Herpes simplex virus-1 (HSV-1) is a common virus; the World Health Organization has estimated that as many as two-thirds of people worldwide who are under age 50 may be infected. HSV-1 can spread through oral contact and these infections may cause blisters or cold sores. Usually, symptoms are mild. But scientists have also shown that HSV-1 is able to move to the eye or nervous system in some cases and may lead to serious, chronic symptoms. New research has now revealed that nasal herpes infections can cause anxiety, cognitive issues, or motor impairment. The findings have been reported in mBio.
In nasal infections, pathogens don't have far to travel to get to the central nervous system.
"If an infected individual is shedding virus via tears, it could reach the nasal cavity, where it could go more directly to the brain," said corresponding study author Professor Deepak Shukla of the Univeersity of Illinois at Chicago. "I think it's underdiagnosed and understudied, but the neurological consequences, we believe, are much more severe than you would normally see with fever blisters or ocular infection."
This work determined that in HSV-1 infections, the virus can harness an enzyme called heparanase, which is made in host cells. This can lead to behavioral symptoms. When heparanase was deactivated in animal models, there were fewer behavioral symptoms after HSV-1 infection compared to animals with functional heparanase. The researchers suggested that heparanase is, therefore, related to the neurological symptoms of HSV-1 infections.
Animals with HSV-1 infections showed high levels of neuronal damage and inflammation within days of exposure. For several months after the infection, which can equate to decades of a human life, there were impairments in motor coordination and memory, and these animals showed symptoms of anxiety.
"There is definitely nerve damage if you take the intranasal route, and the effects are long-term, which is alarming," Shukla said.
The research has highlighted the importance of preventing and treating HSV-1 infections, which are likely carried by billions of people.
Sources: University of Illinois at Chicago, mBio