Scientists reported Thursday what they say is the first direct evidence linking a virus known to cause neurological disease in horses and sheep to psychiatric problems in humans.

A team of American and German researchers examined the relationship between the Borna virus and human illness by copying a piece of the virus's genetic material and seeing if it spurred formation of antibodies in blood taken from people with schizophrenia or personality disorders.The body's immune system produces proteins called antibodies to help fight off infectious agents like viruses. Because it is difficult to detect viruses themselves, doctors test for the specific antibodies they trigger to determine if someone is infected with a particular virus.

In a study published in the journal Science, the researchers found blood samples from three of seven German psychiatric patients produced antibodies to the Borna virus, compared with none of the samples from healthy people.

The people involved in the study were from parts of Germany where the Borna virus is widespread in horses and sheep. Researchers do not know exactly how the virus, which appears to invade the nervous system, is transmitted.

Previous studies of humans and rats have indirectly linked the virus to psychiatric and behavioral disorders.

Recent work in Germany and the United States showed psychiatric patients have antibodies in their bloodstream similar to those triggered by the laboratory-made piece of virus. However, those studies could not directly link those antibodies to the Borna virus.

With the latest results in hand, researchers are now trying to identify the whole Borna virus both in animals and humans.

"Once we do that, this information could lead to diagnostic tests for infected patients and eventually to anti-viral agents to treat the infection and possibly psychiatric disorders as well," said Janice Clements of Johns Hopkins Medical Institutions, a co-author of the study.