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Researchers say they've documented the first ever cases of Alzheimer's disease being transmitted between humans — and though it only took place in extremely rare and unusual circumstances, it could provide valuable clues into the underlying mechanisms of the terrible disease.

Their findings, published as a study in the journal Nature Medicine, detail how eight adult patients, only five of whom are still alive, likely acquired the disease through a banned medical procedure performed on them as children in which they were administered human growth hormone extracted from a cadaver's brain. Decades later, they're now showing early signs of dementia, with the earliest experiencing symptoms as young as 38 years old.

The researchers suggest that the procedure inadvertently transmitted a protein called amyloid beta that's considered to have a central role in the development of Alzheimer's disease.

They stress that this does not mean Alzheimer's is transmissible like a cough or the flu. It was spread "iatrogenically" StatNews notes, or only as the result of a medical practice — in this case a very seldom used but tragically misguided one.

"I should emphasize these are very rare occurrences, and the majority of this relates to medical procedures that are no longer used," study senior author John Collinge, the director of the University College London Institute of Prion Diseases, said in a news brief, as quoted by CNN.

The practice of using cadaver growth hormones — today they're synthesized — was banned in the 1980s because it caused another terrifying brain disorder: Creutzfeldt-Jakob disease (CJD), which is transmitted through "misfolded" proteins called prions.

CJD, like Alzheimer's, causes dementia but is even more severe and always fatal. Since both can stem from the same medical procedure, the researchers say their findings support the idea that the beta-amyloids behind Alzheimer's can be transmitted in similar ways to a prion disease, which have been long known to pass between humans.

"It looks like what's going on in Alzheimer's disease is very similar in many respects to what happens in the human prion diseases like CJD, with the propagation of these abnormal aggregates of misfolded proteins and misshapen proteins," Collinge told Stat News.

A key detail is that none of the patients were shown to have genetic mutations known to cause early-onset dementia, despite all of them experiencing symptoms at a relatively young age and well before the cutoff age of 65.

Only one patient had genetic data known to cause late-onset dementia. In addition, none were found to have elevated levels of a protein called tau, Stat notes, which is associated with cognitive decline. As it stands, the only known common factor between the patients is the HGH procedure they received.

The sheer rarity of the circumstances means that it will be tough to bear out the study's findings. Nonetheless, it's already raised useful questions over the nature of Alzheimer's, the exact cause of which remains elusive.

More on neuroscience: Smoking Cigarettes Does Something Horrifying to Your Brain, Scientists Find

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