Introduction and objective:
The aetiology of encephalitis frequently remains undetermined. Infections of the central nervous system (CNS) with Borrelia burgdorferi sensu lato and Toxoplasma gondii are particularly challenging to confirm due to inconsistent serological findings and low sensitivity of polymerase chain reaction (PCR) in cerebrospinal fluid (CSF). The chemokine CXCL13 has been proposed as a supportive marker for neuroborreliosis. The aim of the study is to investigate the presence of immunological and molecular markers of B. burgdorferi s.l. and T. gondii in patients with encephalitis of unknown origin, and to evaluate their potential association with CNS involvement.

Material and methods:
A retrospective analysis was conducted on serum and CSF samples from 31 patients with a clinical diagnosis of encephalitis of unknown aetiology. Anti-Borrelia and anti-Toxoplasma antibodies and CXCL13 levels were measured using ELISA. Pathogen DNA was assessed by real-time PCR.

Results:
Anti-Borrelia antibodies were detected in 12 (38.7%) serum samples. Elevated CXCL13 levels (>100 pg/mL) were observed in 2 CSF samples. Anti-T. gondii IgG was found in 14 (45.2%) serum samples, with 5 (16.1%) also positive in CSF. High-avidity IgG was present in 9 (29.0%) serum and 3 (9.7%) CSF samples. Low-avidity IgG was found in 3 cases, with variable IgM status. No Borrelia or T. gondii DNA was detected.

Conclusions:
The presence of antibodies against B. burgdorferi s.l. and T. gondii in serum and/or CSF, occasionally accompanied by elevated CSF CXCL13, suggests a potential role of these pathogens in encephalitis of unknown aetiology, even in the absence of detectable DNA, and highlights the diagnostic value of combining serological and molecular testing.