Large Study Finds Link Between Viral Infections and Future Brain Illness

An illustration of varicella zoster virus, the cause of chickenpox and shingles.

An illustration of varicella zoster virus, the cause of chickenpox and shingles.
Illustration: Shutterstock (Shutterstock)

Common viral infections may be having far-reaching effects on our brain health, new research suggests. The study found a link between dozens of different viral exposures and a later increased risk of Alzheimer’s disease and other brain disorders. More research will be needed to untangle the true role, if any, these infections could play in causing these conditions, however.

The research comes from scientists with the US National Institutes of Health. They analyzed data from two existing and nationally representative biobank projects tracking the long-term health of residents in Finland and the UK, respectively, collectively involving around 450,000 people. They looked for links between viral infections that led to hospitalization and six neurodegenerative diseases: Alzheimer’s disease (the most common form of dementia), ALS, multiple sclerosis, Parkinson’s disease, vascular dementia, and generalized dementia.

In the Finland data, they initially identified 45 types of viral exposure potentially linked to a greater risk of neurodegenerative illness. To double-check these results, they then ran the same sort of analysis on the UK data and found a similar relationship for 22 types of viral exposure across both datasets.

Some of these exposures involved specific viral infections, such as influenza, varicella zoster virus (the cause of chickenpox and shingles), and herpes simplex viruses. Others concerned where an infection or its harmful effects took place, such as viral encephalitis or meningitis, types of brain inflammation that can be caused by many different viruses. For some exposures, the risk of subsequent brain illness extended up to 15 years later, while the strongest link was seen between viral encephalitis and Alzheimer’s. The team’s findings were published earlier this month in Neuron.

This is far from the first research to suggest that viral infections can possibly cause later neurological conditions. Studies in recent years have linked herpesviruses to Alzheimer’s in particular. The authors were explicitly motivated to dig deeper by research published last year showing that Epstein-Barr infection is likely the leading cause of multiple sclerosis, as well as concerns that covid-19 can sometimes cause lingering neurological issues (covid-related hospitalizations were not included in the analysis, but the authors did find the same link between Epstein-Barr infection and multiple sclerosis). While many studies have looked at the relationship between infections and brain diseases, though, the authors say theirs is the first to systematically investigate multiple pairings of germs to later neurological illnesses.

Much of this growing body of research, including this study, has only been able to show a correlation between infection and later brain illness, not a direct cause-and-effect link, however. There will likely be many other aspects of this risk to consider, even if it’s genuine.

Some suspected culprits, like herpesviruses, may cause trouble while they’re infecting us but largely lying dormant in our nervous system, for instance. The increased risk posed by other exposures might represent the scars caused by a serious infection that was successfully cleared. And there are almost certainly other factors that predispose people to developing neurological illness working in tandem with these infections. Just about everyone catches Esptein-Barr at some point in their lives, for instance, but less than 1% of the population ultimately develops multiple sclerosis.

Still, even if these common infections play only a small part in why people get dementia or other brain diseases, that added risk could still be substantial on a population level. If further research continues to validate these links, it would further emphasize the need to develop and provide effective treatments that can prevent the worst effects of these infections.

“As vaccines are currently available for some of the associated viruses, vaccination may be a way to reduce some risk of neurodegenerative disease,” the authors note.

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