Objective To understand the genetic and evolutionary characteristics of Brucella abortus in Ningxia and provide genomic evidence to support precise source tracing and evidence-based prevention and control of local brucellosis.

Methods Fetal membrane, vaginal swab, blood of sick animals and soil samples were collected in livestock breadingfarms in Ningxia between 2023 and 2024 for bacterial isolation, species/ biovar identification, and in vitro antimicrobial susceptibility testing. Whole-genome sequencing (WGS) was performed on the isolated strains, and bioinformatics analyses were used to determine average nucleotide identity (ANI), virulence factors, antibiotics resistance genes, pan-genome composition, functional enrichment, and phylogenetic relationships.

Results  Seven Brucella abortus strains were isolated, all of which were identified as biovar 3. All the strains were sensitive to 12 commonly used clinical antibiotics. The average genome size was 3.26 Mb, with a genome completeness >99.80% and a mean GC content of 57.19%. The ANI values between the isolate and reference genomes of B. abortus from different sources were all >99.86%. The strains carried the same virulence factors, including Pmm, VirB, BtpA/BtpB, and BvrS/BvrR, as well as intrinsic resistance genes, including BEPC, BEPD, BEPE, BEPF, BEPG, and MPRF. Pan-genome analysis showed that the local strains had highly conserved genomic structure, with only a small number of accessory genes and genes of unknown function. Phylogenetic analysis showed that the strains belonged to predominant Eurasian lineage, with close genetic relation to strains from Mongolia and Russia. Within this clade, the local strains formed two smaller sub-clades, suggesting that the seven B. abortus strains identified in this study might not share a single genetic origin.

Conclusion Biovar 3 was predominant among the Brucella abortus strains identified in this study. These strains had conserved genomic structure and carried stable virulence and intrinsic resistance genes, and exhibited cross-regional genetic relation with strains from neighboring countries, highlight the need to strengthen inter-regional joint prevention and control and source-tracing of brucellosis.