LONDON -- German scientists said on Friday they had lengthened the life of mice with scrapie, a prion illness similar to mad cow disease, with a compound that could provide a new approach to treating affected humans.

In research reported in **** The Lancet **** medical journal, they described how they injected the rodents with a product designed to stimulate an immune response.

Mice given the experimental treatment lived 38 percent longer than those who received a placebo.

The scientists from Ludwig-Maximilians University and the Technical University in Munich said they were not sure how the compound, CPG oligodeoxynucleotides, worked -- but they said it was safe for humans.

"CPG oligodeoxynucleotides have not been shown to have adverse effects to humans and could therefore be considered as a therapeutic choice after prion infection," said Professor Hans Kretzschmar of Ludwig-Maximilians University.

CPG molecules mimic some properties of bacterial DNA and activate immune responses.

Scrapie, like mad cow disease -- or bovine spongiform encephalopathy (BSE) -- and its human equivalent, creutzfeldt Jakob disease (VCJD), are caused by rogue prion proteins.

Prions exist normally but when they mutate and take on an abnormal shape they cause the brain-wasting diseases in animal and humans, for which there is no cure.

There have been scores of deaths in Britain from VCJD, which scientists suspect is linked to eating meat infected with BSE.

Because of an incubation period that can last decades and genetic factors that could influence susceptibility to the illness, it is impossible to predict how many people will get the disease.

Dr. George Carlson, of the McLaughlin Research Institute in the U.S. state of Montana, was quoted by Reuters as saying more research was needed but the hypothesis about how the CPG compound works was reasonable.

"Such an approach may offer a route to prevent disease," he said in a commentary on the research.