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

## TL;DR

This study analyzed adult tick-borne encephalitis cases in Denmark from 2015 to 2023, finding increased incidence and identifying key clinical features and outcomes.

## Contribution

The study provides a nationwide prospective analysis of TBE in Denmark, including incidence trends and comparison with HSV-1 encephalitis outcomes.

## Key findings

- The incidence of TBE in Denmark increased from 0.03/100,000 in 2015 to 0.48/100,000 in 2023.
- At 3-month follow-up, 32% of TBE patients had an unfavorable outcome compared to 75% in the HSV-1 cohort.
- Common residual symptoms included headache, cognitive impairment, and fatigue.

## Abstract

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.

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.

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.

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.

The online version contains supplementary material available at 10.1007/s00415-025-12986-5.