Aanalyzing risks of incidence of tick-borne encephalitis in areas with different climatic and geographical conditions

Abstract

Ticks are natural reservoirs and vectors of a virus that is an infectious agent of tick-borne encephalitis, a communicable disease with great medical and social significance. Tick-borne encephalitis (TBE) is widely spread in Arkhangelsk region (AR) located in the Arctic zone in Russia where substantial climatic changes are taking place at the moment. Our research involved examining spatial and temporal distribution of numbers of people bitten by ticks, a number of people bitten by ticks per 100 thousand, a number of TBE cases and TBE incidence in districts and settlements in AR. We calculated relative risks of TBE incidence among people bitten by ticks in AR from 1980 to 2019. We analyzed dynamics of indicators showing numbers of people bitten by ticks per 100 thousand and TBE incidence among people living in Arkhangelsk region. The analysis revealed that a number of bitten people grew slowly in 1980–1990, then there was an exponential growth in 1990–2010, and then the trend stabilized in 2010–2019. Dynamics of TBE incidence was completely in line with changes in number of bitten people up to 2014 but there was a substantial drop in TBE incidence after that. Spatial distribution of numbers of bitten people and TBE incidence revealed that average number of bitten people amounted to 25.1 per 100 thousand in the northern districts in 1980–2019 and was statistically significantly lower than in the central and southern districts (р < 0.001). Average long-term incidence was the highest (7.9 per 100 thousand) in the southern districts in comparison with the central (3.0 per 100 thousand; р<0.001) and northern ones (0.7 per 100 thousand; р<0.001). Maximum relative risks of TBE incidence was detected in the southern districts in 1990–1999 (38.8) in comparison with the northern ones. We made an assumption about probable reasons for declining TBE incidence in Arkhangelsk region detected over the last years given the growing numbers of bitten people.