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The annual incidence of Parkinson's disease was 50% higher than previous estimates in data from five epidemiological cohorts in the U.S. and Canada.

The new incidence rate was nearly 90,000 cases annually for people 65 and older, reported Allison Willis, MD, of the University of Pennsylvania in Philadelphia, and co-authors.

This contrasted sharply with the previous incidence rate of 40,000-60,000 new cases a year based on smaller studies, the researchers wrote in .

Not surprisingly, as the primary risk factor for Parkinson's is age, incidence rates increased with age throughout the study. At all ages, incidence was higher in men than women.

Within the U.S., a cluster of counties with higher Parkinson's incidence emerged at the juxtaposition of the Midwestern and Southern regions of the country. Incidence rates also were elevated in southern California, southeastern Texas, central Pennsylvania, and Florida. The first spotted in 2005 persisted in the data.

"Unique to this study, we found that Parkinson's disease incidence estimates have varied for many reasons, including how cases are identified and the geographic location of the study," Willis said in statement.

"The persistence of the Parkinson's disease belt in the U.S. might be due to population, health care, or environmental factors," she noted. "Understanding the source of these variations will be important for health care policy, research, and care planning."

Willis and colleagues identified Parkinson's disease incidence in a common year -- 2012 -- across five cohorts, using data from the U.S. Medicare program, Ontario Health Care in Canada, Kaiser Permanente North California, the Honolulu-Asia Aging Study, and the Rochester Epidemiology Project in Olmsted County, Minnesota. Ascertainment and diagnostic criteria included new documentation of Parkinson's symptoms or a Parkinson's diagnosis in 2012.

The researchers calculated age- and sex-adjusted incidence of Parkinson's disease among people 65 and older for each dataset, then repeated the analyses for people 45 and older in three datasets (Ontario Health Care, Kaiser Permanente, and Rochester Epidemiology) that routinely captured information about younger people.

The five cohorts contained data for 6.7 million person-years of adults 45 and older and 9.3 million person-years of those 65 and older.

For seniors, adjusted incidence estimates ranged from 108 to 212 per 100,000. Among men, those estimates were 162 to 277 per 100,000; among women they were 66 to 161 per 100,000. For people 45 and older, estimates ranged from 47 to 77 per 100,000 and were higher in men.

Incidence varied across the cohorts, in part due to case ascertainment and diagnosis methods. Other factors -- including the prevalence of genetic risk factors in certain populations or environmental toxin exposure in some locations -- also may have played a role, the researchers noted.

Limitations of the study included possible confounding, misclassification, and selection bias, Willis and colleagues acknowledged. Estimates for the current year may be higher due to a lower prevalence of smoking, an increased prevalence of some risks like pesticides, and greater recognition of Parkinson's symptoms, they suggested.

A estimated that approximately 930,000 people in the U.S. had Parkinson's disease in 2020. That figure was expected to climb to 1,238,000 in 2030, based on Census Bureau population projections.

Willis and colleagues suggested their results may help inform strategies for clinical trials and shed light on Parkinson's "hot spots" where more resources may be needed.

"The growth in those diagnosed and living with Parkinson's disease underscores the need to invest in more research toward better treatments, a cure, and one day, prevention," said co-author Brian Fiske, PhD, of the Michael J. Fox Foundation. "It's also a clear call to lawmakers to implement policies that will lessen the burden of Parkinson's disease on American families and programs like Medicare and Social Security."

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