The common cold, influenza, COVID-19—no matter what it is, enduring a viral infection is no fun. After all, these viral illnesses can cause symptoms that affect different parts of the body, including the respiratory and digestive systems. However, viral infections can also impact the brain and potentially increase the risk of neurodegenerative diseases like Alzheimer’s and Parkinson’s, according to new research published in the peer-reviewed journal Neuron.
“The results of this study support the idea that viral infections and related inflammation in the nervous system may be common – and possibly avoidable – risk factors for Alzheimer’s disease (AD) and other neurodegenerative disorders,” Michael Nalls, PhD, study senior author with the NIH’s Center for Alzheimer’s and Related Dementias, told Seasons in an email.
Specific viral infections linked to neurodegenerative diseases
Researchers from the National Institute of Health analyzed medical records and data for about 335,000 participants who were a part of FinnGen, a nationwide Finnish biobank. They looked for diagnoses of common neurodegenerative diseases – including Alzheimer’s, amyotrophic lateral sclerosis, dementia, vascular dementia, Parkinson’s disease and multiple sclerosis – and determined if the participants with those diagnoses had a previous viral infection that led to hospitalization. (They did not include COVI—19 hospitalization in their analysis.)
From this, they were able to identify 45 significant associations between a neurodegenerative disease diagnosis and a previous viral infection. Some of these exposures were also linked with a higher risk of neurodegeneration up to 15 years later after an infection.
For the second part of the study, Nalls and his colleagues replicated and refined 22 of these associations using data from the U.K. Biobank, which included more than 100,000 participants. They determined generalized dementia had the most associations, with links to six different virus exposures, including all influenza, flu and pneumonia, viral pneumonia, viral encephalitis, viral warts, and other viral diseases.
Nalls said their research also showed participants who had viral encephalitis were at least 20 times more likely to be diagnosed with Alzheimer’s. He also said those who were hospitalized due to influenza and pneumonia were about twice as likely to be diagnosed with Alzheimer’s than those who did not experience the flu virus.
“What this study shows is how data mining and other ‘Big Data’ approaches to medical research can help scientists gain a better understanding of the risks associated with neurodegenerative disorders and, ultimately, develop better treatments and preventive measures,” Nalls said.
How viral infections may impact the brain and risk of cognitive diseases
While more studies are needed, the researchers said one explanation could be that some viruses are considered neurotrophic—meaning they can cross and invade the blood-brain barrier and infect the central nervous system.
“This suggests that these viruses may increase neurodegenerative disease risk by lowering cognitive reserve (resilience to neurodegeneration and the ability to carry out complex mental tasks) by contributing to the inflammation of the brain,” the authors wrote.
Nikhil Palekar, MD, medical director of the Stony Brook Center of Excellence for Alzheimer’s Disease and director of the Alzheimer’s Disease Clinical Trials Program, Stony Brook Medicine, told Seasons that viruses – especially those infecting the brain and causing viral encephalitis – can result in increased inflammation or neuroinflammation in the brain.
He said this is “associated with increased risk for neurodegenerative disorders like AD as well as contributing towards the progression of cognitive symptoms in patients with AD and other dementias.”
While researchers found an association between certain viral infections and neurodegenerative diseases, Palekar said many factors can contribute to the development of these illnesses.
“Being exposed to these viruses might contribute toward the risk of developing AD, but it’s unclear how significant that added risk is based on this current study,” he said. “We need more longitudinal studies that cannot only show a strong association between viral exposure and AD, but the specific mechanisms by which the virus increases the risk for developing AD.”
Whether a patient has been diagnosed with viral infections or not, Nalls said clinical decisions on what to do about that should not be made based on their findings alone.
“We would caution against making clinical decisions based on this work; there is a long way to go before we understand what mechanism could underlie these associations,” Nalls added. “It is important to remember that the individuals that were part of this study were so sick from a viral illness that they had to go to the hospital.”
Prevention and treatment of viral infections
Early recognition, prevention and treatment of viral diseases that affect the brain may mitigate the later risk of dementia and other neurodegenerative disorders.
For example, treatments that can reduce hospitalizations, such as vaccinations, could potentially decrease some of the risk from viral exposure in the future.
“Given the easy availability of vaccines that can help lower the rates of these viral infections, it would be a good recommendation for older adults to get vaccinated and potentially lower their risk for AD,” Palekar said.
Beyond getting vaccinated, experts say maintaining a healthy lifestyle can also help to prevent and lower the risk of getting viral infections.
“Regular exercise, cognitive and social activities, a well-balanced healthy brain diet like the MIND diet, and well-controlled chronic medical conditions such as hypertension, obesity and diabetes can help improve the brain’s resilience against neurodegenerative disorders in later years,” Palekar said.