Clinical Characteristics of Tick-Borne Encephalitis in Adult Patients: A 10-year Retrospective Study in Stockholm, Sweden

Background: The incidence of tick-borne encephalitis (TBE) has increased during the last decades in Europe. Our aim was to assess the clinical characteristics and outcome of patients with TBE in Region Stockholm, as a high-risk area in Sweden.

Methods: The notification database at the regional Department of Communicable Disease Control and Prevention was used to identify TBE cases during 2006-2015. Clinical data were retrieved from the included patients' medical records. The associations of specific variables to predefined outcomes of disease severity were evaluated with multivariate logistic regression models.

Results: Of 1004 identified TBE cases, 703 adult patients were included. Sixty-one percent were men, and the median age was 50 years (range, 18-94 years). The majority of patients were nonvaccinated. Comorbidity was present in 34%, and 4% were receiving immunomodulatory therapy. Seventy-five percent were hospitalized, and 11% had severe disease. More than 70% of the 79 patients followed up for >6 months had persisting symptoms. The case fatality rate was 1.4%, 15% in the group with immunomodulatory treatment. In the multivariate analysis, severe disease was associated with underlying comorbid conditions, age ≥50 years, and previous complete TBE vaccination.

Conclusions: This is the largest cohort of patients with TBE in Scandinavia. Our findings of a more severe course of disease in older patients, those receiving immunomodulatory therapy, those with comorbid conditions, and those with vaccination breakthrough infections must be interpreted in the context of hospitalized patients. Optimized prevention is needed for patients receiving immunomodulatory therapy, given the considerable case fatality rate. Follow-up visits and rehabilitation should be better standardized.