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A treatment to reverse Alzheimer's Disease could be available in five years, it has been revealed.

Experiments on mice have indicated that a new vaccine not only halts the advance of the disease, but repairs damage already done.

It could also be given to patients whose families have a history of Alzheimer's, to prevent them developing the disease.

The research by British, American and Canadian scientists, was being hailed last night as the most significant breakthrough yet. Harry Cayton, chief executive of the Alzheimer's Society, said: 'This really does make us optimistic.'

A growing number of elderly and even middle-aged people are being struck down by the degenerative brain disease, which has some 500,000 sufferers in Britain alone. It causes untold misery to families who are left to care for loved ones who may no longer recognise them.

The vaccine attacks the build-up of a protein called beta-amyloid, which forms a damaging waxy plaque on brain cells. The latest research, reported today in the scienctific journal Nature, suggests the drug not only removes the proteins but can restore mental functions.

Clinical trials of the vaccine, which is
called Betabloc and made by Dublin-based Elan Pharmaceuticals, are already under way in the UK.

Preliminary results appear to show it is safe and has no side-effects. About 80 patients with mild to moderate forms of Alzheimer's are taking part in a second set of safety trials, which are close to completion.

The breakthrough came as researchers worked with mice genetically engineered to develop a disease similar to Alzheimer's.

Scientists already knew that the vaccine could reduce plaques, but they used a new test, devised by experts at Edinburgh University, to evaluate whether it could also improve memory and cognition.

Professor Richard Morris, of Edinburgh's department of neuroscience, said past attempts to find a cure for Alzheimer's had been hampered because there was no way of testing short-term memory in animals given experimental treatments.

He said: 'Patients in the early stage of Alzheimer's disease are often good at remembering events from early in their lives, but keeping track of more recent times is a real problem.

'It is easy enough to distinguish a human's short-term memory, but until now we could not test the same function in rodents.'

The new test involves monitoring the mice in an elaborate 'water maze' where the way out to dry areas is changed frequently.

Researchers in both Toronto and Florida found dramatic improvements in the memories of mice given the drug over several months.

Dr Peter St George-Hyslop of the University of Toronto said: 'Not only were we able to clean up the brain tissue, but we also prevented the behavioural consequences of Alzheimer's.'

The scientists believe their work provides final proof of exactly how Alzheimer's is initiated by chemical changes in the brain, although other factors play a role in its development.

Professor Morris said: 'We know that Alzheimer's sufferers express an abnormal amount of beta-amyloid, which collects in the brain and forms plaques.

'It seems a person's ability to learn and remember things declines as these plaques grow, and we know this is true for both humans and mice.'

The experts stressed that the vaccine could not become generally available until it has been through large-scale clinical trials.

But Mr Cayton, who described the research as 'extremely exciting', said: 'It would not be unrealistic to say that we might see a treat-ment within five years.'