Isolation and Comprehensive Characterisation of <i>Edwardsiella anguillarum</i> From Diseased Tilapia ( <i>Oreochromis niloticus</i> ) Cultured in Mexico: Biochemical, Genetic and Genomic Insights and Dose‐Dependent Pathogenicity

Edwardsiella anguillarum is an emerging aquaculture pathogen responsible for economic losses across several cultured fish species. This study reports the isolation, identification, and characterisation of four isolates recovered from two infectious outbreaks in farmed Nile tilapia (Oreochromis niloticus) in Mexico. In 2021 and 2022, mortalities events occurred at two aquaculture facilities located in Querétaro and Estado de México, where affected fish exhibited clinical signs including skin darkening, ataxia, erratic swimming, haemorrhages at the operculum base, and scale loss. Internally, ascites and small nodules in the spleen and liver were observed. Bacteriological cultures from infected spleen tissue yielded four isolates, all conclusively identified as E. anguillarum through phenotypic, genetic, and phylogenomic analyses, clustering with the type strain E. anguillarum ET080813^T, differentiating them from other Edwardsiella species. Draft genome sizes are approximately 4.12 Mbp, with GC contents of about 58.8%. Comparative genomics revealed near-clonal relatedness among isolates, with Average Nucleotide Identity based on BLAST (ANIb) values > 99.98% and identified 49 to 77 putative genomic islands enriched in mobility and virulence-associated genes. Pathogenicity tests using isolate S-002-23 resulted in mean cumulative mortality of 97% and 53% at 1.3 × 10⁶ and 3.7 × 10⁴ CFU/fish, respectively. Infected tilapia displayed clinical signs and necrotising granulomas typical of edwardsiellosis. These findings expand the known geographic distribution of E. anguillarum, confirming its virulence and genomic plasticity, and suggesting a potential impact on tilapia aquaculture.