The long-term outcome of tick-borne encephalitis in Central Europe.

Background: Information on the long-term outcome of tick-borne encephalitis (TBE) is limited.

Objectives: To assess the frequency and severity of post-encephalitic syndrome (PES) at different time points after TBE, and to determine the parameters associated with unfavourable outcome.

Methods: Adult patients diagnosed with TBE in Slovenia in the period 2007-2012 were followed-up for 12 months and also examined 2-7 years after TBE. Each patient was asked to refer a person of similar age without a history of TBE to serve as control.

Results: A total of 420 patients and 295 control persons participated in the study. The proportion of patients with PES (defined as the presence of ≥ 2 subjective symptoms that newly developed or worsened since the onset of TBE and which had no other known medical explanation, and/or ≥ 1 objective neurological sign) was higher (P < 0.001) at the follow-up visit 6 months after the acute illness (127/304, 42%, 95% CI: 36-47%) than at 12 months (68/207, 33%, 95% CI: 26-40%); the proportion at 12 months was the same as at 2-7 years after TBE (137/420, 33%, 95% CI: 28-37%). However, the proportion of severe PES at the last two time points differed (9.7% vs 4.3%, P = 0.008). Multivariate logistic regression showed that unfavourable outcome at 6 months was associated with CSF leukocyte count (OR = 1.003, 95% CI: 1.001-1.005%, P = 0.017), at 12 months with the disease outcome at 6 months (OR = 115.473, 95% CI: 26.009-512.667%, P < 0.001), and at the final visit with disease outcome at 6 months (OR = 3.808, 95% CI: 1.151-12.593%, P = 0.028) and 12 months (OR = 26.740, 95% CI: 8.648-82.680%, P < 0.001). Unspecific symptoms that occurred within the four weeks before the final examination were more frequent and more constant in patients than in the control group.

Conclusions: The frequency of PES diminished over time and stabilized 12 months after the acute illness, whereas the severity of PES continued to decline. Unfavourable outcomes at 12 months and at the final visit were strongly associated with the presence of PES at previous time points.