Viral infections possibly linked to Alzheimer's, Parkinson's, researchers say

National Institutes of Health researchers found a correlation between viral infections, including influenza, and neurodegenerative disorders such as Parkinson's, Alzheimer's and amyotrophic lateral sclerosis, Science reported Jan. 19.

The study, published in Neuron, focused on 22 associations found between viral infections and neurodegenerative disorders after analyzing electronic medical records from two large European databases. 

Nearly all of the viral-disease pairings involved "neurotropic" viruses, such as herpes simplex and some strains of influenza, which can invade the central nervous system.

The study found five key correlations:

  1. Influenza that progressed to pneumonia most commonly had positive associations with all neurodegenerative diseases, except multiple sclerosis.

  2. Dementia was most commonly associated with viral infections. It was linked to six infections, including flu with or without pneumonia and viral encephalitis.

  3. The strongest association, with 31-fold risk elevation, was between Alzheimer's and viral encephalitis.

  4. Influenza was associated with an approximately fivefold increased risk of dementia.

  5. No viral infections were associated with protective effects against neurodegenerative disease.

However, there are many caveats to the study, experts told Science. Most importantly, the pairings were associations based only on medical records. There may be genetic and environmental reasons why someone is more susceptible to viruses and neurodegenerative disorders, the report said.

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