Aims: To determine the seasonality of clinical disease onset and month of birth in type 1 diabetes mellitus (DM) in the southern hemisphere.
Patients: Two hundred and seventy-five children with type 1 DM in the South Island of New Zealand were studied. The total live births (91,394) of the same period were used as control data.
Methods: Seasonal rhythms were analyzed using the 12 month cosinor method.
Results: The month of birth pattern of the patients with DM showed a statistically significant peak (p < 0.01) in summer, whereas the disease onset had a significant peak in winter (p < 0.01), similar to that registered in countries of the northern hemisphere, but in different months of the year. The total live births had no significant rhythm. The different seasonality of birth of the children who subsequently developed type 1 DM from that of the total live births is suggestive of the initiation of the autoimmune process in utero or perinatally.