Analysis of effectivity of the tickborne encephalitis virus detection methods in ixodid ticks.

Tick-borne encephalitis (TBE) is transmissible viral disease widely common in temperate zone of Eurasia. ELISA and PCR are used for express identification of the vector's infection, but the results of the two methods often do not agree. Aim of the work is comparative analysis for TBE virus of Ixodid ticks from nature using complex of methods, including ELISA, PCR, and isolation of the virus in laboratory mice. 18608 Ixodid ticks were collected during 2013-2019 in TBE natural foci of the Baikal Region. The ticks suspensions were examined individually, using ELISA (n=17610) and PCR (n=2999). Suckling mice were inoculated with the suspensions positive in the both tests. The TBEV antigen was found in 1.2 % of ticks in average. All ticks positive in ELISA were examined in PCR (Group 1). Randomly selected part of negative-ELISA samples were examined in PCR too (Group 2). The PCR results were positive in 68.9±3.13 % of the Group 1, with average Ct index 24.6±0.38. Positive results of PCR in Group 2 accounted for just 2.7±0.31 % with average Ct index 31.0±0.70. The average Ct margin of the Groups 1 and 2 is statistically significant (p < 0.001; df = 118). Isolation of strains was significantly more successful in Group 1 (21.7±2.77 %), than in Group 2 (8.2±5.26 %; p < 0.05; df = 50). ELISA is more useful for examining large amounts of ticks. To get a more complex picture about epidemically dangerous part of the vectors in TBE natural foci, the results of the two express-methods is better to sum. The isolation of the virus is useful to carry out of the samples positive in ELISA and PCR concurrently.

Tick-borne encephalitis virus isolates from natural foci of the Irkutsk region: clarification of the genotype landscape.

The Irkutsk region is the unique territory where all known subtypes of tick-borne encephalitis virus (TBEV) circulate. In the last years, the phenomenon of changes in TBEV subtypes (substitution of the Far-Eastern subtype by the Siberian one) was noted in some regions of the Russian Federation. The results of individual investigation of 11522 Ixodes persulcatus ticks and brain specimens from 81 small mammals collected in natural foci of the Irkutsk region during 2006-2014 are presented in the article. More than 60 TBEV strains have been isolated and studied by virological methods; E gene fragments (1193 b.p.) of 68 isolates have been typed. The majority of the strains (irrespective of subtype) were of high virulence for laboratory mice (LM) in case of both intracerebral and subcutaneous inoculation of virus. All isolates from warm-blooded small mammals and humans were of high virulence for LM, but placed in the same clusters of the phylogenetic tree with ticks collected in the same area. Tick-borne strains of different virulence also did not form separate clusters on the tree. Phylogenetic analysis showed that modern TBEV genotypic landscape of the studied territory is changing toward absolute predominance of the Siberian subtype (94.1%). This subtype is represented by two groups with prototype strains "Zausaev" and "Vasilchenko". The "Vasilchenko" group of strains is spread on the whole territory under study; the strains of "Zausaev" group were isolated previously in the Irkutsk suburbs. The European subtype of TBEV circulates in natural foci of Pribaikalie permanently (at least 5% of the random sampling); the strains are of high virulence for LM. The Far-Eastern TBEV subtype was not found within the group of isolates collected in 20062014. The phylogenetic relationship of the strains under study had a higher correlation with the place of isolation than with the year or source.