Infectious proteins called prions cause a disease resembling BSE, according to scientists who say they have "final proof" to support the idea.

Based on test tube experiments, researchers have long thought BSE and similar disorders were caused by prions. But they've never had scientific proof that the proteins are capable of causing damage in animals or humans.

Despite the lack of definitive proof, Stanley Pruisner's "prion hypothesis" experiments in the 1980s led him to win the Nobel Prize in Medicine in 1997. He proposed the term "prion" to describe the infectious agent.

Model of a prion.

Now Claudio Soto, of the University of Texas Medical Branch, has closed an experimental loop to show prions cause disease.

The finding could potentially help researchers to develop better diagnostic tests or treatments for diseases like BSE or variant Creutzfeldt-Jakob disease in people.

First, researchers needed to show prions taken from an infected animal can cause illness in other animals.

In this case, Soto's team used prions isolated from the brains of hamsters experimentally infected with a BSE-like illness called scrapie that normally harms sheep.

Using a new laboratory technique developed by Soto's team, researchers "copied" scrapie prions from infected hamsters.

When the copies were injected back into healthy hamsters, the rodents developed symptoms of scrapie within four months, and died in six months, on average.

The goal of the experiment was to show prions can cause disease in living cells, not just test tubes, to rule out the role of another pathogen like a virus.

"The evidence in favor of the prion hypothesis was strong, but the final proof was still missing," said Soto. "Now we have supplied this proof."

Nevertheless, scientists cannot rule out that another pathogen is involved in causing the prion diseases, known scientifically as transmissible spongiform encephalopathies.

The study appears in the April 21 issue of the journal Cell.