Geographic variations of multiple sclerosis prevalence in France: The latitude gradient is not uniform depending on the socioeconomic status of the studied population

Abstract

BackgroundIn France, two studies analysed multiple sclerosis prevalence nationwide: one was carried out in farmers, and the other one in employees. A south-north gradient of prevalence was found solely in farmers.ObjectiveIn order to better describe the latitude gradient in France, which is not uniform depending on the studied population, we assessed whether a gradient exists in another population than farmers and employees: independent workers. The same methods of case ascertainment have been used.MethodsAltogether 4,165,903 persons insured by the French health insurance scheme for independent workers were included. We searched the database for (a) long term disease status ‘multiple sclerosis’, (b) domicile, (c) gender and (d) age.ResultsA total of 4182 cases of multiple sclerosis were registered giving a prevalence of 100.39/100,000. Adjustment by age and sex and spatial smoothing with a Bayesian analysis showed a gradual increase of prevalence from the southwest to the northeast of France. Standardised morbidity ratio was correlated with latitude and longitude (p<0.0001; p = 0.0031; adjusted R2 = 0.3038).ConclusionA discrepancy of geographic distribution between farmers and independent workers on the one hand and employees on the other cannot be attributable to environment. Assuming that socioeconomic status by itself is not associated with multiple sclerosis risk, employees’ geographic mobility at adulthood for professional reasons could have interfered with the gradient effect.

Environmental and genetic risk factors for MS: an integrated review

“…That MS prevalence increases with greater distance from the equator led to the hypothesis that lower exposure to ultraviolet (UV) radiation and subsequently, lower vitamin D status (blood concentration of 25‐hydroxyvitamin D (25OHD)), increases the risk of MS. Latitude gradients have reduced over time in some populations, but not all; this may be due to changes in exposure to latitude‐related factors, for example, sun exposure, diet, in some populations, but not others. Higher latitude and lower summer sun exposure in adolescence have been linked to a younger age of onset.…”

Geographical Heterogeneity of Multiple Sclerosis Prevalence in France

“…Furthermore, the lowest prevalence was found in the departments of the Auvergne region, located in the center of France. Therefore, these results do not show a clear northeast to southwest gradient among the 21 departments under study, as was previously suggested in the 2003 national study of data for French farmers [3] and more recently among independent workers [19], but the results are closer to those in the 2004 national study of data for the main national health insurance system (CNAMTS) [13]. Moreover, the high prevalence in Haute-Garonne (SW) is consistent with the results obtained in the capture–recapture study conducted in Haute-Garonne in 2005 [15].…”

“…As compared with previous studies, these studies revealed increased prevalence in these five departments. More recently, a study of a population of independent workers that involved a Bayesian method found a decreasing northeast to southwest gradient on departmental scale [19]. …”

Small-area distribution of multiple sclerosis incidence in western France: in search of environmental triggers

“…However, as a low Gini coefficient was found, the authors concluded that the causes of MS are pervasive and shared across population groups [ 6 ]. In French territory, several studies found geographic variations in MS but at much larger geographical scales than the municipality level ( département or region), highlighting higher prevalence or incidence rates in north-eastern France [ 8 , 24 , 25 ].…”