The distribution range of the taiga tick, Ixodes persulcatus, has increased in the last decade in Northern Europe. We performed samplings at nine locations in Nordland County, east and northeast of Brønnøysund, south of the Arctic Circle in July 2023. Four of 62 ticks collected were identified as I. persulcatus (one female, two males and one nymph), the remaining were identified as Ixodes ricinus (9 females, 2 males and 47 nymphs). This is the first report of I. persulcatus in Norway. Given the importance of I. persulcatus as vector of Borrelia burgdorferi sensu lato and more pathogenic subtypes of the tick-borne encephalitis virus, more research should be done focusing on its distribution, ecology and associated pathogens in Norway.

Tick-borne encephalitis virus (TBEV) is an emerging pathogen that initially causes flu-like symptoms and can progress to central nervous system (CNS) infections. Tick-borne encephalitis (TBE) is an endemic disease in southern coastal counties with regular human cases, while the causative agent, TBEV, is prevalent in ticks in most of the coastal regions of Norway. This study was aimed to understand TBEV infection status across Norway including both TBE endemic and non-endemic areas. For this, we analyzed a total of 1940 residual serum samples from 19 counties of Norway (as of 2016). The samples were initially screened by ELISA, followed by virus neutralization tests for TBEV confirmation. We found a similar TBEV seroprevalence of 1.7% in TBE endemic and 1.6% in non-endemic areas. Since TBE cases are only reported from endemic regions, our findings suggest a potential subclinical or asymptomatic infection and underdiagnosis in non-endemic areas. Notably, only 43% of the ELISA-positive samples were confirmed by virus neutralization tests indicating that not all ELISA positives are true TBEV infections. Additionally, 137 samples of patients presenting with symptoms of CNS infections from a non-endemic area were included. Of these samples, 11 ELISA-positive samples were analyzed for cross-reactivity among flaviviruses. Cross-reactivity was detected with Dengue virus, West Nile Virus, and non-specific reactions. This underscores the importance of using multiple diagnostic tests to confirm TBEV infections. None of the patients with CNS infection was found to be TBE positive, and in the whole cohort, we found a low TBEV seroprevalence of 0.7%.

Background: Borrelia burgdorferi sensu lato (s. l.) and Neoehrlichia mikurensis are the most prevalent pathogenic bacteria found in Ixodes ricinus in Norway. Here we evaluate the prevalence of both bacteria and the density of infected ticks, key factors to assess the human infection risk. The study was performed in one location in southern Norway. Results: An overall prevalence of 12% for B. burgdorferi s. l. and 8% for N. mikurensis was found. The most prevalent B. burgdorferi s. l. genospecies was Borrelia afzelii , followed by Borrelia burgdorferi sensu stricto and Borrelia garinii. The highest density of infected nymphs with B. burgdorferi s. l. was in May and the lowest was in November. In the case of N. mikurensis , the highest density of infected nymphs was found in September and the lowest in November. B. burgdorferi s. l. prevalence was negatively correlated with tick density. Conclusions: Our results highlight the importance of assessing not only pathogen prevalences but also tick population densities. Extended periods of surveillance in questing ticks are necessary to understand the intricate dynamics of ticks and tick-borne pathogens.

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.

Tick-borne encephalitis virus (TBEV) is a medically important orthoflavivirus endemic across Europe and Asia, transmitted primarily by Ixodes ticks but also through ingestion of unpasteurized milk from infected ruminants. While milk-borne human infections are well documented, experimental evidence for natural mother-to-offspring transmission in livestock remains scarce. Here, we experimentally assessed the potential for lactogenic transmission of TBEV in sheep. Eight ewes were infected subcutaneously with the European subtype TBEV strain Hochosterwitz and co-housed with 16 three-week-old lambs (two per ewe) for an 18-day observation period. Clinical parameters, hematology, and virological and serological profiles were monitored. All animals remained asymptomatic, with no hematological abnormalities. TBEV RNA was detected in ewes' serum and milk from day one post-infection, and in lamb serum from day five. Virus-specific antibodies emerged in ewes from day four and in lambs from day 13. These findings provide direct experimental evidence that TBEV can be transmitted via milk from ewes to their suckling lambs throughout the lactation period, underscoring a potential but underappreciated route for virus transmission in endemic areas.