Microorganisms in Ixodes ricinus ticks in southern and southwestern Norway.

Tick-borne diseases constitute a major group of emerging vector-borne infections and represent a growing public health concern in many European countries. In Europe, the most clinically significant agents transmitted by Ixodes ricinus are tick-borne encephalitis virus (TBEV) and Borrelia burgdorferi sensu lato, the causative agents of tick-borne encephalitis and Lyme borreliosis, respectively. Ixodes ricinus also serves as a vector for other pathogenic microorganisms, including louping-ill virus (LIV) and Anaplasma phagocytophilum. Various endosymbionts, such as Wolbachia pipientis and Midichloria mitochondrii, are also found in ticks. In this study, we assessed the prevalence of TBEV, LIV, B. burgdorferi s. l., A. phagocytophilum, W. pipientis, and M. mitochondrii in 3437 I. ricinus ticks collected from five locations in Norway. The estimated pool prevalence of TBEV ranged from 0 to 0.4% in nymphs, while the prevalence in adults ranged from 0 to 7.1%. In contrast, no LIV was detected. Borrelia burgdorferi s. l. was detected at all sites, with prevalences ranging from 3.4% to 14.3% (overall prevalence of 5.7%), representing four genotypes (B. afzelii, B. garinii, B. valaisiana and B. burgdorferi sensu stricto). Estimated pooled prevalence of A. phagocytophilum was 5.1% in nymphs, while the overall prevalence in adults was 19.8%. In adult ticks, W. pipientis and M. mitochondrii occurred at overall prevalences of 11.0% and 84.1%, respectively. In only 12.3% of ticks, none of the agents investigated were detected. However, 32.6% of the ticks showed co-occurrence of infectious agents and/or the endosymbionts. Thus, this underscores the need for improved understanding of microbial interactions influencing tick-borne disease outcomes.