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Parkinson’s disease, a neurodegenerative disorder that affects the central nervous system, is caused by destruction of nerve cells in the part of the brain called the substantia nigra. Symptoms of Parkinson’s disease typically manifest later in life and are characterized by tremors and a shuffling gait. Findings from a new study indicate that influenza infection may increase a person’s risk for Parkinson’s disease.

Bacterial and viral infections typically resolve quickly, but in some cases, they elicit long-term adverse effects on human health. For example, streptococcus bacterial infection, which causes strep throat, increases a person’s risk for rheumatic fever, causing fatigue, joint pain, and a dangerous buildup of fluid around the heart. Similarly, human papilloma virus infection, a generally mild sexually transmitted disease, can increase a person’s risk for certain types of cancer. And a growing body of evidence indicates that infection with SARS-CoV-2, the virus that causes COVID-19, is associated with long-term complications that last several weeks or months, a phenomenon previously referred to as “long COVID” and now known as “Post-Acute Sequelae after SARS-CoV-2 infection.”

The authors of the current study drew on data from the Danish National Patient Registry, a longitudinal registration of detailed administrative and clinical data used exclusively for research. They analyzed more than 61,000 patient records spanning nearly 40 years (1977 to 2016) to identify people who had been diagnosed with influenza and/or Parkinson’s disease.

They found that more than 10,000 people had been diagnosed with Parkinson’s disease during the study period. Those who were diagnosed with influenza (but not other viral infections) were 73 percent more likely to be diagnosed with Parkinson’s more than ten years later, compared to people who had never had an influenza diagnosis. When the researchers restricted the time frame of when the people were diagnosed with influenza to the peak influenza season (when it was less likely to be a false-positive diagnosis), the association with Parkinson’s disease was even stronger.

These findings suggest that influenza infection increases the risk of developing Parkinson’s disease. The authors of the study posited that the mechanisms that drive this association may be related to inflammatory responses during a viral infection that could promote subsequent neurodegeneration, but they caution that their findings were observational and therefore not causal.

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