A common virus may slink into the brain through the nose. After setting up shop in people’s nasal mucus, human herpesvirus-6 may travel along olfactory cells right into the brain, researchers report online the week of August 8 in the Proceedings of the National Academy of Sciences.
Most people’s first bout with HHV-6 comes at a tender age: It causes the common childhood infection roseola, marked by a chest rash and a high fever. “Everyone is exposed to this,” says study coauthor Steven Jacobson of the National Institute of Neurological Disorders and Stroke in Bethesda, Md. “You have it. I have it.”
Despite its ubiquity, very little is known about the virus. HHV-6 may live in tonsils and shed in saliva, some studies suggest. And in some people (researchers don’t know how many), the virus can infect the brain, where some researchers believe it may contribute to neurological disorders such as multiple sclerosis, encephalitis and a form of epilepsy.
Other viruses such as herpes simplex, influenza A and rabies can invade the brain by shooting through the nose, so Jacobson and his team wondered whether HHV-6 could do the same trick.
The researchers found high levels of HHV-6 in the olfactory bulb, a smell-related part of the brain, in two of three autopsy brain samples. The team then looked at nose mucus and found the virus in 52 of 126 different samples. “We were surprised to find so much in the nasal mucus,” Jacobson says.
And in a lab dish, the team found, specialized cells that help connect nerves to the brain were susceptible to HHV-6 infection. These cells might be a route of entry for the virus, Jacobson says.
“Viruses take advantage of whatever they can,” says neurologist Avindra Nath of the National Institute of Neurological Disorders and Stroke, who was not involved in the study. “They’ll try to gain entry any way they can, so it’s not surprising that they’d use nasal mucosa to do so.”
Yet the results should be interpreted cautiously, says neuroimmunologist Robyn Klein of Washington University School of Medicine in St. Louis. Since the study presents correlative data on a small number of samples, it’s hard to say whether HHV-6 really travels along olfactory pathway into the brain. “Am I convinced that this is how it gets in? No,” she says. “Is it a possibility? Sure.”
Confirming the nose-to-brain passage is important, Klein says, because a virus’s entry point to the brain may have a big impact on the infection’s outcome. Because each part of the brain is distinct, an infection in one part could cause very different outcomes than infection in another part.
Virologist Dharam Ablashi, who codiscovered HHV-6 and now is scientific director of the HHV-6 Foundation in Santa Barbara, Calif., says the results may be just the tip of an iceberg. “As research techniques improve,” he says, “we may find that most neurological conditions are caused by viruses that enter the brain through the nasal passages.”