A recent study analyzing about 500,000 medical records highlighted that serious viral infections, such as encephalitis and pneumonia, may elevate the risk of developing neurodegenerative diseases like Parkinson’s and Alzheimer’s.
Researchers identified 22 links between viral infections and neurodegenerative conditions while examining around 450,000 individuals.
Interestingly, those treated for viral encephalitis had a staggering 31-fold increase in likelihood of developing Alzheimer’s. To put it another way, among the 406 cases of viral encephalitis, 24 eventually developed Alzheimer’s—approximately 6 percent.
Moreover, people who were hospitalized for pneumonia following the flu appeared to be at greater risk for Alzheimer’s, dementia, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS).
Infections in the intestines and meningitis, often caused by viruses, along with the varicella-zoster virus responsible for shingles, also showed connections to several neurodegenerative diseases.
The influence of viral infections on neurological health was noted to persist for up to 15 years in some instances. Notably, there were no cases indicating that viral exposure provided any protective benefits.
Roughly 80 percent of the viruses linked to brain diseases were categorized as ‘neurotrophic,’ meaning they can cross the blood-brain barrier.
Interestingly, vaccines are available for some of these viruses, including those causing influenza, shingles, and pneumonia, according to the researchers. They highlighted that while vaccines don’t prevent all illnesses, they significantly reduce hospitalization rates, suggesting vaccination might lessen the risk of neurodegenerative diseases.
In another study conducted in 2022 with over 10 million participants, a connection was found between the Epstein-Barr virus and a 32-fold increase in the risk of multiple sclerosis.
Michael Nalls, a neurogeneticist at the National Institute on Aging, pointed out that scientists had traditionally looked for individual links between specific neurodegenerative disorders and viruses, but this study took a more data-driven approach by analyzing medical records for broader connections.
The team first looked at the medical records of about 35,000 Finns with various neurodegenerative diseases and contrasted this with a control group of 310,000 individuals who were healthy. This led to 45 potential links between viral exposure and neurodegenerative diseases, which was then narrowed to 22 after further analysis of 100,000 medical records from the UK Biobank.
Although this retrospective observational study doesn’t establish a cause-and-effect relationship, it adds to the existing body of research suggesting a link between viruses and conditions like Parkinson’s and Alzheimer’s.
Co-author Andrew Singleton, who is a neurogeneticist and Alzheimer’s researcher, emphasized that neurodegenerative disorders encompass diseases with limited effective treatments and numerous risk factors. He noted that their findings support the notion that viral infections and associated inflammation in the nervous system could be widespread—and potentially avoidable—risk factors for these disorders.
This study was published in Neuron.
An earlier version of this article was published in January 2023.
