Tick-borne encephalitis (TBE) is a viral infection transmitted by ticks. TBE is the third most common cause of encephalitis in France. Across France, numerous cases of lymphocytic meningitis remain without a definitive diagnosis, notably at the Nancy University Hospital. The objective of this study is to assess the impact of a change in diagnostic practice on the work-up of undocumented lymphocytic meningitis and meningoencephalitis at Nancy University Hospital.

Tick-borne encephalitis (TBE) is a viral infection transmitted by ticks. It is an emerging infectious disease and has been notifiable in France since 2021. TBE is the third most common cause of encephalitis in France, following herpes simplex virus and varicella-zoster virus infections (ENCEIF data).

According to initial data collected by Santé publique France, eastern France is the most affected region, with over half of the reported cases occurring in the Rhône-Alpes and Alsace regions. However, this infection remains relatively unfamiliar to many clinicians and is infrequently investigated during routine diagnostic procedures.

Across France, numerous cases of lymphocytic meningitis remain without a definitive diagnosis, notably at the Nancy University Hospital, where approximately 150 such cases are recorded annually. It is plausible that a proportion of these cases are secondary to TBE infection.

The hypothesis underlying this study is based on these observations. Diagnostic practices at the Nancy University Hospital are currently evolving. The infectious diseases team is actively working to increase clinicians' awareness of TBE, encouraging them to consider this diagnosis when presented with compatible clinical scenarios. Specifically, in cases suggestive of TBE, the infectious diseases team now routinely recommends testing for this pathogen.

This change in practice may lead to an increased rate of documented TBE cases and potentially improve patient management.

The primary objective of this study is to assess the impact of this change in diagnostic practice on the work-up of undocumented lymphocytic meningitis and meningoencephalitis at Nancy University Hospital.

Inclusion Criteria:

• Individuals who have received full information regarding the organisation and conduct of the study
• Adults or children
• Hospitalised in a department of Nancy University Hospital with a diagnosis of lymphocytic meningitis or meningoencephalitis
• Cerebrospinal fluid showing more than 5 cells/mm³, with either a negative FilmArray result or no FilmArray performed

Exclusion Criteria:

• Patients diagnosed with non-lymphocytic meningitis or meningoencephalitis
• Patients with documented lymphocytic meningitis or meningoencephalitis of a known cause (excluding TBE) prior to the date the advisory was issued
• Patients who have expressed opposition to the use of their data for this study

Inclusion Criteria:

• Individuals who have received full information regarding the organisation and conduct of the study
• Adults or children
• Hospitalised in a department of Nancy University Hospital with a diagnosis of lymphocytic meningitis or meningoencephalitis
• Cerebrospinal fluid showing more than 5 cells/mm³, with either a negative FilmArray result or no FilmArray performed

Exclusion Criteria:

• Patients diagnosed with non-lymphocytic meningitis or meningoencephalitis
• Patients with documented lymphocytic meningitis or meningoencephalitis of a known cause (excluding TBE) prior to the date the advisory was issued
• Patients who have expressed opposition to the use of their data for this study

The datasets generated and analysed during the current study are not publicly available due to restrictions by the French law, but are available from the last author on reasonable request at the following email address: b.lefevre@chru-nancy.fr.