Importance of surveillance of tularemia natural foci in the known endemic area of Central Europe, 1991-1997

Introduction and purpose: Marked activation of natural foci of tularemia in the known endemic area of Central Europe, comprising the borderland of Slo vakia, Austria and the Czech Republic, led to an epidemic outbreak in weste rn Slovakia and an increase in the number of human tularemia cases in the a djoining regions of northeastern Austria and southern Moravia from 1995 to 1997. The aim of this paper was to present the results of a longitudinal st udy on the prevalence of infection with Francisella tularensis in small mam mals and ticks in six localities of western Slovakia and northeastern Austr ia from 1991 to 1997, and to draw attention to the importance of systematic surveillance of natural foci for epidemiologic prognoses and for taking pr eventive measures. Methods: In Slovakia, three localities of the Zahorie lowland were monitore d for 3 to 6 years (1991 to 1996) for the presence of F. tularensis in smal l terrestrial mammals. In Austria, small mammals and ticks from three local ities were investigated for 1 to 3 years (1994 to 1997). Spleens of live-trapped animals (at 1- to 2-month intervals) and ticks coll ected by flagging (in 6- to 8-week intervals) were examined by inoculation of pools into white mice, or individually by cultivation. Results: In four localities under investigation (three localities in wester n Slovakia and one in Austria), a nearly simultaneous flare-up of tularemia epizootics was recorded in the autumn of 1994. The highest mean prevalence of infection in small mammals was 3.9 % in the last quarter of the year, w hich along with isolations of F. tularensis from Dermacentor reticulatus ti cks collected from vegetation in the locality of Austria (1.3% positivity), indicated the high epizootic activity of foci. F. tularensis was isolated from five rodent species - Apodemus flavicollis, A. sylvaticus, Clethrionom ys glareolus, Microtus arvalis and Sorer araneus. From 1995 to 1996, the pe rsistent activity of the focus was repeatedly confirmed by isolations of F. tularensis from D. reticulatus ticks. In 1995 a natural focus of tularemia was detected in a further locality of Austria, where the agent was isolate d from D. reticulatus. The increased epizootic activity of foci in the ende mic region of Central Europe indicated a higher risk of acquiring tularemia for humans and was, in fact, followed by an epidemic outbreak in western S lovakia as well as by a large number of cases in Austria and the Czech Repu blic. Conclusion: Our findings clearly demonstrate that long-term surveillance of natural foci in endemic regions provides useful information on the activat ion of tularemia foci before the onset of epidemic outbreaks, thus permitti ng timely epidemiologic prognoses and the institution of preventive measure s.