Seasonal variation in Type 1 diabetes incidence was already reported in the 1920s when higher rates of 'acute diabetes' were found during the late autumn, winter and early spring (1). Peaks in incidence, with one peak in the winter months and the other during the late summer, were detected in northern Sweden among children aged 0-14 registered during 1938 to 1977 (106). Several other epidemiologic studies have described seasonal patterns in the onset (or better at diagnosis) of new cases of insulin-dependent diabetes in children (17,22,49,51,53,78,81,82,86,97,107-111,113). Most studies have reported higher occurrence of insulin-dependent diabetes during the cold autumn and winter months than during the warmer spring and summer months, but these findings are difficult to compare because of differences in methodology. Overall the seasonal variation in the month of diagnosis of the disease has been reported in regions where seasons are well-defined summer or winter.
The months/seasons of highest incidence have varied across populations; however, the low incidence during the warm months has been consistent. There were some exceptions in the seasonal pattern; in Europe in France (89) and in Western Siberia (94) there were no seasonal differences in diabetes incidence. In the US Virgin Islands (56) there was a noticeable peak in incidence in June, but the number of cases was small and the difference in the yearly temperature is minimal at this latitude. In Finland, where the incidence of Type 1 diabetes in children is the highest in the world and steeply rising over the last four decades, a statistically significant seasonal pattern could be confirmed for males but not for females. During a calendar year, one cycle with a decreased incidence of insulin-dependent diabetes in June was found among younger boys. Among older boys there were two distinct cycles with a decreased incidence, the first in June and the second during November-December. The most visible seasonal pattern was a lower number of cases diagnosed in June, while during the rest of the year the incidence remained relatively stable and high (114).
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