Study of 500,000 medical records repeatedly links viruses to Alzheimer's: ScienceAlert

A study of around 500,000 medical records found that severe viral infections such as encephalitis and pneumonia increase the risk of neurodegenerative diseases such as Parkinson's and Alzheimer's.

In the study of around 450,000 people, researchers found 22 connections between viral infections and neurodegenerative diseases.

People treated for a brain inflammation called viral encephalitis were 31 times more likely to develop Alzheimer's disease. (Of 406 viral encephalitis cases, 24 developed Alzheimer's disease – about 6 percent.)

Those 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 effects of viral infections on the brain lasted up to 15 years in some cases. And there were no cases where exposure to viruses was protective.

Around 80 percent of viruses involved in brain diseases were thought to be “neurotrophic,” meaning they could cross the blood-brain barrier.

“Notably, there are currently vaccines against 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 hospitalizations. These findings suggest that vaccinations can partially reduce the risk of neurodegenerative disease.”

In 2022, a study of more than 10 million people linked the Epstein-Barr virus to a 32-fold increased risk of multiple sclerosis.

“After I read it [this] “In the study, we realized that for years, scientists had been looking one after another for links between a single neurodegenerative disease and a specific virus,” said lead 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. “Using medical records allowed us to systematically search all possible links at once.”

First, the researchers analyzed the medical records of around 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 revealed 45 associations between virus exposure and neurodegenerative diseases, which were narrowed down to 22 associations 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 wealth of research pointing to the role of viruses in Parkinson's and Alzheimer's disease.

“Neurodegenerative diseases 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 disease researcher and director of the Center for Alzheimer's and Related Dementias.

“Our results support the notion that viral infections and associated inflammation in the nervous system may be common – and potentially preventable – risk factors for these types of diseases.”

This study was published in Neuron.

An earlier version of this article was published in January 2023.