Comprehensive Laboratory Analysis of a Scrub Typhus and H1N1 Influenza Co-Infection: A Case Report from Hainan, China

Co-infection of <i>Orientia tsutsugamushi</i> and influenza A virus complicates diagnosis and treatment in endemic regions because of overlapping clinical features and potential synergistic inflammation. We describe a 68-year-old woman from Hainan, China, who presented with five days of high fever (39.2 °C), nonproductive cough, eschar formation, lymphadenopathy, cytopenias, elevated liver enzymes, and raised inflammatory markers. On the day of admission, influenza A was confirmed by rapid antigen test and <i>Orientia tsutsugamushi</i> IgM/IgG was detected via colloidal-gold immunochromatography, prompting concurrent oseltamivir and doxycycline therapy. Quantitative PCR on day 2 measured an <i>Orientia tsutsugamushi</i> load of 2.85 × 10⁴ copies/mL (Cq 28.86), and targeted next-generation sequencing on day 3 revealed a high H1N1pdm09 viral burden (>1 × 10⁶ copies/mL) with low-level human herpesvirus 1 co-detection. Nested PCR and Sanger sequencing assigned <i>Orientia tsutsugamushi</i> to the Karp_A lineage and influenza A to clade 6B.1A.5a.2a. The patient defervesced by hospital day 2, laboratory indices normalized by day 3, and radiographic abnormalities resolved by day 6. This first documented <i>Orientia tsutsugamushi</i>-influenza A co-infection in China highlights the value of integrating rapid serology, qPCR quantification, nested PCR genotyping, and tNGS for early, precise dual-pathogen identification. Systematic multi-pathogen screening during overlapping transmission seasons is recommended to guide timely combination therapy and enhance epidemiological surveillance.