Parkinson disease may have become more common over the past 30 years, at least according to a study in one Minnesota county.

“This is the first evidence that shows an increasing trend of Parkinson incidence, confirmation is needed,” said Dr. Honglei Chen of the National Institute of Environmental Health Sciences in Triangle Park, North Carolina, who wrote an editorial accompanying the new results.

Parkinson disease takes decades to develop, so it can be difficult to identify reasons for the trend and a number of factors may play a role, Chen said.

“However, if the trend is confirmed, one may speculate roles of environmental or other non-genetic factors,” he told Reuters Health by email.

Senior author Dr. Walter A. Rocca of the Mayo Clinic in Rochester, Minnesota, and his coauthors studied trends in Parkinson disease and symptoms like resting tremors, rigidity, impaired reflexes and slowness of movement in Olmsted County, Minnesota, between 1976 and 2005.

During that time, 906 patients developed symptoms seen in Parkinson disease (a condition known as parkinsonism) and 464 developed Parkinson disease. Half of people were over age 73 at onset.

For men, parkinsonism incidence increased from 39 to 56 cases per 100,000 people per year between the 1976-1985 decade and the 1996-2005 decade. Parkinson disease cases also increased from 18 to 30 cases per 100,000 people per year.

The increasing trend was driven by men over age 70. There was no increasing trend for women over the 30-year period, as reported in JAMA Neurology.

It’s possible that doctors are just getting better at diagnosing Parkinson disease as time goes on, Chen said, but that wouldn’t explain the differing trends by sex.

“There is a long history and ongoing debate about a paradoxical finding that smokers are less likely to have Parkinson disease,” Chen said. He noted that smoking rates declined during the period of the study.

“Cigarette smoking has numerous adverse health effects, but its inverse association with Parkinson disease was observed in almost every epidemiological study,” Chen added.

Nicotine seems to be beneficial for animals in parkinsonism studies, he said.

“On the other hand, some scientists are still concerned that this observation was due to non-biological reasons, for example, individuals at risk for Parkinson disease are less likely to start smoking early in life, or if they started, they are more likely to quit,” Chen said.

People need not be concerned about this potential trend until it’s been confirmed by multiple additional studies, he said.