Connecting the Dots: How Viruses Could Play a Role in Neurodegenerative Diseases

 

Viral infections can be a risk factor for neurodegenerative diseases

Neurodegenerative ailments are capable of causing damage to various parts of the nervous system, including the brain, which can lead to issues concerning memory, thinking, and/or movement. Alzheimer's disease (AD), multiple sclerosis (MS), and Parkinson's disease (PD) are some examples of such diseases, which usually occur later in life. Unfortunately, there are very few effective treatments available for these conditions.

Previous research findings have indicated that viruses may play a crucial role in certain neurodegenerative diseases. For instance, a recent study found a correlation between Epstein-Barr virus infection and the risk of MS. Additionally, there are concerns about the cognitive impacts of SARS-CoV-2, the virus responsible for COVID-19.

To examine the links between viruses and neurodegenerative disease more generally, a team of researchers led by Drs. Mike Nalls, Kristin Levine, and Hampton Leonard of NIH's Center for Alzheimer’s and Related Dementias conducted an analysis of data from the FinnGen project, a repository of biomedical data, or biobank, comprising information from more than 300,000 individuals in Finland. The researchers searched the biobank for individuals who had been diagnosed with one of six different conditions, including AD, ALS, generalized dementia, vascular dementia, PD, and MS, and then checked how many of them had been hospitalized for a viral illness previously. To confirm their findings, the team also looked for the same associations in the UK Biobank, which contains data from almost 500,000 people in the United Kingdom. The results of this research appeared in Neuron on January 19, 2023.

The researchers identified 45 associations between viruses and neurodegenerative diseases in FinnGen. Of these, 22 also appeared in the UK Biobank. The strongest association observed was between viral encephalitis, which is characterized by brain inflammation caused by a virus, and AD. A person who had viral encephalitis in the FinnGen database was 30 times more likely to be diagnosed with AD than someone who had not experienced encephalitis. The results from the UK Biobank were similar, with individuals with viral encephalitis being 22 times more likely to develop AD than those without.

In addition, the team identified the same association between Epstein-Barr virus and MS in FinnGen that was previously described. However, this association was not observed in the UK Biobank, which could be due to differences in how the two biobanks used hospital diagnostic codes; Epstein-Barr viruses are common and therefore are not always noted.

Influenza with pneumonia was associated with all of the neurodegenerative diseases, except for MS. The researchers only included cases of influenza that were severe enough to necessitate hospitalization in the study, and therefore, these associations only apply to the most severe cases of influenza.

FinnGen contains data on the same individuals over time, which allowed the researchers to examine how the associations depended on the time elapsed since infection. They discovered that certain viral infections were associated with an increased risk of neurodegenerative disease for up to 15 years after infection.

The researchers pointed out that vaccines exist for some of the viruses they identified, including influenza, varicella-zoster (which causes chickenpox and shingles), and certain pneumonia-causing viruses. Therefore, vaccination could potentially reduce some of the risk associated with the conditions they examined.

"The results of this study provide researchers with several new critical pieces of the neurodegenerative disorder puzzle," Nalls said. "In the future, we plan to use the latest data science tools to not only find more pieces but also help researchers understand how those pieces, including genes and other risk factors, fit together."