Background The aetiology of appendicitis, the commonest cause of acute abdomen, is unknown. Infection has been proposed but the evidence has been unconvincing. The purpose of the present study was to investigate if temporo-spatial clustering and outbreaks, characteristics of infectious diseases, could be found in appendicitis cases in a defined Swedish population. Methods. Temporo-spatial clustering among 1155 cases from three districts of Jonkoping County in 1984-1990 was identified by Knox space-time cluster analysis. Outbreaks were identified by retrospective study of temporal variations in 3590 consecutive cases of acute appendicitis from the city of Jonkoping in 1969-1990. Results. Space-time clustering was found among patients with operations less than 60 days apart (observed/expected [O/E]-ratio 1.46, P = 0.016). Subset analysis revealed clustering to be commonest among patients from the same households (O/E-ratio 6.52, P = 0.012), among patients younger than 15 years (O/E-ratio 3.61, P = 0.004) and among females (O/E-ratio 2.28, P = 0.004). Three outbreaks with a significantly increased number of cases were observed during the 22-year study period (O/E-ratio 1.6-2.2, P = 0.001-0.049). Conclusions. The finding of temporo-spatial interaction and outbreaks among appendicitis cases supports the concept that appendicitis may be caused by infectious agents.
