What if mental illness is catching?
Although it sounds far-fetched and remains controversial, this theory got another boost from a study published recently in the journal Molecular Psychiatry. Using a new diagnostic tool to screen blood for a pathogen known as the Borna virus, a team of German researchers from major academic institutions found that it infects up to 30% of healthy people and up to 100% of people with severe mood disorders.
Borna disease is common in horses, where it can cause encephalitis. It's also been known to strike birds, cows, sheep, cats and dogs, producing behavior changes that are eerily similar to depression and other neuropsychiatric disorders in humans. Named after a town in Saxony (now Germany), where an outbreak of encephalitis in horses crippled the Prussian army in the late 1800s, Borna disease has been recognized in recent years as an emerging illness among humans.
In 1996, scientists at Scripps Research Institute in La Jolla found the first evidence that the Borna disease virus can infect human brain tissue. All of those infected had a history of mental disorders involving memory loss and depression.
There are widely varying infection rates among animals and people in Europe, North America and parts of Asia. The link between neuropsychiatric disorders and infection in humans varies as well.
Researchers in South Korea, for example, found no link between Borna disease infection and mental illness, while investigators in Taiwan found a high rate of infection in people with schizophrenia--and among their family members and mental health workers.
"The fact that you find evidence of an infection in one population or another does not allow you to conclude that there is a causal relationship," says W. Ian Lipkin, professor of neurology at UC Irvine and head of a lab that unraveled the Borna disease virus genome.
In other words, Lipkin says, scientists still cannot determine which comes first: the infection or the mental disorder. (Various mental illnesses can suppress the immune system and make individuals more vulnerable to certain microbes.)
And they also don't know how close the association is. It's possible, for example, to have Borna disease without depression and depression without Borna disease.
How Borna disease is transmitted is also still a mystery, although there's evidence in animals that it might spread via nasal passages, Lipkin notes in an article in this month's issue of Trends in Microbiology. The article, written by Lipkin and two of his Irvine colleagues, notes that the link between the Borna virus and human disease remains controversial but warrants continued investigation.
The latest findings from Germany are likely to boost interest in Borna disease, Lipkin said. The new results "are interesting," he said. "If this can be replicated and the linkage [between Borna disease and depression] can be established, this would be very important."
Even so, no one suggests that Borna disease might be the only cause of depression and other mental disorders. Although researchers have discovered a link between certain types of infection and heart disease, they have not concluded that one causes the other. A similar association between Borna disease and psychiatric disease would not prove cause, but it might yield insight into mental illness.
"We have learned a great deal and are beginning to understand that there are a whole host of environmental factors that cause a wide range of disease, whether we're talking about cardiovascular disease, diabetes, multiple sclerosis or schizophrenia and depression," Lipkin says.
"It's a whole new way of looking at things."