According to the National Institute of Mental Health (NIMH), 4.4 percent of the population of the United States will have bipolar disorder at one point in their lives.
Another 16.2 million, or around 6.7 percent of all U.S. adults, will experience at least one episode of severe depression in their lives.
While the precise causes of such, often debilitating, psychiatric conditions remain unknown, scientists do know that both genes and the environment play a role.
For instance, a recent study identified 44 genetic loci thought to raise the risk of depression, while another has suggested that 80 percent of schizophrenia risk can be attributed to genes.
New research that now appears in the journal Frontiers in Microbiology highlights the fact that environmental factors such as viruses may be the driving force behind these disorders.
An international team of scientists led by Bhupesh Prusty — from the Department of Microbiology at the University of Würzburg in Germany — discovered that in the brains of people who lived with bipolar and major depression, a class of neurons called Purkinje cells was infected with the herpesvirus HHV-6A.
Purkinje neurons are inhibitory brain cells located in the human cerebellum, which is the brain area responsible for controlling movement, muscles, balance, and posture.
However, some research has also tied this brain region to language, cognition, and mood.
How HHV-6 may cause depression, bipolar
Prusty and team started from the hypothesis that the human herpesviruses HHV-6A and HHV-6B may drive the development of psychiatric disorders.
So, they examined two large cohorts of brain biopsies from the Stanley Medical Research Institute in Kensington, MD.
“We were able to find active infection of HHV-6 predominantly within Purkinje cells of human cerebellum in bipolar and major depressive disorder patients,” Prusty reports.
“Inherited factors,” he continues, “have long been known to increase the risk of developing several types of psychiatric disorders including bipolar disorder, major depressive disorder, and schizophrenia.”
However, continues Prusty, environmental factors such as viruses can also contribute by triggering neuroinflammation in early life. “Pathogens may disrupt neurodevelopment and cross-talk with the immune system at key developmental stages,” he suspects.
The findings of this study suggest that the herpesvirus HHV-6 could infect brain cells and cause cognitive and mood disorders.
Prusty also explains that the results of the study contradict the belief that latent viruses — that is, viruses thought to be inactive, laying dormant in organs and tissues — are completely harmless.
“Studies like ours prove this thinking as wrong,” says Prusty, who points to the mounting evidence that shows that human herpesviruses may cause other neurological conditions.
For instance, a study that Medical News Today recently reported on found “strong evidence” to suggest that the human herpesviruses HHV-6A and HHV-7 may cause Alzheimer’s disease.
A much higher number of these viruses were found in the brains of people who had lived with the disease. Another study that we covered offered “the first population evidence for a causal link between herpes virus infection and Alzheimer’s disease.”
Next, Prusty and his colleagues plan to study the molecular mechanisms that could explain exactly how HHV-6A damages Purkinje cells, and how this could lead to psychiatric disorders.
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