Transient HBV entry into cerebrospinal fluid following blood-brain barrier disruption: a case of HSV-2 encephalitis with polymicrobial detection elucidated by serial targeted next-generation sequencing

Hepatitis B virus (HBV) is traditionally considered a strictly hepatotropic virus, and its neuroinvasive potential remains controversial. When targeted next-generation sequencing (tNGS) simultaneously detects multiple viruses in the cerebrospinal fluid (CSF), distinguishing the true pathogen from bystander viruses poses a clinical challenge. We report a rare case of herpes simplex virus 2 (HSV-2) encephalitis with concurrent detection of HBV and human gammaherpesvirus 4 (HHV−4) in the CSF, and explore the mechanism of HBV entry into the central nervous system (CNS) through serial tNGS before and after treatment, combined with routine CSF analysis. A 42-year-old male with untreated hepatitis B presented with acute fever, frontal and temporal headache for 10 days, unresponsive to empirical anti-infective therapy. EEG revealed increased theta activity in bilateral frontal regions after hyperventilation, suggesting encephalitis. Initial CSF analysis (Day 1) showed marked pleocytosis (white blood cells 1849 × 10 6 /L) and hemorrhagic appearance (red blood cells 1000 × 10 6 /L), indicating acute blood-brain barrier (BBB) disruption. CSF tNGS detected sequences for HSV-2 , HHV-4 and HBV . High level of serum HBV DNA with positive HBsAg confirmed active HBV replication. The patient was treated with acyclovir and tenofovir alafenamide (TAF). Repeat lumbar puncture (LP) on Day 17 demonstrated normalized CSF parameters (red blood cells 0 × 10 6 /L, white blood cells 143 × 10 6 /L) and no viral sequences on tNGS. Serum HBV DNA decreased by approximately 2 log 10 on the same day. The patient showed significant clinical improvement. Serial CSF tNGS before and after therapy showed synchronous clearance of three viruses with restored BBB integrity and reduced serum HBV DNA, confirming transient passive HBV entry during HSV-2 -induced barrier disruption and creating a false polymicrobial picture. These findings highlight serial tNGS as a valuable tool for validating etiological hypotheses and guiding precision therapy in complex CNS infections.