Risk factors and tick-borne encephalitis spatio-temporal variation: a case study in northern Italy

Introduction Tick-borne Encephalitis (TBE) is a zoonosis of public health relevance in many European countries. Italy is considered at low risk of incidence, although in some endemic areas (foci the risk of TBE infection is high. The circulation of the TBE virus is characterized by a high variability both in term of location and impact. In the Province of Trento (Italy) the incidence rate had a 5-fold increase in the last six years (2012-2017) compared to the 20 years before (1992-2011), and also the geographical distribution of local foci experienced a northward shift. Objectives Our aims were to determine the factors affecting TBE virus circulation among rodent hosts; assess the prevalence of TBE virus in questing ticks in order to estimate the current TBE infection risk in the study area. Materials & methods We investigated the pattern of tick infestation and TBE virus seroprevalence in a population of yellow-necked mouse, Apodemus flavicollis, in a known TBE focus in the Province of Trento (Italy). We also performed a wide screening of TBEV prevalence in ticks by real-time RT-PCR. Results The number of co-feeding ticks on rodents and TBE seroprevalence were affected by the combination of climatic condition, in particular the autumnal cooling, with the abundance of the feeding tick hosts, in particular rodents and deer. The screening of questing ticks for TBE virus confirmed the same very low prevalence which has been previously recorded in the area . Sequencing of the virus from the positive ticks showed that the Western subtype of TBE virus is circulating in this area. Conclusion Complex factors shape the pattern of TBE disease focal distribution. Variation of climatic condition, tick host abundance and feeding interactions could affect the circulation and presence of the virus shaping its distribution at local scale. More information on the new TBE foci are needed also in the public health perspective point of view as TBE vaccination could be implemented.