Soleng et al.
Microorganisms in Ixodes ricinus ticks in southern and southwestern Norway. Int J Parasitol Parasites Wildl. 2026;101252. doi:10.1016/j.ijppaw.2026.101252
Ixodes ricinus is the predominant tick species affecting human and animal health. However, knowledge of the microorganisms carried by these ticks in Norway is limited. A study was conducted at five locations in southwestern Norway and on two islands in southern Norway to assess the prevalence of selected pathogens and commensal microorganisms in nymphs and adult ticks.
Borrelia burgdorferi s.l. was detected at all locations with an overall prevalence of 5.7% (range: 3.4–14.3%) in adult ticks. Four genospecies were detected: B. afzelii (23.1%), B. garinii (61.5%), B. burgdorferi s.s (7.7%), and B. valaisiana (7.7%).
Endosymbiotic Wolbachia pipientis was detected for the first time in Norway, and Midichloria mitochondrii was detected at all locations.
The estimated pool prevalence of Anaplasma phagocytophilum in nymphs was 5.1% (range: 2.1–11.1%). Anaplasmosis is not a notifiable disease in Norway, but it has been estimated that 300,000 lambs are infected annually.
In the present study, no louping-ill virus could be detected in a total of 2,490 analysed ticks. Nearly 50 years ago, an outbreak of louping-ill occurred at a sheep farm on the southwestern coast, where Site 1 of this study is located. Since 1991, there have been no clinical reports of LIV-infected animals in Norway.
The estimated pool prevalence of TBE virus in nymphal ticks was 0.1% (range: 0–0.4%), and the overall prevalence in adult ticks was 3.3% (range: 0–7.1%). The prevalence reported in this study was consistent with previous findings in Norway. Notably, TBE virus was detected at two locations on the west coast, only 50 metres apart. No virus could be detected at a third investigated location about 800 m from these two sites , confirming the highly focal distribution of TBE virus.
Only 12.3% of the ticks were negative for all investigated microorganisms.
The detection of TBE virus in ticks confirms previous data about the circulation of TBE virus in Norway (see also Snapshot week 23/2026 and 16/2026). Notably, the findings also confirm the circulation of TBE virus in geographically restricted areas, referred to as TBE foci.
The authors highlight the need for further epidemiological investigations because the annual number of reported TBE cases has significantly increased during the last decade (from approximately 10–20 cases per year to around 100 cases annually) (see TBE Book, chapter 13).