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 3,437 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.