Seeds of Alzheimer's disease can potentially attach to surgical instruments and be transferred from one person to another during certain medical procedures, a study suggests.
The highly controversial research, reported in the leading journal Nature, provides the first evidence of dementia transmission in humans via microscopic protein fragments.
British scientists stumbled on the discovery while investigating a rare form of "iatrogenic" Creutzfeldt-Jakob Disease (iCJD), a brain-destroying condition known to be spread by contaminated surgical instruments and procedures.
They inspected the brains of eight patients who died from the disease after receiving pituitary growth hormone extracted from cadavers.
Unexpectedly, six bore a clear molecular hallmark of Alzheimer's - sticky clumps of fragmented protein called amyloid beta.
In four cases, the amyloid deposits were widespread and only one patient was not affected at all.
All eight individuals were relatively young, aged from 36 to 51, and none had genetic variants associated with early-onset Alzheimer's disease.
The evidence points to the hormone carrying "seeds" of the Alzheimer's protein into the patients' brains as well as CJD.
Lead scientist Professor John Collinge said there was increasing evidence that neurodegenerative diseases including Alzheimer's might, in rare circumstances, be "acquired" by people being "exposed to a medical accident".
"The seeds will potentially stick to metal surfaces whatever the instrument is," he said.
"Certainly, there are potential risks with dentistry where it's impacting on nervous tissue, for example root canal treatment."
He stressed there was no epidemiological evidence at all to suggest that Alzheimer's could be transmitted via blood transfusions.
Later he appeared to backtrack on dental treatments, issuing a statement saying the current data had "no bearing" on dental surgery and "certainly do not argue that dentistry poses a risk of Alzheimer's disease".
Prof Collinge said "much further research" was needed to see if the findings could apply "some other medical or surgical procedures".
Meanwhile, he urged people not to be concerned about planned medical procedures and to dismiss any notion of Alzheimer's being "contagious" in the same way as flu, for instance.
The UK's chief medical officer Professor Dame Sally Davies maintained there was "no evidence" that Alzheimer's disease could be transmitted in humans through any medical procedure.
Treatment of people with pituitary growth hormone taken from dead donors was stopped in 1985.
Professor Nigel Hunt, dean of the faculty of dental surgery at the Royal College of Surgeons, said: "This study alone does not provide any conclusive proof that Alzheimer's disease can be transmitted from person to person. Dental practice carries no more risk than any invasive clinical procedure."
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