How common viral illnesses could be silently sabotaging your brain health.

According to a study recently published in the journal Neuron, common viral illnesses make people more susceptible to Alzheimer’s disease and five other catastrophic neurodegenerative (ND) diseases. Unexpected associations have also been found between dementia, Parkinson’s disease, multiple sclerosis (MS), Lou Gehrig’s disease, or amyotrophic lateral sclerosis (ALS), and vascular dementia.

The first study, which focused on data from blood samples from 10 million American soldiers, showed that it is highly unlikely that multiple sclerosis would develop without infection with the Epstein-Barr virus. These results prompted researchers at the US National Institutes of Health Center on Alzheimer’s Disease and Related Dementia (NIH CARD) to investigate other important links between viral infections and ND.
In Finland and Great Britain, alarming connections were found based on the analysis of the medical records of almost a million people.


Which viruses are associated with a decline in brain function?

Biomedical data researchers process data from 300,000 patients at the FinnGen biobank in Finland and 500,000 patients at the UK biobank. The findings, published in the journal Neuron, discovered up to 22 links between a previous hospital-acquired viral infection and a diagnosis of neurodegenerative disorder (NDD) 5 to 15 years later.
The researchers were able to identify five significant infection-NDD associations from the FinnGen dataset Hits from the UK Biobank were then examined, and 22 associations could be replicated, which was the focus of the study.
A positive association was found between the worsening of influenza to pneumonia and the five NDs mentioned above, except for multiple sclerosis. People who were treated for encephalitis, or inflammation of the brain, were almost 31 times more likely to develop Alzheimer’s disease.

To clarify, six percent of patients with viral encephalitis developed Alzheimer’s disease in the following years. In addition, intestinal infections and the herpes virus that causes chicken pox (varicella zoster virus) have been found to cause several neurodegenerative diseases. In 
of six NDs, dementia was found to be most frequently associated. Eighty percent of the replicated associations were due to neurotrophic viruses—viruses that enter the central nervous system through the outer surface of the brain and spinal nerves. All viruses were found to increase the risk of ND, and none was associated with a protective effect.


Can flu shots help dementia?

Although these findings seem surprising, the study has some unavoidable limitations. A data-science approach examines all possible links simultaneously. The resulting pairs are data connections only, and causation cannot be confirmed. It also does not take into account susceptibility due to genetics, age, lifestyle, or environmental factors.
The study must be expanded to include those individuals who were infected but not hospitalized. NIH CARD Director Andrew Singleton agrees that mechanistically linking viral exposure to ND pathogenesis requires rigorous follow-up studies.
However, the research raises an issue that could have significant implications for public health. It also protects against the need for vaccines, as they significantly reduce hospitalizations. Fortunately, vaccines are available for some of the viruses involved and may be able to reduce the risk of developing ND.
Influenza, pneumonia, and shingles vaccines reduce dementia. However, the role of vaccines also requires more research to determine their effectiveness in slowing down and possibly preventing the development of ND.

This study was published in the journal Neuron.

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