Surveillance of tick-borne encephalitis in Europe and case definition.

The study by Stefanoff et al raises two important questions concerning tick-borne encephalitis (TBE) virus infections. First, the lack of a generally accepted case definition and secondly the quality of national surveillance of TBE cases. Ideally, reported cases should be confirmed and the clinically relevant cases with central nervous system (CNS) disease should be separated from febrile cases without CNS manifestations. The surveillance of TBE in the European countries is not uniform and not always mandatory. Efforts to reach a final diagnosis, especially in less severe cases and in children, varies as well as the awareness of the disease in low endemic regions. The only relevant and stable basis for national surveillance is cases with established CNS disease, although immunity to TBE virus after less severe febrile illness is of interest on individual basis. The ratio of non-CNS disease to CNS disease is generally believed to be about three, but there are regional differences in virulence. Significantly, age related differences are basically unknown.