Maharjan et al.
Novel variants of tick-borne encephalitis virus from patient and tick samples in Norway. Ticks Tick Borne Dis. Published online July 7, 2025. doi:10.1016/j.ttbdis.2025.102501
The number of reported tick-borne encephalitis (TBE) cases in Norway has increased markedly over the past two decades (see, for example, Vikse et al., TBE in Norway, in TBE Book, 8th edition, edited by Dobler et al., 2025, Chapter 13: Countries with Confirmed TBE). To date, only one complete genome sequence of a TBE virus isolate from Norway – strain Mandal-2009 – obtained from ticks has been published.
In 2022, a patient residing in southern Norway developed severe monophasic TBE. Ten days after symptom onset, the patient was diagnosed with hemophagocytic lymphohistiocytosis (HLH) and treated with corticosteroids and intravenous immunoglobulin. Bronchoalveolar lavage fluid revealed the presence of Aspergillus fumigatus and Pneumocystis jirovecii. The patient responded well to targeted antimicrobial therapy and required mechanical ventilation for nine days. TBE virus-specific IgM antibodies were detected on day 29, while IgG antibodies became detectable on day 87. This delayed serological response may have been influenced by underlying subclinical lymphoma and/or immunosuppressive treatment for HLH.
TBE virus was successfully isolated from both the patient and from ticks collected approximately 16 kilometers from the site of the tick bite. The viral sequences demonstrated 99.7% nucleotide homology. Phylogenetic analysis showed that these sequences were closely related to a TBE virus strain previously identified in Kumlinge, Åland, Finland.
These findings confirm the presence of novel TBE virus variants circulating in the endemic regions of Telemark and Vestfold Counties in southern Norway. Notably, this study presents the first TBE virus genome sequence derived from a human patient in Norway.