Seasonal variation in prevalence of Borrelia burgdorferi sensu lato and Neoehrlichia mikurensis in Ixodes ricinus nymphs in southern Norway

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.