Geographical mapping of type 1 diabetes in children and adolescents in south east Sweden

Study objective: As earlier studies have shown space-time clusters at onset of type 1 diabetes in the south east region of Sweden we investigated if there also has been any geographical clusters of diabetes in this region. Design: The place of residence (coordinates) at the time of diagnosis were geocoded in a geographical information system (GIS). All children diagnosed with type 1 diabetes up to 16 years of age at diagnosis between 1977-1995 were included. The population at risk was obtained directly from the population registry for the respective years and geographical area levels. Setting: South east region of Sweden containing 5 counties, 49 municipalities, and 525 parishes. Main results: A significant geographical variation in incidence rate were found between the municipalities (p<0.001) but not between the counties. The variation became somewhat weaker when excluding the six largest municipalities (p<0.02). In municipalities with increased risk (>35.1/100 000) the major contribution comes from children in age group 6-10 years of age at diagnosis. There were no obvious differences between the age groups in municipalities with decreased risk (<20.1/100 000). Boys and girls had about the same degree of geographical variation. Conclusions: Apart from chance, the most probable explanation for the geographical variation in the risk for children and adolescents to develop type 1 diabetes between the municipalities in the region is that local environmental factors play a part in the process leading to the disease.