Nationwide study of Parkinson's disease incidence between 2010 and 2015 among the French agricultural population

Introduction The role of farming and professional pesticide exposure is well established in Parkinson's disease (PD). PD is multifactorial disease, and there are other risk/protective factors that may have an influence on the incidence of the disease in the agricultural population. The impact of professional pesticide exposure on the spatial distribution of the disease is not well known. Our objectives were to estimate PD incidence among the French agricultural population and to compare it to PD incidence in the general population, and to examine its spatial distribution. Methods We used French national drug claims databases to identify PD cases each year between 2010 and 2015 using a validated algorithm. We defined the agricultural population based on affiliation to the “Mutualite sociale agricole” (MSA), the only health insurance for farmers and agricultural workers. We compared PD incidence in MSA farmers, MSA workers, and affiliates to the General scheme (GS, that represents about 91% of the French population) of the social security using Poisson regression adjusted for overdispersion, 5-year groups of age, sex, age × sex, and year, and estimated relative risks (RRs) and 95% confidence intervals (CIs). We also examined multiplicative interactions of agricultural status with age, sex, and year. We estimated PD incidence ratios standardized for age, sex, and year in each French “departement” among MSA farmers and MSA workers and estimated the intraclass correlation coefficient (ICC), a measure of the proportion of the variance in PD incidence that is due to between-departement variation. Results During the study period, between 2253 and 2628 MSA affiliates were annually identified as newly treated for PD; the total number of incident PD cases was of 14,464 (72% farmers, 28% workers). The crude annual incidence ranged from 0.715 to 0.791 per 1,000 person-years. PD incidence was 7% higher in 2010 ( P  = 0.001) than in the rest of the period but no linear trend was observed between 2011 and 2015 ( P for trend = 0.560). The mean age was 75 years (SD = 12) and 7990 (55%) were males. PD incidence was 12% higher among MSA affiliates compared to GS affiliates (RR = 1.12; 95% CI: 1.09–1.15). Among MSA affiliates, farmers had a higher risk of PD than other workers (RR = 1.10; 95% CI: 1.05–1.15). The increased PD risk in farmers depended on age ( P for interaction = 0.004), with a stronger association at older ages, but not on sex ( P for interaction = 0.085) or calendar year ( P for interaction = 0.443). Regarding the spatial distribution of PD incidence, we observed a more pronounced between-departement variation among MSA farmers (ICC = 87%) than among other MSA workers (ICC = 79%). Conclusion Higher PD incidence among French farmers compared to the general population and agricultural workers, as well as the marked spatial variation of PD incidence in this population are consistent with the role of occupational pesticide exposure in PD. These findings warrant the epidemiological surveillance of PD among French farmers.