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Can the Flu Contribute to Parkinson’s Disease?

Need more incentive to get a flu shot, or to keep taking extra precautions this flu season? A new study suggests there may be a link between influenza infection and an increased risk for Parkinson’s disease.

For decades, neurologists have suspected there may be a link between the flu and Parkinson’s disease, a chronic and progressive disorder of the nervous system marked by problems with movement, cognitive changes and a range of other symptoms. Several earlier studies, for example, reported a sharp increase in Parkinson’s cases following the 1918 influenza pandemic. Some cases of Parkinson’s have been linked to environmental exposures to pesticides and other toxic chemicals, and genetics may also play a role, but most cases of Parkinson’s have no known cause. Treatments for Parkinson’s can help delay its progression, but there is no known cure.

The new study, using Danish health care databases, included 10,231 men and women who had been diagnosed with Parkinson’s between 2000 and 2016. Researchers compared them with 51,196 controls who were matched for age and sex. The researchers tracked influenza infections beginning in 1977 using hospital and outpatient discharge records. The report appeared in JAMA Neurology.

Parkinson’s takes years, if not decades, to develop, and initially may produce only subtle symptoms like a hand tremor. It may take years for doctors to diagnose the condition, so any connection between a flu infection and the disease would be evident only many years later.

The researchers found that compared with people who had not had a flu infection, those who had the flu had a 70 percent higher risk of Parkinson’s 10 years later, and a 90 percent higher risk 15 years after.

“The association may not be unique to influenza, but it’s the infection that has gotten the most attention,” said the lead author, Noelle M. Cocoros, a research scientist at the Harvard Pilgrim Health Care Institute. “We looked at other infections as well, and there are several specific ones — hepatitis C and others — that may be associated with Parkinson’s. But we didn’t have large enough numbers to analyze them.”

Some researchers have speculated that infections may contribute to Parkinson’s by causing inflammation of the central nervous system. Several studies, for example, have found that appendicitis may increase the risk of Parkinson’s, though other research has failed to find this link.

Dr. Cocoros cautioned that the results of this latest study should not be over-interpreted. “We’ve couched our findings with appropriate limitations,” she said. “This is not evidence of a causal link between flu infection and Parkinson’s. Our study adds to a broader literature, and we shouldn’t overstate the results.”

Dr. Kelly A. Mills, an associate professor of neurology at Johns Hopkins who was not involved in the research, said that many mechanisms for the development of Parkinson’s are being explored, most of which involve a combination of genetic and environmental factors.

“The size of this study and the magnitude of the association they found is intriguing,” he said. “It’s a well-done study that brings up some important points to keep investigating.” But, he cautioned, it has the same limitations as any other retrospective cohort study, including the possibility that the researchers did not account for certain variables, such as lifestyle characteristics or socioeconomic conditions, that might contribute to Parkinson’s.

Dr. J. Timothy Greenamyre, a neurologist who had no part in the study and director of the Pittsburgh Institute for Neurodegenerative Diseases at the University of Pittsburgh, pointed out some other limitations. The researchers, he said, did not have laboratory-confirmed diagnoses of flu, so they may have included cases that were not actually influenza. And the flu differs from year to year, so different strains might have different effects on the risk for Parkinson’s.

Still, he said, “All in all, despite its limitations, I believe this is a solid, well-done epidemiological study. I think this paper provides the strongest evidence to date that exposure to influenza increases the risk for Parkinson’s disease.”

Will a flu vaccination reduce the risk? “There are many other good reasons to get a flu shot,” Dr. Cocoros said. “But if there is an association with Parkinson’s, then vaccination would lessen your risk. Still, it’s pretty evident that Parkinson’s can be caused by many things. Infection may be one of the many causes.”

“On a purely scientific level, it’s really interesting to think about the long-term consequences of an infection we usually just experience, recover from and then forget about,” she added. “On a broader level, it’s helpful to understand what it means for how we respond to infections.”

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