Scientists have found that harmful tangles of proteins that cause diseases such as Alzheimer's can be transmitted from one brain to another, spreading and causing damage after being injected into the brains of mice. The researchers stressed, however, that Alzheimer's was not contagious and said it could not be caught, for example, through blood transfusions.

Alzheimer's and similar neurodegenerative diseases can be caused by the build-up in the brain of tangled masses of a type of protein called tau. They destroy brain function and, when they damage large amounts of tissue, can lead to dementia.

In experiments on mice, researchers found that the tau tangles spread in the brain as though they were an infectious agent, after they were injected in tissue from the brain of an affected mouse into the brain of a healthy one. The research is published tomorrow in the journal Nature Cell Biology, and gives scientists a much better idea of how to target therapies for neurodegenerative diseases.

Michel Goedert of the Medical Research Council's Laboratory of Molecular Biology in Cambridge, who took part in the study, said the work opened up new avenues in understanding and allowing scientists to experiment with the causes of dementia.

"This research in mice does not show that tau pathology is contagious or it can spread easily from mouse to mouse – what it has revealed is how tau tangles spread within brain tissues of individual mice," he said. "It suggests that tangles of proteins that build up in the brain to cause symptoms could have some contagious properties within brain tissue but not between mice that haven't been injected with tissue from another mouse and certainly not between people." Though they are also bits of protein, tau tangles do not transmit in the same way as prions, the proteins that cause diseases such as vCJD and mad cow disease by destroying brain tissue, because they cannot be passed easily between individuals.

Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "This greater understanding of how tangles spread in Alzheimer's may lead to new ways of stopping them and defeating the disease." Abnormal tangles build up in the brain during Alzheimer's and other diseases of the brain. It's not clear how that happens - but it is clear that Alzheimer's itself is not contagious. We desperately need more research like this to find answers to dementia, a cruel condition that affects 700,000 people in the UK."

There is still much unknown about the changes in tau protein that lead to tangle formation in humans and, eventually, widespread brain cell death.

But Susanne Sorensen, head of research at the Alzheimer's Society, said: "Each new piece of knowledge helps build a better picture and takes us closer to the point where we can stop loss of brain tissue and dementia for good."