Clinical Characteristics and Neutralizing Antibody Levels in a Japanese Patient with Tick-borne Encephalitis: A Case Report.

Tick-borne encephalitis (TBE) is a severe neurological disease caused by infection with the TBE virus (TBEV), primarily transmitted to humans through tick bites. As there is no effective treatment, prevention through vaccination and avoidance of tick bites is essential. In Japan, only five cases had been reported up to 2023, all from Hokkaido, the northernmost main island. Here, we report the sixth case of TBE in Japan. The patient was a 58-year-old male, in whom magnetic resonance imaging revealed enhancing lesions in the brain and spinal cord, accompanied by severe neurological symptoms. Despite intensive treatment, his condition deteriorated, and he died 5 months after the onset. At the initial consultation, no tick-bite sites were identified. However, interviews with the patient's family revealed he had engaged in outdoor activities before disease onset. Based on the clinical course, epidemiological history, and detection of anti-TBEV antibodies, the patient was diagnosed with TBE. During hospitalization, serum neutralizing antibody titers gradually decreased but remained extremely high for 5 months. This case underscores the importance of epidemiological investigation, including history-taking for possible tick exposure, and highlights the need to raise awareness of tick-borne diseases among healthcare workers, particularly in endemic areas.