Researchers in the UK say they have found evidence Alzheimer's disease can be transmitted between people, for example through contaminated injections of human growth hormone.
But experts are playing down the claim, saying the medical procedures that led to the deaths of the patients whose brains were studied are no longer used, and there are other possible explanations for the findings.
Brains belonging to eight victims of Creutzfeldt-Jakob disease (CJD), the human form of 'mad cow disease', were looked at by researchers at the National Hospital for Neurology and Neurosurgery.
All had developed CJD after being given contaminated injections of human growth hormone extracted from cadavers. Aged between 36 and 51, none of the patients were expected to show signs of Alzheimer's – but six of the eight had "some degree of amyloid beta pathology", with four "widespread".
"Such pathology is rare in this age range and none of the patients were found to have mutations associated with early-onset Alzheimer's," the researchers said in a statement.
In contrast, 116 other patients looked at – who were of a similar age range and had diseases similar to CJD, but never received a growth hormone injection – showed no signs of developing Alzheimer's.
The researchers' hypothesis is that a protein in the growth hormones, produced by the pituitary gland at the base of the brain, is to blame, and that it can be transferred from one person to another.
The study, detailed in today's issue of journal Nature, has been hailed as "extraordinary" and "very interesting" by medical experts, but lacking a "smoking gun" to directly link Alzheimer's with cadaver-derived human growth hormone transplants, which ended in the 1980s.
"It suggests that pituitary glands of humans used to make the human growth hormone contained seeds that caused the amyloid beta pathology observed," says Dr Bryce Vissel of the Neurodegeneration Diseases Research Laboratory in Missouri.
"This matters enormously as it raises the possibility that other routes of transmission, including surgical instrument use and blood transfusion, could be relevant to the transmission of Alzheimer's disease… and other neurodegenerative diseases."
Neuroscience professor David Allsop of the University of Lancaster is more sceptical, saying the build-up of the amyloids linked to Alzheimer's could be a side-effect of CJD.
"It is very well-known from other studies that one type of rogue protein can predispose to accumulation of another. There is no evidence that Alzheimer's disease can be transmitted from one person to another, or through use of contaminated surgical instruments, and these results should be interpreted with a great deal of caution."
Neuroscience professor John Hardy at London's University College says more research is needed, but there is some merit to the claims because similar results have been found in trials on mice.
"I think we can be relatively sure that it is possible to transmit amyloid pathology by the injection of human tissues which contain the amyloid of Alzheimer's disease," he says.
"Does it have implications for blood transfusions? Probably not, but this definitely deserves systematic epidemiological investigation. Does it suggest Alzheimer's disease is infectious through contact? Almost certainly not."
Dr Doug Brown, director of research at the UK's Alzheimer's Society, says the presence of the protein doesn't guarantee Alzheimer's will follow, and the procedure – extracting human growth hormone from the pituitary glands of cadavers – was ended in the 1980s after the link between the procedure and CJD was discovered.
An estimated 159 New Zealanders received cadaver-derived human growth hormone injections in the 1960s and '70s, with six known cases of CJD developing as a result.
"The work we are doing in my laboratory at the Centre for Brain Research is directly related to understanding the route of entry and early origins of degenerative diseases, and so I can see the value in pursuing the line of research this group has taken," says Maurice Curtis, deputy director of the Human Brain Bank at the University of Auckland.
Since the early 1980s, human growth hormone has been synthesised using human DNA and bacteria, without the need for cadavers.