A new study published in the journal Neuron has found that some viral infections can be significantly associated to the onset of neurodegenerative disease such as Alzheimer’s and Parkinson’s disease.
The research used real-world bio bank scale data – analysing about 450,000 electronic health records to see if there was a link between infections with influenza and other common viruses and an increased risk of having a neurodegenerative condition later in life.
The analysis found at least 22 links between viral infections and neurodegenerative diseases. Researchers said that the largest effect association was between exposure to the rare virus encephalitis and Alzheimer’s disease. People with encephalitis were about 31 times more likely to develop Alzheimer’s later in life than were people who did not.
Others links were less startling: people who had flu followed by pneumonia were four times more likely to develop Alzheimer’s than were people who didn’t develop both flu and pneumonia.
However, severe influenza with pneumonia was significantly associated with five of the six neurodegenerative diseases that were studied – namely Alzheimer’s, dementia, Parkinson’s Disease, amyotrophic lateral sclerosis, vascular dementia, and multiple sclerosis.
The research also replicated a link previously found between the Epstein-Barr virus and multiple sclerosis association. Researchers noted that some of the viral exposures were associated with an increased risk of neurodegeneration up to 15 years after infection.
“It’s startling how widespread these associations seem to be, both for the number of viruses and number of neurodegenerative diseases involved,” Matthew Miller, a viral immunologist at McMaster University in Hamilton, Canada told Nature.
However, the authors of the study caution that the data show only a possible association, and that it’s still not clear whether the infections trigger an onset of neurodegenerative conditions. .
No viruses were associated with a protective effect in the study. The overwhelming majority of replicated associations include viruses commonly considered neurotrophic (81%), which means they can invade the central nervous system through peripheral nerves or by crossing the blood-brain barrier.
One of the strongest associations was between viral encephalitis, a rare inflammation of the brain that can be caused by multiple types of virus, and Alzheimer’s.
There were no pairings that suggested a protective link between viral infection and brain disease.