Long-term monitoring of tick-borne viral encephalitis and tick-borne borreliosis in the Komi Republic

Abstract

Our study was aimed at analyzing data combining objective epidemiological, zoological and entomological research intended to determine the environmental and epidemiological patterns of tick-borne encephalitis (TBE) and Lyme borreliosis (LB) foci, as well as the role of anthropogenic factors in epidemic manifestations to improve anti-epidemic activities. Long-term data on the abundance of ticks flagged from vegetation in various districts of the Komi Republic (RK) have been analyzed. The prevalence of tick-borne encephalitis virus (TBEV) and Borrelia burgdorferi (agent of LB) in ticks picked from vegetation, humans and domestic animals in various districts of the RK was measured by using enzyme-linked immunosorbent assay and polymerase chain reaction (PCR-ELISA). In addition, the data on 1994–2017 tick-bite incidence in RK, 1970–2017 TBE incidence, and 1997–2017 LB incidence were also examined and correlated with the impact of forest transformation on the TBE incidence. The rise of tick population in the southern districts of RK and its northward expansion were noted. It is also shown that the RK territory, where tick bites are registered, expanded significantly. Mean annual tick-bite incidence rate in 2006–2017 vs. 1994–2005 timeframe was increased by 4-fold. RK regions where deforestation took place, the tick-bite incidence was increased, and the local TBE incidence elevated distinctly. In recent years, there was a rise in TBEV prevalence in ticks, collected either from vegetation or from humans and animals. In 2006–2017 and 1970–1981, mean annual TBE incidence rate increased by a 11- and 36-fold as compared to that one registered in 1982–1993. In recent years, the most severe focal TBE forms began to be recorded. In 1996–2017, the TBE mortality rate was 2.3%. The main features of LB epidemiology mimic those found in TBEV. In particular, the prevalence of Borrelia burgdorferi in ticks flagged from vegetation increased from 9.5% to 34.7% in 2010 vs. 2017, respectively. A sharp rise in LB incidence rate and profound northward shift for bacterial range, where this infection started to be registered only in recent years, were noted. Both TBE and LB demonstrated marked occupation-related pattern. The need in conducting task-oriented prophylaxis of tick-borne infections is necessitated by taking into account geographic specifics. The RK northern regions should be considered as potentially endemic, where tick bites in human population has recently begun to be reported.