Prevalence and genetic variability of tick-borne encephalitis virus in host-seeking Ixodes ricinus in northern Italy

Tick-borne encephalitis (TBE) is a severe disease that has been endemic in north-east Italy since 1992. Over the past two decades, there has been an increase in the number of human cases reported in many European countries, including Italy. To assess the current TBE infection risk, questing ticks were collected from known TBE foci, as well as from a site in northern Italy where no human infections have been reported previously. A total of 1739 Ixodes ricinus (1485 nymphs and 254 adults) was collected and analysed for TBEV prevalence by a real-time RT-PCIR targeting the 3' untranslated region. Phylogenetic analyses of the partial envelope gene were conducted on two newly sequenced TBE virus (TBEV) strains and 28 previously published sequences to investigate the genealogical relationships of the circulating TBEV strains. These phylogenetic analyses confirmed a previous report that the European TBEV subtype is the only subtype circulating within the TBE foci in north-east Italy. Interestingly, nucleoticle sequence analysis revealed a high degree of divergence (mean 2.54%) between the TBEV strains recovered in the Italian province of Trento, despite the circulation of a single TBEV subtype. This elevated genetic variability within a single TBE focus may reflect local differences in the long-standing evolutionary dynamics of TBEV at this site relative to previously characterized sites, or more recent and continuous reintroduction of various TBEV strains.