Seasonality and environmental drivers of tick-borne encephalitis virus prevalence in Ixodes ricinus ticks from Southern Norway.

The tick-borne encephalitis virus (TBEV) is the causative agent of neurological disease in humans, and it is mainly transmitted by the tick Ixodes ricinus in western Europe. In Norway, previous surveillance studies have detected the virus along the coast from Nordland County in the north to the Swedish border in the southeast. However, all human cases of tick-borne encephalitis are restricted to the southernmost part of the country, presenting a significant increase in the incidence in the last decades. The incidence and prevalence of TBEV is consistently linked to climatic factors. We evaluated the monthly variation in prevalence of TBEV during six years, in nymphal and adult ticks from Mandal, southern Norway; a TBEV endemic area with high density of ticks. In addition, we evaluated the relationship between the virus prevalence and the accumulated heat and relative humidity. A marked seasonality of the virus prevalence was observed, peaking in the spring-early summer in nymphs and in summer-early autumn in adult ticks. The accumulated heat correlated positively with the prevalence in nymphs and adults, and the relative humidity correlated negatively with the prevalence in adults. Future research should focus on understanding the impacts of host dynamics, climate change and land use in the circulation of this emerging virus in Norway.