# Tick-borne encephalitis infections without CNS involvement: An observational study in Latvia, 2007–2022

## TL;DR

This study in Latvia found that 20% of tick-borne encephalitis cases lacked central nervous system symptoms but still caused severe illness requiring hospitalization.

## Contribution

The study provides the first population-based evidence of non-CNS TBE cases and their clinical significance.

## Key findings

- 20% of TBE cases in Latvia had no CNS involvement but required hospitalization for a median of 7 days.
- 14.1% of non-CNS TBE patients underwent lumbar puncture, indicating suspicion of CNS involvement.
- Non-CNS TBE cases are likely underreported and may represent a larger health burden than previously recognized.

## Abstract

Tick-borne encephalitis (TBE) is a human viral infectious disease involving the central nervous system (CNS). It is caused by the tick-borne encephalitis virus (TBEV). At present, there is very limited information regarding the clinical importance and health burden of TBE infections without signs of CNS inflammation. Moreover, such cases are omitted from official TBE surveillances and there are no reports of population-based studies.

A nationwide population-based study was conducted in Latvia by intensively searching for symptomatic TBEV infections recorded in outpatient and hospital settings between 2007 and 2022. In total, 4,124 symptomatic TBEV infections were identified, of which 823 (20.0%) had no CNS involvement. Despite the lack of neurological symptoms, non-CNS TBE patients still experienced severe health conditions that required management in a hospital setting for a median duration of 7 days. Furthermore, lumbar puncture information was available for 708 of these patients, with 100 (14.1%) undergoing the procedure, suggesting a high suspicion of CNS involvement.

Clearly, non-CNS TBE has the potential to negatively impact the health of patients. The actual burden of non-CNS TBEV cases may be higher than we think as these cases are omitted from official TBE surveillances and are challenging to recognize.