LONDON, Sept 9 (Reuters) - British scientists have found evidence that the biological seeds of Alzheimer's disease could be passed on through medical procedures - though specialists said the risk of transmission was largely theoretical.

Research published on Wednesday found evidence that suggested one of the hallmark proteins of Alzheimer's - that could go on to develop into the brain disease - spread to a group of patients via a now banned form of hormone treatment.

"This was very surprising," said John Collinge, a University College London professor and director of the Prion Unit who led the studies and published them in the journal Nature.

The growth treatment, using human-derived hormones, is no longer used due to the risk of contamination. But Collinge said studies are now needed into whether other procedures, such as blood transfusions and the repeated use of surgical instruments, pose a risk.

"We do need to ask that question," he said, noting that previous experiments on laboratory mice and monkeys have already shown that transmission of the Alzheimer's protein is theoretically possible. "There is evidence from animal studies that it is not implausible."

Experts asked to comment on Collinge's work said it was scientifically intriguing, but should not cause undue concern.

"Although a very interesting paper, I don't think we need to worry excessively," said Simon Lovestone, a professor of Translational Neuroscience at Oxford University.

"This form of (human growth hormone) treatment stopped 20 years ago and there is no evidence from this paper or any other work I am aware of that any other form of treatment would result in exposure to amyloid."

Masud Husain, an Oxford neurology specialist added: "While this is a beautiful piece of investigative medicine, we have to keep the findings in context.

"These results certainly do not provide sufficient evidence to believe Alzheimer's disease is a transmissible illness."

In what external experts praised as a landmark study, Collinge and other specialists from the Medical Research Council's Prion Unit discovered the Alzheimer's protein in the brains of seven out of eight patients they studied who had died of Creutzfeld Jacob Disease (CJD), another brain disease.

The patients, aged 36 to 51, had contracted CJD from contaminated growth hormone given to treat growth problems, the researchers said, but autopsies also showed their brains had significant levels of the Alzheimer's protein amyloid beta.

That finding suggested people treated with human growth hormone in the past and currently well may be at risk not only of CJD, but also of developing Alzheimer's seeded by an accidental medical transmission of amyloid protein, Collinge said.

At a briefing in London, he noted that while Alzheimer's is a common disease of the elderly, it is highly unusual to see amyloid deposits in the brains of relatively young people.

"You simply don't see that sort of pathology in this age group," he said.

Use of human-derived growth hormone has been banned since 1985 after doctors found CJD may spread in contaminated tissues.

Around 450 people worldwide have died of CJD transferred in this way, but patients are now treated with a synthetic hormone which avoids the risk.