Some schizophrenia patients harbor retroviruses in their cerebrospinal fluid and brain, suggesting that viruses might trigger the disease, according to a study published in today's Proceedings of the National Academy of Sciences. More than two million Americans suffer from schizophrenia, which typically manifests itself in the late teens or twenties. Researchers believe the disease has both genetic and environmental origins, and some have suggested that retroviruses, which integrate their genetic material into DNA, play a role.

To find out, scientists from the U.S. and Germany tested 35 newly diagnosed patients for retroviruses. They discovered that almost one third of these individuals contained genetic material from a certain class of retroviruses called HERV-W in their cerebrospinal fluid. In contrast, only 5 percent of chronic sufferers and none of the controls¿a mix of patients with other neurological disorders and healthy people¿showed traces of the viruses. What's more, the level of HERV-W virus was elevated in brains from dead schizophrenic patients, compared with healthy controls.

The scientists do not know if the retroviruses were endogenous, or naturally present in the genome and inherited, or if they entered the brain through infection. They also caution that some of the study groups were from geographically different areas, making them more difficult to compare. Furthermore, they are not sure by what mechanism HERV-W viruses may cause schizophrenia. Nerve cells making viral proteins may fuse (one HERV-W protein is known to cause cell fusion). In any case, retroviruses are likely to represent just one of several factors leading to schizophrenia.