by A review of approximately 500,000 medical records suggested that serious viral infections such as encephalitis and pneumonia increase the risk of neurodegenerative diseases such as Parkinson’s and Alzheimer’s.
Researchers found 22 links between viral infections and neurodegenerative diseases in the study of about 450,000 people.
People treated for a type of brain inflammation called viral encephalitis were 31 times more likely to develop Alzheimer’s disease. (For every 406 cases of viral encephalitis, 24 developed Alzheimer’s disease – about 6 percent.)
Those who were hospitalized with pneumonia after contracting the flu appeared to be more susceptible to Alzheimer’s disease, dementia, Parkinson’s disease and amyotrophic lateral sclerosis (ALS).
Intestinal infections and meningitis (both often caused by a virus), as well as the varicella-zoster virus, which causes shingles, have also been implicated in the development of several neurodegenerative diseases.
The impact of viral infections on the brain lasted for up to fifteen years in some cases. And there were no cases where exposure to viruses was protective.
About 80 percent of the viruses involved in brain diseases were considered “neurotrophic,” meaning they could cross the blood-brain barrier.
“Remarkably, vaccines are currently available for some of these viruses, including influenza, shingles (varicella-zoster) and pneumonia,” the researchers wrote in their paper published last year.
“Although vaccines do not prevent all cases of disease, they are known to dramatically reduce hospitalization rates. This evidence suggests that vaccination may somewhat reduce the risk of developing neurodegenerative diseases.”
In 2022, a study of more than 10 million people linked Epstein-Barr virus to a 32-fold increased risk of multiple sclerosis.
“After reading [this] study, we realized that for years scientists had been looking one-by-one for links between an individual neurodegenerative condition and a specific virus,” said senior author Michael Nalls, a neurogeneticist at the National Institute on Aging in the US.
“That’s when we decided to try a different, more data science-based approach,” he said. “By using medical records, we were able to systematically look for all possible connections at once.”
First, the researchers analyzed the medical records of about 35,000 Finns with six different types of neurodegenerative diseases and compared them with a group of 310,000 controls who had no brain disease.
This analysis identified 45 links between viral exposure and neurodegenerative diseases, and this was reduced to 22 links in a subsequent analysis of 100,000 medical records from the UK Biobank.
Although this retrospective observational study cannot prove a causal relationship, it adds to the pile of research pointing to the role of viruses in Parkinson’s and Alzheimer’s disease.
“Neurodegenerative disorders are a collection of diseases for which there are very few effective treatments and many risk factors,” said co-author Andrew Singleton, a neurogeneticist and Alzheimer’s researcher and director of the Center for Alzheimer’s and Related Dementias.
“Our results support the idea that viral infections and related inflammation in the nervous system may be common – and potentially preventable – risk factors for these types of conditions.”
This study was published in Neuron.
An earlier version of this article appeared in January 2023.
