Epidemiology and ecology of tick-borne encephalitis in the eastern part of Germany between 1960 and 1990 and studies on the dynamics of a natural focus of tick-borne encephalitis.

From 1960 to 1990, attempts to isolate TBE virus from ticks and small mammals were made and investigations to detect TBE antibody in small mammals, game and humans were performed in the five new federal Länder of Germany. The confirmed TBE cases for which the site of exposure could be determined were also registered. As a result of these epidemiological and ecological investigations, a map is presented showing the natural foci of TBE which are primarily located in the subatlantic and mountainous climatic regions. TBE was endemic in the area of investigation from 1960 to 1990 showing a morbidity of up to 0.7 per 100,000 inhabitants which decreased in recent years to 0.02 per 100,000 inhabitants. The natural foci of TBE virus in eastern Germany showed a high activity between 1960 and 1970. Since that time, there have hardly been any cases of human disease and TBE virus could no longer be detected, neither in ticks nor in small mammals. Taking the natural focus on the Island of Usedom as an example, attempts have been made to elucidate whether such foci have become extinct or whether they have persisted. From 1983 to 1989, a surveillance programme was performed to detect antibodies to TBE virus in small mammals and game and to attempt to culture virus from ticks and small mammals. The attempts to isolate virus from a total of 8200 ticks were negative. Attempts to isolate virus from the brains of small mammals were also negative. Antibody prevalence in 446 small mammals and 500 animals of game was ca. 1%. At one site, the sero-positive reactions converted from 0% (1983-1988) to 4.5% (1989) among small mammals population. The investigations performed on the Island of Usedom have shown that this natural focus has not disappeared but is in a state of endemic latency. Moreover, the seroconversion observed in the small mammals population shows that further surveillance of such foci is necessary. This becomes obvious by the sporadic occurrence of single TBE cases as well as by a low antibody prevalence of 1% in small mammals and game. The epidemiological situation in eastern Germany is thus completely different from that in western Germany where an average of 70-120 TBE cases per year are registered, occurring mainly in Bavaria and Baden-Württemberg.