Alzheimer's Disease may be contractible through surgical instruments during medical procedures such as dentistry, researchers have claimed.
New research has found that contaminated instruments or injections, such as human growth hormone, may pose a rare but potential risk.
But health experts have been quick to reassure people and urged them to not cancel any impending medical procedures, after the research was published in medical journal, Nature.
The hypothesis comes from post-mortem brain studies in eight patients while British scientists were investigating a rare form of "iatrogenic" Creutzfeldt Jakob Disease (iCJD), a brain-destroying condition known to be spread by contaminated surgical instruments and procedures.
A study into Alzheimer's found that the condition could be contracted
The eight adults, aged between 36 and 51, all died of Creutzfeldt-Jakob disease (CJD) after receiving contaminated hormone injections as children.
Autopsies on their brains also revealed that seven of them harboured the misfolded proteins associated with the early stages of 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, director of the Medical Research Council Prion Unit at University College London, said there was increasing evidence that neurodegenerative diseases including Alzheimer's might, in rare circumstances, be "acquired", the Press Association reports.
Speaking on a phone-link to the British Science Festival taking place at the University of Bradford, he said: "You could have three different ways you have these protein seeds generated in your brain. Either they happen spontaneously, an unlucky event as you age, or you have got a faulty gene, or you've been exposed to a medical accident. That's what we're hypothesising."
He said that, like CJD prions, amyloid beta protein fragments stick to metal surfaces and resist conventional sterilisation.
Professor John Collinge, director of the Medical Research Council Prion Unit at UCL
Experiments on laboratory mice and monkeys has shown that transmission of the Alzheimer's protein is theoretically possible.
When liquified brain tissue from dead Alzheimer's patients was injected into the central nervous systems of the animals, they developed the brain changes associated with the disease.
Yet UK experts have stressed that their findings are inconclusive and do not mean Alzheimer's is infectious. Furthermore, people cannot catch Alzheimer's by coming into contact with other people with the condition.
While blood donations are not considered a meaningful risk, this should be investigated as a precaution, researchers say.
Questioned specifically about dentistry, Prof Collinge said: "The seeds will potentially stick to metal surfaces whatever the instrument is. With prions, we know quite a lot about that.
"Certainly, there are potential risks with dentistry where it's impacting on nervous tissue, for example root canal treatment, and special precautions are taken with reamers that are used in root canal treatment for that reason, in the UK at least.
"If you are speculating that amyloid beta seeds might be transferred by instruments, one would have to consider whether certain types of dental procedure might be relevant."
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".
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 chief medical officer, Professor Dame Sally Davies, also stepped in to reassure the public, maintaining there was "no evidence" that Alzheimer's disease can be transmitted in humans through any medical procedure.
She added: "This was a small study on only eight samples. We monitor research closely and there is a large research programme in place to help us understand and respond to the challenges of Alzheimer's.
"I can reassure people that the NHS has extremely stringent procedures in place to minimise infection risk from surgical equipment, and patients are very well protected."
Speaking to Radio 4's Today programme on Thursday, Dr Eric Karran, chief scientist at the charity Alzheimer's Research UK, said: "There is absolutely no evidence at all that you can receive Alzheimer's disease from having surgery. There is no evidence at all that a blood transfusion will give you Alzheimer's disease."
But he added: "Absence of evidence is not evidence of absence."
The biggest risk factor for Alzheimer's is age, genetic and lifestyle factors, he said.
Dr Doug Brown, director of research at Alzheimer's Society, said: "Injections of growth hormone taken from human brains were stopped in the 1980s. There remains absolutely no evidence that Alzheimer's disease is contagious or can be transmitted from person to person via any current medical procedures."
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."