Is mental illness infectious.

For centuries, people blamed mental illness on witchcraft. Then Freud and other psychotherapists emphasized repressed experiences from a troubled past.

The focus has since shifted to biochemical abnormalities that affect brain function. Now, a group of German scientists suggests that some forms of mental illness may be caused by a virus.

For the first time, a microbe linked to behavioral abnormalities in animals has been isolated in humans, says Liv Bode of the Robert Koch Institute in Berlin.

Bode and his colleagues obtained the Borna virus from three people who had been diagnosed with mood disorders. When injected into laboratory animals, the virus caused the animals to develop behavioral problems, the team reports in the July Molecular Psychiatry.

"These findings open up a new area of investigation of mental disorders," say the researchers.

"This is the first solid clue that an infectious agent may be linked to mental illness," says R. Michael Hendry of the California Department of Health Services in Berkeley.

Earlier studies showed that the Borna virus has evolved over eons into distinct strains, each with a predilection for a specific animal species. The human version has also proved to be distinct, suggesting that the individuals in the German study were infected with a Borna virus that circulates in the human population.

Other studies have identified antibodies to animal strains of the virus in up to 5 percent of healthy animals, Hendry notes. Furthermore, Borna virus antibodies have been reported in nearly one-third of people with certain mental illnesses, including obsessive-compulsive disorder and schizophrenia, he says.

Because the tests now in use were designed to detect infection with animal strains of the Borna virus, they may fail to find antibodies to the human version. Scientists armed with the genetic sequence of the newly found human virus, however, can devise a test that will zero in on the human version. If such a test is developed for general use, it could measure the prevalence of the virus among the mentally ill.