Large study finds link between viral infections and future brain disease

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

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

Common viral infections can have far-reaching consequences for 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. However, more research is needed to unravel the true role of these infections, if any.

The research comes from scientists at the US National Institutes of Health. They analyzed data from two existing and nationally representative biobank projects that track the long-term health of residents in Finland and the UK, respectively, involving about 450,000 people together. They looked for links between viral infections leading 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 Finnish data, they initially identified 45 types of viral exposure that may be linked to a greater risk of neurodegenerative disease. To check these results, they then performed the same kind of analysis on the UK data and found a similar relationship for 22 types of viral exposure in both datasets.

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

This is far from the first research to suggest that viral infections could potentially cause later neurological disorders. Studies in recent years have linked herpesviruses to Alzheimer’s disease in particular. The authors were explicitly motivated to dig deeper by research published last year that showed just that Epstein-Barr infection is probably the leading cause of multiple sclerosis, as well as concerns that covid-19 can sometimes cause persistent neurological problems (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 disease, the authors say their study is the first to systematically examine multiple links of germs to later neurological disease.

Much of this growing body of research, including this study, has only been able to show an association between infection and subsequent brain disease, but not a direct cause-and-effect relationship. There will probably be many other aspects of this risk to consider, even if it is real.

Some suspected culprits, such as herpes viruses, can cause problems while infecting us, but lie largely dormant in our nervous system, for example. The increased risk from other exposures may represent the scarring caused by a serious infection that has been successfully cleared. And there are almost certainly other factors that predispose people to developing neurological disorders that interact with these infections. For example, almost everyone gets Esptein-Barr at some point in their lives, but less than 1% of the population eventually develops multiple sclerosis.

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

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

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