Influenza infection is linked to a subsequent diagnosis of Parkinson’s disease (PD) more than 10 years later, resurfacing a long-held debate about whether infection increases the risk for movement disorders over the long term.
In a large case-control study, investigators found and by more than 70% for PD occurring more than 10 years after the flu.
“This study is not definitive by any means, but it certainly suggests there are potential long-term consequences from influenza,” study investigator Noelle M. Cocoros, DSc, research scientist at Harvard Pilgrim Health Care Institute and Harvard Medical School, Boston, said in an interview.
The study was published online Oct. 25 in JAMA Neurology.
Ongoing debate
The debate about whether influenza is associated with PD has been going on as far back as the 1918 influenza pandemic, when experts documented parkinsonism in affected individuals.
Using data from the Danish patient registry, researchers identified 10,271 subjects diagnosed with PD during a 17-year period (2000-2016). Of these, 38.7% were female, and the mean age was 71.4 years.
They matched these subjects for age and sex to 51,355 controls without PD. Compared with controls, slightly fewer individuals with PD had chronic obstructive pulmonary disease (COPD) or emphysema, but there was a similar distribution of cardiovascular disease and various other conditions.
Researchers collected data on influenza diagnoses from inpatient and outpatient hospital clinics from 1977 to 2016. They plotted these by month and year on a graph, calculated the median number of diagnoses per month, and identified peaks as those with more than threefold the median.
They categorized cases in groups related to the time between the infection and PD: More than 10 years, 10-15 years, and more than 15 years.
The time lapse accounts for a rather long “run-up” to PD, said Dr. Cocoros. There’s a sometimes decades-long preclinical phase before patients develop typical motor signs and a prodromal phase where they may present with nonmotor symptoms such as sleep disorders and constipation.
“We expected there would be at least 10 years between any infection and PD if there was an association present,” said Dr. Cocoros.
Investigators found an association between influenza exposure and PD diagnosis “that held up over time,” she said.
For more than 10 years before PD, the likelihood of a diagnosis for the infected compared with the unexposed was increased 73% (odds ratio [OR] 1.73; 95% confidence interval, 1.11-2.71; P = .02) after adjustment for cardiovascular disease, diabetes, chronic obstructive pulmonary disease, emphysema, lung cancer, Crohn’s disease, and ulcerative colitis.
The odds increased with more time from infection. For more than 15 years, the adjusted OR was 1.91 (95% CI, 1.14 - 3.19; P =.01).
However, for the 10- to 15-year time frame, the point estimate was reduced and the CI nonsignificant (OR, 1.33; 95% CI, 0.54-3.27; P = .53). This “is a little hard to interpret,” but could be a result of the small numbers, exposure misclassification, or because “the longer time interval is what’s meaningful,” said Dr. Cocoros.