Geographical clustering of mortality from systemic sclerosis in the Southeastern United States, 1981-90.

OBJECTIVE: To determine whether elevated rates of mortality from systemic sclerosis (SSc) in the Southeastern United States result from local, multicounty clusters of the disease.
METHODS: Detection of spatial clusters of SSc mortality by applying the method of Kulldorff and Nagarwalla to death certificate data from 955 counties in 12 southeastern states.
RESULTS: From 1981 to 1990, significant excess mortality from SSc in the Southeastern US occurred among white males [standardized mortality ratio (SMR) = 1.2; p = 0.0004] and black males (SMR = 1.2; p = 0.04), but not among white females (SMR = 0.98; p = 0.55) or black females (SMR = 1.1; p = 0.06). When the cluster detection algorithm was applied to data for white males, 3 significant clusters were identified. The primary cluster (p = 0.001) was centered around Coffee, Tennessee. Two smaller clusters overlapped the primary cluster -- one centered at Calhoun, Alabama, (p = 0.008) and another centered at Chattooga, Georgia, (p = 0.04). Analysis of data for black males resulted in a single significant cluster (p = 0.02) centered at Northampton, North Carolina. When data for white or black females were analyzed, no clusters reached statistical significance. In combination, excess SSc mortality in the detected clusters accounted for 79.0 and 66.2%, respectively, of the excess deaths among white and black males across the whole Southeast.
CONCLUSION: Elevation of SSc mortality rates in the Southeastern US results from local clusters of concentrated mortality. These clusters may be artifacts of regional variation in death certificate quality. If not, distinctive environmental factors in these areas may provide new insights into the etiology of SSc.