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

Abstract

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-PCR targeting the 39 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, nucleotide 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 longstanding evolutionary dynamics of TBEV at this site relative to previously characterized sites, or more recent and continuous reintroduction of various TBEV strains. INTRODUCTIONTick-borne encephalitis virus (TBEV) is a zoonotic arbovirus of significant medical importance in both Europe and Asia, causing 4500 and 11 000 cases, respectively, of human encephalitis annually (Randolph, 2006). The ecology of TBEV involves a rodent-tick transmission cycle where uninfected ticks acquire TBEV from infected ticks while co-feeding on the same competent reservoir host, mostly small mammals of the genus Apodemus (Labuda et al., 1993(Labuda et al., , 1997. The non-viraemic transmission among co-feeding ticks represents the major amplification route for TBEV, which contributes to the local specificity of infection risk and the heterogeneity of the spatial distribution of endemic sites (Randolph et al., 1999). Additionally, systemic viraemia in rodents is not sufficient to ensure TBEV transmission to ticks, as the virus itself causes a high mortality rate in the host before ticks are presented the opportunity to complete a blood meal (Randolph et al., 1996). Furthermore, the distribution of endemic tick-borne encephalitis (TBE) sites can also be influenced by multiple abiotic and biotic factors, which affect tick abundance, tick phenology, tick infestation prevalence and tick human contact rate, as well as differences in surveillance efforts (Randolph et al., 2008).TBEV belongs to the genus Flavivirus and its genome consists of a single-stranded, positive-sense RNA of approximately 11 kb, which encodes a polyprotein that is cleaved by cellular and viral proteases into three structural -the capsid, membrane and envelope proteins -and seven non-st...

Ancient Ancestry of KFDV and AHFV Revealed by Complete Genome Analyses of Viruses Isolated from Ticks and Mammalian Hosts

“…Ticks sampled on the same day in one city (Najran) harbor a more genetically diverse AHFV population than isolates obtained from human cases over the course of 15 years throughout Saudi Arabia. Similarly, an analysis of the European subtype of tick-borne encephalitis virus (TBEV) isolated from ticks shows relatively high variation in a limited geographic area and temporal period, perhaps due to importation of TBEV on migratory birds [31], [ 32]. The notable genetic heterogeneity of AHFV in Najran may be explained similarly, as camel and other livestock traders often travel large distances to Najran to market their animals.…”

Tick-Borne Flaviviruses, with a Focus on Powassan Virus

“…Replication in the vertebrate host might play an important role in virus overwintering (58). The significance of different tick species in overwintering of TBEV has also been reported (59,60).…”

The role of viral persistence in flavivirus biology

“…Deer may stabilize and maintain TBFVs at levels that are important for transmission (Pugliese et al, 2007;Carpi et al, 2009;McGee et al, 2010;Dobler et al, 2012). While deer are important sources of blood meals for ticks, they develop low virus titer and are therefore not competent in transmission of TBFVs.…”