A virus that most adults on Earth have been exposed to has a sneaky route into the brain, and a world-first study has now demonstrated that it can cause behavioral changes. While this form of infection thankfully appears to be rare, the authors do believe it merits more attention.
Herpes simplex virus type 1 (HSV-1) is an extremely prevalent virus that’s usually associated with cold sores. The World Health Organization estimates that 64 percent of the global population under the age of 50 is infected, and most will go for long periods of time – or even forever – with no symptoms at all.
While for most adults HSV-1 infection has not historically been considered a “big deal” – though some of its consequences can be very unpleasant – recent evidence suggests that this virus could have a bit more to answer for than we realized. Studies began to find an association between herpesviruses and neurodegenerative disease. As well as setting up camp for life in our peripheral nerves, it seems these viruses have the potential to travel to the brain itself.
As well as shedding more light on just how they do it, a new study is the first to indicate the possibility that these brain infections may lead to behavioral changes.
There’s one particularly stealthy way in which HSV-1 can reach the central nervous system, as lead author Dr Deepak Shukla of the University of Illinois Chicago explained in a statement: “If an infected individual is shedding virus via tears, it could reach the nasal cavity, where it could go more directly to the brain.”
If you’re feeling uneasy at the thought of a virus working its way up your schnozz to do heaven knows what inside your brain, that’s more than fair.
“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,” Shukla added.
To examine these possibilities in more detail, the team infected mice with HSV-1. To mimic a chronic infection in humans, the mice were observed for six months, equivalent to several human years.
“The virus reactivates throughout life; it’s a lifelong infection. So, I think this awareness will be really important among the large population which is carrying this virus,” explained co-author Dr Chandrashekhar Patil.
In the immediate aftermath of the infection, the team observed signs of neuronal damage and inflammation. Over time, the infected animals performed worse than their control counterparts on tests of coordination and memory, and they showed more signs of anxiety.
“There is definitely nerve damage if you take the intranasal route, and the effects are long-term, which is alarming,” said Shukla.
But there was a bit of good news. Previous research indicated that HSV-1 might make use of an enzyme in our cells called heparanase (HPSE) to help it invade brain cells successfully. Mice with the Hpse gene knocked out did not see the same neurological issues, which could indicate a potential therapeutic avenue to prevent these damaging effects.
Scientists are learning more all the time about the link between this all-too-common virus and neurological issues. While this particular pathway to the brain is rare in humans, the authors are concerned about the potential for harm given it’s such a direct route.
These findings in mice may not directly apply to humans, it’s important to say; but according to the team, their results are a basis for future research that could reveal the scale of this potential issue in humans, as well as how to tackle it.
The study is published in the journal mBio.
