HLA-related heterogeneity in seasonal patterns of diagnosis in Type 1 (insulin-dependent) diabetes.

HLA type, time of year of diagnosis, and age at diagnosis were studied in 52 new cases of Type 1 diabetes in Seattle, Washington. Diagnosis was found to be seasonal in diabetic patients positive for DR3 (p less than 0.005), with the expected marked reduction in new cases during the summer months. This seasonality was not age-related (p greater than 0.13). Cases who were DR3-negative did not show significant seasonality of diagnosis (p greater than 0.5). However, when age at diagnosis was adjusted for, a seasonal effect was found in the DR3-negative group (p less than 0.006), with older cases favouring a spring onset and younger cases favouring an autumn onset. Thus, DR3-positive cases showed a seasonal diagnosis pattern that did not depend on age, while DR3-negative cases showed an age-dependent seasonal pattern. These differences may reflect the predominance of different aetiological mechanisms in these two genetic groups.