Scientists are urging research into the risk of "accidental" medical transmission of Alzheimer's, after a study revealed several possible cases.

People injected with hormones extracted from cadaver brains in a long-abandoned procedure may have received "seeds" of Alzheimer's disease, according to a new study.

Published in the journal Nature on Wednesday, the research claims to provide evidence for the hypothesis that the protein fragments which assemble into Alzheimer's-causing plaques, can be passed between humans via diseased tissue transfer.

But this does not mean that Alzheimer's is contagious, the study authors and independent commentators stress.

"This relates to a very special situation where people have been injected with essentially extracts of human tissue," said co-author John Collinge of University College London (UCL).

"I don't think there needs to be any alarm that we're saying in any way that you can catch Alzheimer's Disease."

Further research, however, would be "prudent," he said during a telephone press briefing.

"We should think about whether there might be accidental routes in which these diseases might be transmitted by medical or surgical procedures."

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While conducting research into an unrelated disease, Collinge and a team examined the brains of eight people who had received injections in childhood of a hormone to treat dwarfism.

The hormone had been extracted from pituitary glands harvested from thousands of human cadavers.

This practice was halted in 1985 when doctors realised it could transmit a variant of Creutzfeldt-Jakob disease (CJD) - the human version of "mad cow" disease. Eight subjects in the study in fact died from this ailment.

Collinge and colleagues, "very much to our surprise," found that seven of the eight had brain deposits of Alzheimer's-linked amyloid beta (Abeta) fragments - with four of them having high concentrations.

Strikingly the patients were 36-51 years old, whereas such deposits are normally seen in elderly people.

"We think the most likely explanation is that the growth hormone preparations with which these people were treated as children, in addition to being contaminated with CJD prions (a different protein type), was probably also contaminated with Abeta seeds."

Previous laboratory studies showed that Abeta in Alzheimer's-ridden brain tissue, when transferred to mice or monkeys, could infect the host animal brain - even when it had been injected into their abdomens.

"So there are mechanisms to transport these protein seeds to the brain," said Collinge.

"We don't know what they are, but clearly it can happen."

Experts who were not part of the study underlined there was no evidence of any modern-day medical treatment, including dental surgery or blood transfusions, raising the Alzheimer's risk.