[Development of early-summer meningoencephalitis (FSME) in the Thurgau region 1990-1995--a new endemic area?].

Tick-borne encephalitis (TBE) is a viral infection transmitted by bites of infected ticks. The clinical course is mostly mild, but death occurs in 1-2% of TBE infections and nearly half of patients with meningitis/meningoencephalitis show residual disease, above all chronic headache. TBE-infected ticks occur only in endemic areas. A knowledge of the endemic areas is very important for immunoprophylaxis of TBE. In recent years between 26 and 97 cases of TBE have occurred in Switzerland. The largest endemic areas are in Canton Schaffhausen, the northern part of Canton Zurich and the north-west of Canton Thurgau. Another endemic area is known in the region of Thun in Canton Berne. Another possible endemic area is known in the Zurich Oberland around Elgg, only 7 km from Aadorf in Thurgau. Up to now, Diessenhofen in the north-west of Thurgau was the only known possible endemic area. In 1994 and 1995 we observed an accumulation of TBE infections in western Thurgau. The question was whether there are other endemic areas in Thurgau. In this retrospective analysis we studied the TBE cases in Thurgau between 1990 and 1995 with data derived from the cantonal health authorities reports. Clinical data were taken from case histories of the two cantonal hospitals in Frauenfeld and Münsterlingen, completed by data from family doctors and patients. Between 1990 and 1995 30 TBE infections (1990; 1, 1991: 4, 1992: 3, 1993: 1, 1994: 4 certain, 3 uncertain, 1995: 14) were observed. TBE infections appeared between May and October (maximum in May). 14 patients remembered a bite by a tick several weeks before onset of the illness. 7 bites occurred in the area of Frauenfeld/Aadorf. Only one bite occurred in Diessenhofen. 2 patients were infected in well-known endemic areas in Canton Zurich, a vicinal region in the west of Thurgau. In 1995 the incidence of TBE in Thurgau was 5.4/100000 population. In 9 of the 14 patients recalling a bite by a tick (64.4%), the bites occurred near their domicile. No bite was seen east of a line between Steckborn and Weinfelden. The incidence of TBE in Thurgau in 1995 was clearly higher than the average in Switzerland in recent years (0.46/100000), and higher than in the well-known endemic areas in the vicinity (Schaffhausen 3.95, Zurich 1.31). Based on our data, the region Frauenfeld/Aadorf must be declared a new endemic area for TBE. Probably the well-known endemic area in the Zurich Oberland in the vicinity of Elgg has spread eastward. Persons who are often in the forests of this region should be advised to be vaccinated.