It has long been suspected by scientists that the flu (influenza) might play a role in developing Parkinson’s disease (PD) later in life. The first and possibly most famous example of this connection was the 1918 Spanish flu outbreak. People born during the Spanish flu had a two- to three-fold-increased risk of later developing PD compared to those born before 1888 or after 1924 — suggesting that early-life exposure to the flu boosted PD risk.
A 2011 United Kingdom study reported an association between influenza infections and people developing PD symptoms such as tremor, but not with an increased risk of developing PD. There was also a small study in Canada in 2012 that found an association between having had severe influenza 10 years prior increased the risk of developing PD. Is there a connection?
Recently published in the journal, JAMA Neurology, “Long-term Risk of Parkinson Disease Following Influenza and Other Infections” (Cocoros et al., 2021), a large-scale, case-controlled study sought to rigorously investigate whether:
- Previous flu infections are associated with an increased risk of PD more than 10 years after infection
- Is the association between the flu and elevated PD risk specific to the flu — as opposed to being triggered by other types of inflammatory infections, such as urinary tract infections.
The study pulled data from the Danish National Patient Registry, which analyzed information from 10,271 men and women (average age 71.4) diagnosed with PD between 2000 and 2017. The data from this group was then compared to 51,355 controls (people without PD) of similar age, sex, and preexisting conditions such as cardiovascular disease, diabetes and lung cancer. The data of both groups was analyzed for cases of the flu (and other infections) between the years 1977 and 2016 and categorized by time from infection to PD diagnosis. Of note: given that diagnostic codes were the only method available to identify flu cases — rather than actual laboratory-confirmed cases — the study authors limited their focus to flu cases diagnosed during peak flu season.
Compared to those who were not diagnosed with the flu:
- There was a 70% higher risk of PD for those who had the flu 10 or more years earlier.
- There was a 90% higher risk of PD for those who had the flu 15 or more years earlier.
- There was a 19% higher risk of PD for those who had a urinary tract infection 10 or more years earlier.
- Other types of infections such as gastrointestinal infection, septicemia (blood poisoning from bacteria), male genital infections, appeared to be associated with PD within 5 years of infection, but not after 10 or more years.
What does this mean?
All common colds and flu strains cause inflammation. The symptoms we experience — from stuffy noses to coughing and body aches — are the result of inflammation in the body. Inflammation is how the immune system works to combat infections. We also know that inflammation has also been linked to neurodegeneration in the brain.
This study has demonstrated a strong association between PD and having had the flu within the previous 10 or more years. However, these findings do not necessarily mean that having the flu causes PD. This study also found a 19% higher risk of PD following urinary tract infections occurring the previous 10 or more years before a PD diagnosis. Other infections, such as gastrointestinal infection, septicemia, and male genital infections, which by nature also cause inflammation, were associated with PD within only five years after infection.
Inflammation is clearly a factor in PD – and influenza is known to trigger an extreme inflammatory response in the body. Whether influenza directly causes PD remains unclear. In light of the COVID-19 pandemic, and its known neurological consequences such as brain fog and loss of smell, continued robust research into how inflammation impacts the brain is warranted.
The Parkinson’s Foundation believes in empowering the Parkinson’s community through education. Learn more about infections and PD by visiting the below Parkinson’s Foundation resources, or by calling our free Helpline at 1-800-4PD-INFO (473-4636) for answers to your Parkinson’s questions.
- Top Questions and Answers on COVID-19 and Parkinson’s Disease
- Covid-19 & Parkinson’s
- PD Hospitalization and Coronavirus Preparedness Fact Sheet: Download this document that has crucial information for healthcare professionals in the case you are hospitalized during the COVID-19 pandemic.
- Is Parkinson’s on the Rise?