Potential drivers of human tick-borne encephalitis in the Örebro region of Sweden, 2010-2021

Abstract

Abstract Incidence of tick-borne encephalitis (TBE) has increased during the last years in Scandinavia, but the underlying mechanism is not understood. TBE human case data reported between 2010 and 2021 (N = 81) was aggregated into postal codes within Örebro County, south-central Sweden, along with tick abundance and environmental data to analyse spatial patterns and drivers of TBE. We identified a substantial and continuing increase of human TBE incidence in Örebro County during the study period. Although the spatial pattern varied between years, spatial cluster analyses showed significant hotspots (higher number of cases than expected) in the southern and northern parts of Örebro County, whereas a cold spot (lower number of cases than expected) was found in the central part comprising Örebro municipality. Generalised linear models showed that the risk for residents of acquiring TBE increased by approximately 141% for every percent increase in the proportion of wetland forest, however models had relatively low goodness of fit (R2 < 0.25). Results suggest that incidence of TBE in Örebro County is spatially clustered, however variables used in this study, i.e., forest cover, water, tick abundance, sheep as indicator species, alone do not explain these patterns.