People who get common viral infections like pneumonia, influenza, and shingles may be at increased risk for neurodegenerative diseases like Alzheimer’s and Parkinson’s down the line, a new study suggests.
For the study, published in the journal Neuron, researchers analyzed medical records for about 450,000 adults in Finland and the United Kingdom. They searched for connections between a wide range of viral infections and six common neurodegenerative conditions: Alzheimer’s disease, amyotrophic lateral sclerosis (ALS, also called Lou Gehrig's disease), dementia caused by stroke, other types of dementia, Parkinson’s disease, and multiple sclerosis (MS).
Overall, scientists found 22 different connections between specific viral infections and these neurodegenerative conditions.
Viral Infections Requiring Hospitalization Raise the Risk of Later Neurodegenerative Conditions
Dementia was linked to the most types of infections. It was likely to develop after people had viral encephalitis (an infection of the brain), viral warts, influenza, pneumonia, or a combined infection with both influenza and pneumonia. The risk was strongest for viral encephalitis, which made people at least 20 times more likely to later be diagnosed with Alzheimer’s.
Severe influenza cases were linked to the widest range of risks. Influenza and pneumonia infections were associated with every neurodegenerative disorder studied except multiple sclerosis.
“Keep in mind that the individuals we studied did not have the common cold,” senior study author Michael Nalls, PhD, of the Center for Alzheimer’s and Related Dementias at the National Institutes of Health (NIH) in Bethesda, Maryland, said in a statement. “Their infections made them so sick that they had to go to the hospital.”
Vaccines Can Prevent Severe Illness and Hospitalization for Common Viral Infections
However, widely available vaccines have the potential to reduce the risk of developing or being hospitalized with many of the viral infections associated with neurodegenerative disease risk in the study, Nalls said.
Vaccines available in the United States can prevent infection by viruses that cause influenza and pneumonia, per the Centers for Disease Control and Prevention. Common vaccines can also prevent two other infections associated with an increased risk of neurodegenerative diseases in the study: meningitis and shingles.
“The fact that commonly used vaccines reduce the risk or severity of many of the viral illnesses observed in this study raises the possibility that the risks of neurodegenerative disorders might also be mitigated,” Nalls said.
Viral Infection May Be a Preventable Risk Factor for Degenerative Brain Disorders
While the study wasn’t designed to prove whether or how viral infections might directly cause neurodegenerative diseases, most of the viruses implicated in these diseases have the ability to travel beyond the bloodstream and enter tissue in the brain, according to the study.
It’s possible that once these viral infections enter the brain, they cause inflammation that contributes to the development of neurodegenerative diseases, the study authors write. These diseases involve damage to the brain and nervous system and can lead to a wide range of problems with daily functions controlled by the brain including thought, speech, and movement.
Many previous studies, some dating back decades, have linked certain viruses to common brain disorders. One study from the 1990s, for example, examined autopsied brain tissue and linked the herpes simplex virus to an increased risk of Alzheimer’s disease. Last year, a study of patient medical records and blood samples found the Epstein-Barr virus associated with an increased risk of multiple sclerosis. Another study of health insurance claims connected the flu vaccine to a lower risk of Alzheimer’s disease.
“Neurodegenerative disorders are a collection of diseases for which there are very few effective treatments and many risk factors,” study coauthor Andrew Singleton, PhD, director of the Center for Alzheimer's and Related Dementias at the NIH, said in a statement. “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.”