[Epidemiology of tick-borne encephalitis (TBE) in Switzerland 1984 to 2004].

From 1984 to 2004 a total of 1370 TBE cases have been reported to the Federal Office of Public Health through the notification system. A significant increase of cases was observed in the nineties. Since 1999 the yearly average of reported cases was 101, corresponding to a yearly incidence of 1.4/10(5) inhabitants for all of Switzerland or to an incidence of approximately 6/10(5) if only the population living in endemic regions is taken into account. Male cases were twice as frequent as female ones. The age distribution showed two peaks in the age ranges of 6 to 14 and 60 to 69 years. Cases in children under 6 years of age were very rare with a yearly average of two hospitalised children. The severity of the disease increased with age. In 86% of the patients CNS involvement was present while 14% showed only flu-like symptoms. Ninety percent of patient with CNS involvement were hospitalised, 1,3% died (on average one death per year since 1999). Cases in fully vaccinated persons were very rare. The majority of patients had most probably been infected at their place of residence or in the nearer surroundings. The incidence in the large cities was low (< or = 1/10(5)/year). In smaller towns located directly in an endemic region, the incidence rose to as much as 18/10(5)/year. During the 21 years of observation, the endemic regions in four cantons known in the early eighties had clearly expanded and new infected regions appeared in seven other cantons. A frequent recreational or occupational exposure in wooded areas was mentioned by 85% of the patients. Although 80% of these concerned leisure time activities (mainly walking), the incidence is most probably still higher in occupationally exposed persons. This indicates that not only persons with leisure time exposure but also occupationally exposed persons are still insufficiently vaccinated to achieve an optimal control of this disease in Switzerland.