Pediatric brucellosis with negative serology: Molecular confirmation and implications for epidemiological surveillance in Mexico.

Brucellosis is an endemic zoonosis. Diagnosis is challenging in children due to nonspecific symptoms and low early sensitivity of serological tests. We describe a 3-year-old male patient with prolonged fever, thrombocytopenia, and seizures. Serological tests performed for brucellosis by the national reference laboratory, which are recommended by the Manual of Standardized Procedures for Epidemiological Surveillance in Mexico, were negative. Negative serological results may be due to the early course of infection and the technical sensitivity limitations of routine tests used for standard surveillance. Initial proteomic identification of microbiological strain by blood culture by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF) generated a low-confidence score for Chryseobacterium scophthalmum, inconsistent with the clinical presentation. Therefore, the proteomic spectrum was reanalyzed through a proteomic database, revealing high-confidence spectral matches with B. melitensis from the Koch Institute and Centers for Disease Control and Prevention (CDC), confirming the etiologic agent. In parallel, full 16S rRNA gene sequencing confirmed B. melitensis (98% coverage, 98.25% identity). The patient responded successfully to specific antimicrobial therapy. This case highlights the discrepancy between laboratory tests based on serology and molecular methods, which undermines the current definition of a confirmed brucellosis case in Mexico. It reveals the potential risk of underreporting, diagnostic delays, or absence of targeted therapy. Integrating microbiological and molecular tools into operational definitions is key to improving diagnostic sensitivity and reducing underreporting of confirmed cases. Immunological tests should remain as initial support, but it is necessary to update the national case definition manual to include molecular diagnostic innovations and strengthen epidemiological surveillance of brucellosis.