Tick-borne encephalitis in the Far East focal region of the Eurasian continent

Abstract

The analysis of the activity of natural foci of tick-borne encephalitis (TBE) in the Far Eastern Federal District (Primorsky and Khabarovsk Territories, the Jewish Autonomous and Amur Regions), represented by a single Sikhote-Alin focal region was performed. Endemic territories of China, Korea, Japan are adjacent to this focal region. In the last decade the decrease of the infection rate in ixodid ticks and the morbidity of TBE compared to the 1990s was observed. Data on the infection rates in ixodid ticks based on ELISA, PCR and TBEV isolation are often discrepant. Active circulation of TBEV that involved dominant and less significant species of ixodid ticks and mouse-like rodents has been demonstrated since 1952 in China, since the 1990s in Japan and since 2011 in South Korea. The circulation of TBEV strains of the Far Eastern subtype was determined in all these territories. The exception was 7 strains isolated in South Korea, which appeared to be the European subtype based on the results of molecular genetic analysis. Two new strains of TBEV from the organs of wild rodents Marmota himalayana were isolated in 2017 in the territories of previously unknown natural foci in the Tien Shan, China. The genetic divergence of these strains from three TBEV subtypes allowed us to establish the existance of a new Himalayan TBEV subtype (Him-TBEV), which, according to the authors, has been "hidden" for hundreds of years. One can also speculate the processes of emergence of new TBEV subtypes The most complete picture of the Far Eastern population of TBEV was obtained in the last decade of the twentieth century, which served as the basis and impetus for the study and comparative analysis of the differences in numerous strains of TBEV isolated on the territory of the Eurasian continent.