The nose apparently can be a portal for a cousin of the herpes virus that is linked to brain disorders, scientists have discovered.
These findings reveal a new way the brain can get infected.
Scientists investigated human herpes virus-6 (HHV-6), a member of the family of viruses that includes genital herpes as well as oral herpes, which causes cold sores. HHV-6 is linked with brain disorders such as multiple sclerosis, encephalitis and a form of epilepsy, and causes roseola, a disease common among infants that leads to a high fever and skin rash.
"This is a virus that we've all been exposed to, that we all pretty much acquired in childhood," said researcher Steven Jacobson, a neurovirologist at the National Institute of Neurological Disorders and Stroke in Bethesda, Md. "Most of the time it's utterly benign."
The way this virus entered the brain had remained a mystery, as the seat of our intelligence is largely protected by the so-called blood-brain barrier, which filters out many germs and drugs.
However, researchers had known that other viruses, such as influenza and rabies, apparently could use the sensory network hooked up to the nose as a kind of highway into the central nervous system.
To see how HHV-6 enters the brain, scientists analyzed tissue samples from autopsies, including a patient who had multiple sclerosis. Although viral DNA was seen throughout the brain, it was found largely in the olfactory bulb, the brain region involved in detecting odors.
In addition, the researchers found DNA from HHV-6 in nasal mucus samples from healthy people, those suffering a loss of smell, and people with multiple sclerosis. This suggests the nasal cavity might harbor the virus in both healthy and diseased individuals.
Moreover, in experiments, scientists demonstrated that HHV-6 could infect lab-grown versions of the olfactory ensheathing cells, which help olfactory neurons grow and establish connections in the brain. The researchers believe the virus might use these cells as a bridge across the blood-brain barrier, the first time scientists had evidence these cells could be a route of infection.
"Now researchers can start looking to see if other viruses might use this route as well," Jacobson told LiveScience.
Jacobson cautioned that while this virus might help trigger brain disorders, it was not necessarily the primary cause. "We may all have it, but some might have a special genetic susceptibility to it, or maybe there's an environmental trigger that causes neurologic disease to then occur," Jacobson said.
Further studies could also investigate whether this virus has any effect on behavior. "It all depends on where this virus goes in the brain," Jacobson said. With the new information, researchers could then look for therapies against this virus.
The scientists detailed their findings Monday in the Proceedings of the National Academy of Sciences.