Pathogenicity and virulence of Aeromonas schubertii in farmed Asian seabass (Lates calcarifer) in Thailand.

Background: Aeromonas schubertii, an emerging pathogen primarily recognized in humans, shrimps, and other mammals, is increasingly implicated in aquatic animal diseases. Despite its growing prevalence, reports of its involvement in fish diseases are limited. This study investigates a natural outbreak of A. schubertii associated with high mortality in cultured Asian seabass (Lates calcarifer) in Thailand and suggest its potential pathogenicity in cultured fish species.

Results: To investigate the cause of mortality in Asian seabass at a commercial farm in central Thailand, two A. schubertii isolates-CHULA2021a and CHULA2021b-were recovered from moribund and dead juvenile fish. Naturally infected fish exhibited lethargy and slow swimming behavior, without any visible external lesions. In experimentally infected fish, clinical signs developed within 96 h post-exposure and included nodular lesion, and severe necrosis in the spleen, kidney, and liver. Histological examination revealed extensive infiltration of inflammatory cells, multifocal necrosis, marked hepatic damage, and occasional granuloma-like formations. Immunohistochemistry detected A. schubertii antigens predominantly in gill epithelium and phagocytic cells with the strongest signals observed in the head kidney. Survival analysis demonstrated significantly higher mortality in both immersion (87%) and injection (90%) groups compared to controls. Genomic analysis revealed a 4.2 Mb genome with 61% GC content. Phylogenetic analysis grouped A. schubertii with related Aeromonas species and highlighted distinct genetic features. Most identified virulence genes were related to adherence, particularly flagellar function, followed by a substantial number associated with secretion systems, predominantly type III and type VI.

Conclusions: This study reports the presence of A. schubertii in Asian seabass and its association with significant mortality during a natural outbreak. The integration of experimental infection data, histopathology, and immunohistochemistry contributes to a comprehensive understanding of its pathogenicity and supports the development of effective control measures. Additionally, genome analysis and identification of virulence genes providing molecular insights into potential virulence mechanisms. These findings enhance our understanding of the organism's pathogenic mechanisms and provide insights for future diagnostics and disease management strategies.