by the evolution of human intelligence that sets our species apart. But not all are convinced of that. For instance, one 2008 study in the Journal of Comparative Neurology documented the brain of a 41-year-old chimpanzee, finding both neurofibrillary tangles and amyloid plaques in the neural tissue.
"I am not surprised by it at all," neuropathologist Larry Walker, who led that 2008 study, tells Cross about the results of this latest research, which reinforces to him that his prior research wasn't an "outlier of some sort."
This new study raises the possibility of research into Alzheimer's with chimpanzees, Cross reports, but the window for those studies may have already passed. In 2015, the United States declared chimpanzees endangered, which outlawed most research on the animals. Thus MRI scans, which could be used to track the brains of chimpanzees as they age, are now impossible to do with the remaining pool of former research apes.
"I don’t think there is anything here that is going to cause a fundamental reconsideration of where we are,” former National Institutes of Health official Kathy Hudson tells Cross. "Doing MRI on consenting humans who understand what’s going on is stressful enough."
Raghanti, however, is still optimistic about using this discovery to advance Alzheimer's research. Her team is now studying inflammation in the chimpanzee brain samples, reports Sara Reardon for Nature, and are counting the neurons in them to see if the apes lose brain cells as they age.