According to new research, common viral infections could have far-reaching effects on our brain health. The study found a link between dozens of different viral exposures and a subsequent increased risk of Alzheimer’s disease and other brain disorders. However, more research will be needed to unravel the true role, if any, that these infections might play in causing these conditions.
The research comes from scientists at the US National Institutes of Health. They analyzed data from two existing, nationally representative biobank projects that track 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 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 potentially linked to an increased risk of neurodegenerative disease. To double-check these results, they then performed the same type of analysis on the UK data and found a similar relationship for 22 types of viral exposure in both data sets.
Some of these exposures involved specific viral infections, such as influenza, varicella-zoster virus (causes chickenpox and shingles), and herpes simplex viruses. Others involved 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 disease continued up to 15 years later, while the strongest link was observed 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 can eventually cause later neurological conditions. Studies in recent years have related herpes virus to Alzheimer’s disease in particular. The authors were explicitly motivated to dig deeper by research published last year showing that Epstein-Barr infection is probably the main 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 found the same link between infection of ‘Epstein-Barr and multiple sclerosis). Although numerous studies have examined the relationship between infections and brain disease, the authors say theirs is the first to systematically investigate multiple pairings of germs with subsequent neurological disease.
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Much of this growing body of research, including this study, has only been able to show a correlation between the infection and subsequent brain disease, but not a direct causal link. There will likely be many other aspects of this risk to consider, even if it is real.
Some suspected culprits, like herpes viruses, can cause problems while they infect us, but are largely dormant in our nervous system, for example. The increased risk posed by other exposures could represent scars caused by a severe infection that has been successfully removed. And there are almost certainly other factors that predispose people to developing neurological diseases working in tandem with these infections. Just about everyone gets Esptein-Barr at some point in their life, for example, but less than 1% of the population eventually develops multiple sclerosis.
Yet even if these common infections play only a small role in why people get dementia or other brain diseases, this additional risk could still be significant at the population level. If further research continues to validate these links, it would further underscore the need to develop and deliver 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 diseases,” note the authors.