Tick-borne encephalitis in adults in Denmark: a nationwide prospective cohort study from 2015 to 2023.

Abstract

Background: Our aim was to characterize the clinical presentation and outcome in adults with tick-borne encephalitis (TBE) and to determine the incidence and test activity of TBE in Denmark.

Methods: A nationwide prospective cohort study of all adults hospitalized with TBE at departments of infectious diseases in Denmark from 2015 to 2023. An age- and sex-matched cohort of herpes simplex virus type 1 (HSV-1) encephalitis patients was included to compare outcome.

Results: Fifty-two patients with TBE were included. Median age was 50 years, 32/52 (62%) were men, 1/52 (2%) was fully vaccinated against TBE at the time of infection, 29/52 (56%) were infected in Denmark. Upon admission 25/52 (48%) had meningitis, 27/52 (52%) encephalitis, three of the latter 3/52 (6%) with additional myelitis or radiculitis. Admission to the intensive care unit 6/52 (12%) and death 2/52 (4%) were associated with pre-existing comorbidities and older age. At 3-month follow-up, 16/50 (32%) had an unfavorable outcome (Glasgow Outcome Scale score 1-4) compared to 39/52 (75%) in the HSV-1 cohort. The most common residual symptoms at 6-month follow-up or later were headache, cognitive impairment, and fatigue. The TBE incidence increased from 0.03/100,000 in 2015 to 0.48/100,000 in 2023, and the test rate from 5.5/100,000 in 2015 to 14.4/100,000 in 2023, with a positivity rate of 0.6% in 2015 and 3.3% in 2023.

Conclusion: The incidence of TBE in Denmark increased in the study period, with clinical characteristics and outcome of adult patients comparable to reports from other European countries.