In a study of more than 450 000 people, researchers found a significant 22 connections between viral infections and neurodegenerative conditions, suggesting that people treated for viral encephalitis were 31 times more likely to develop Alzheimer’s disease (for every 406 viral encephalitis cases, 24 later developed Alzheimer’s – around 6%).
Those who were admitted to hospital with pneumonia after being infected with flu appeared more susceptible to Alzheimer’s, dementia, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS), they said, while intestinal infections and meningitis (both often caused by a virus), as well as the varicella-zoster virus, which causes shingles, were also implicated in the development of several neurodegenerative diseases.
The impact of viral infections on the brain persisted for up to 15 years in some cases, reports Science Alert. And there were no instances where exposure to viruses was protective.
Around 80% of the viruses implicated in brain diseases were considered “neurotrophic”, which means they could cross the blood-brain barrier.
“Strikingly, 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 illness, they are known to dramatically reduce hospitalisation rates. This evidence suggests that vaccination may mitigate some risk of developing neurodegenerative disease.”
In 2022, a study of more than 10m people linked the Epstein-Barr virus with a 32-fold increased risk of multiple sclerosis.
“After reading this study, we realised that for years scientists had been searching – one-by-one – for links between an individual neurodegenerative disorder and a specific virus,” said senior author Michael Nalls, a neurogeneticist at the National Institute on Ageing 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 search for all possible links in one shot.”
First, the researchers analysed the medical records of around 35 000 Finns with six different types of neurodegenerative diseases and compared this against a group of 310 000 controls who did not have a brain disease.
This analysis yielded 45 links between viral exposure and neurodegenerative diseases, and this was narrowed down to 22 links in a subsequent analysis of 100 000 medical records from the UK Biobank.
While this retrospective observational study cannot demonstrate a causal link, it adds to the pile of research hinting at 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 the director of the Centre 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 possibly avoidable – risk factors for these types of disorders.”
This study was published in Neuron.
Study details
Virus exposure and neurodegenerative disease risk across national biobanks
Kristin Levine, Hampton Leonard, Cornelis Blauwendraat et al.
Published in Neuron in April 2023
Summary
With recent findings connecting the Epstein-Barr virus to an increased risk of multiple sclerosis and growing concerns regarding the neurological impact of the coronavirus pandemic, we examined potential links between viral exposures and neurodegenerative disease risk. Using time series data from FinnGen for discovery and cross-sectional data from the UK Biobank for replication, we identified 45 viral exposures significantly associated with increased risk of neurodegenerative disease and replicated 22 of these associations. The largest effect association was between viral encephalitis exposure and Alzheimer’s disease. Influenza with pneumonia was significantly associated with five of the six neurodegenerative diseases studied. We also replicated the Epstein-Barr/multiple sclerosis association. Some of these exposures were associated with an increased risk of neurodegeneration up to 15 years after infection. As vaccines are currently available for some of the associated viruses, vaccination may be a way to reduce some risk of neurodegenerative disease.
See more from MedicalBrief archives:
Viral illnesses linked to dementia, neurodegenerative diseases – US study
Flu and pneumonia vaccinations may reduce Alzheimer's risk
More evidence flu jab may guard against Alzheimer’s – Texas cohort study
Corroborating the role of herpes simplex virus in Alzheimer’s