Prevalence and detection of Brucella infection in people with fever of unknown origin in Inner Mongolia, China

Background Brucellosis is a highly endemic zoonosis in Inner Mongolia, yet its contribution to undifferentiated febrile illnesses often remains underestimated due to non-specific clinical presentations and diagnostic limitations. This study aimed to determine the prevalence and diagnostic gaps of Brucella infection among patients presenting with fever of unknown origin (FUO) in Inner Mongolia. Methods A cross-sectional surveillance study was conducted in 2021. A total of 335 febrile patients were enrolled, with 266 meeting the criteria for FUO. Venous blood samples were collected and tested using quantitative real-time PCR (qPCR) followed by digital PCR (dPCR) for absolute quantification of bacterial loads. A subset of serum samples was tested with ELISA for total antibody. Results Of the 335 febrile patients, 228 (68.0%) were farmers or herders and 247 (73.7%) had livestock contact. Brucella was detected in 52 (15.5%), with 40 (15.0%) in 266 FUO patients. A substantial portion of Brucella detection was identified in people (22/107, 18.7%) with professions other than farmer or herder. The history of direct contact with livestock had no association with Brucella detection. The most common clinical symptom among positive patients was muscle, joint, or musculoskeletal pain (84.6%, 44/52), but it lacked the specificity for Brucella detection. dPCR determined the bacterial loads ranging from 1.7 × 10 2 to 1.3 × 10 5 copies/mL of blood, demonstrating a declining trend along with the interval between fever onset and blood draw in FUO patients. ELISA identified three additional cases of Brucella infection in the subset of 236 FUO patients. Conclusion This surveillance confirms that Brucella is a significant causative agent of FUO in the surveillance area, accounting for 15.0% of cases. The sole reliance on serological tests in clinical settings may lead to missed diagnoses. The integration of sensitive qPCR methods is crucial for improving diagnostic accuracy and timely case detection. These findings underscore the need for enhanced clinical awareness and the implementation of routine Brucella screening in FUO algorithms across healthcare facilities in Inner Mongolia to reduce diagnostic delays and prevent chronic complications.