TREND OF TICK-BORNE ENCEPHALITIS (TBE) IN THE THURGAU REGION 1990-1995 - A NEW ENDEMIC AREA

Tick-borne encephalitis (TEE) is a viral infection transmitted by bite s of infected ticks. The clinical course is mostly mild, but death occ urs in 1-2% of TEE infections and nearly half of patients with meningi tis/meningoencephalitis show residual disease, above all chronic heada che. TEE-infected ticks occur only in endemic areas. A knowledge of th e endemic areas is very important for immunoprophylaxis of TEE. In rec ent years between 26 and 97 cases of TEE have occurred in Switzerland. The largest endemic areas are in Canton Schaffhausen, the northern pa rt of Canton Zurich and the northwest of Canton Thurgau. Another endem ic area is known in the region of Thun in Canton Berne. Another possib le 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 TEE infections in western Thurgau. The que stion was whether there are other endemic areas in Thurgau. In this re trospective analysis we studied the TEE cases in Thurgau between 1990 and 1995 with data derived from the cantonal health authorities report s. Clinical data were taken from case histories of the two cantonal ho spitals in Frauenfeld and Munsterlingen, completed by data from family doctors and patients. Between 1990 and 1995 30 TEE infections (1990; 1, 1991: 4, 1992: 3, 1993: 1, 1994: 4 certain, 3 uncertain, 1995: 14) were observed. TEE infections appeared between May and October (maximu m in May). 14 patients remembered a bite by a tick several weeks befor e onset of the illness. 7 bites occurred in the area of Frauenfeld/Aad orf. 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 TEE in Thurgau was 5.4/100 0 00 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 TEE in Thurg au in 1995 was clearly higher than the average in Switzerland in recen t years (0.46/100 000), and higher than in the well-known endemic area s 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 TEE. Probably the well-known endemic area in the Zurich Oberland in th e vicinity of Elgg has spread eastwards. Persons who are often in the forests of this region should be advised to be vaccinated.