Clusters of autochthonous hepatitis A cases in a low endemicity area.

At the University Hospital of Besançon (département of Doubs, France), an unusually high number of patients were hospitalized for hepatitis A during the 1999-2000 period, some of whom had not travelled abroad. This prompted us to conduct an investigation on a population basis and search for clusters of cases possibly related to local sources of contamination. Accordingly, case definition was restricted to autochthonous cases. During the 1999-2002 period, 45 autochthonous cases were classified as possibly originating from local environmental sources. A space-time scan statistic detected one most likely cluster (standardized incidence ratio 20.63, 95% confidence interval 10.6-37.1), consisting of 11 persons (of whom five children had attended the same swimming pool). It remained significant in a sensitivity analysis, strongly supporting the hypothesis of an environmental source of contamination. This study reveals the necessity of regular surveillance for hepatitis A and raises the issue of virological surveys of pool waters.