Rapid Metagenomic Detection of Brucella abortus During a Two-Case Bovine Abortion Investigation in Inner Mongolia,...

Abortion in cattle entails substantial economic loss, and rapid identification of abortigenic pathogens is critical for timely on-farm response and reduction in human exposure risk. In 2024, two Holstein cows from a small farm in Inner Mongolia aborted in close succession without an obvious cause. Vulvar swabs from both cows, one afterbirth sample, and whole blood from one aborted fetus were collected. Shotgun metagenomic sequencing was performed, followed by host-read removal, taxonomic profiling with Kraken2, de novo assembly of Brucella-aligned reads, and whole-genome comparison. Serological tests, Gram-stained smears, and Brucella genus- and species-specific qPCR assays were used as orthogonal verification. Putative resistance and virulence determinants were screened against CARD and VFDB. Brucella reads were detected in all samples, with the highest relative abundance in the 138-afterbirth (96%). qPCR assays detected Brucella DNA and B. abortus-specific signals in all four samples. A draft Brucella genome was assembled from the 138-afterbirth sample and was phylogenetically placed within B. abortus, showing relatedness to previously circulating Chinese lineages. Cows 138 and 198 were RBT-positive with SAT titres of 1:100 (++). No acquired Brucella resistance genes were identified in CARD. Within 72 h of sample receipt, B. abortus was reported to the farm and local authorities and emergency biosecurity measures were implemented. This field investigation shows that metagenomic sequencing, when combined with conventional serology, microscopy, and targeted qPCR, can support rapid etiological investigation when culture is delayed, hazardous, or biosafety level 3 facilities are unavailable.