VERA IRENE ERICKSON: Genomic characteristics of Aeromonas spp. and Edwardsiella ictaluri in striped catfish in Vietnam.

PhD Student: Vera Irene Erickshon
e-mail: vera.erickson@sund.ku.dk, veie@ssi.dk 

THE PROJECT
Vietnam is the world's leading producer and exporter of striped catfish (Pangasianodon hypophthalmus), with aquaculture concentrated in the Mekong Delta. This region, characterised by a high density of farms, frequently experiences disease outbreaks during production cycles, notably bacillary necrosis of Pangasius (BNP) and motile Aeromonas septicaemia (MAS). The diseases impose a significant economic burden for farmers. BNP is caused by the Gram-negative aquatic pathogen Edwardsiella ictaluri, while MAS is attributed to various Aeromonas species, with prevalence varying by location. Samples from diseased striped catfish fingerlings were collected from different Mekong Delta provinces between 2017 and 2021. Thirty representative isolates of each bacterial genus were selected for whole genome sequencing (WGS) based on the year and location of sampling.

THE PURPOSE 
This thesis aims to provide insights into genomic epidemiology, antibiotic resistance patterns, and the virulence of bacteria that cause the disease to help Vietnamese veterinary authorities offer more targeted treatment guidelines for the two diseases.

THE RESULTS  
In Vietnam, Aeromonas dhakensis ST-656 was the primary pathogen causing MAS, responsible for about 80% of outbreaks, while Aeromonas hydrophila ST-251 caused 16%. The impact of A. hydrophila in aquaculture may have been overestimated due to inadequate species identification methods, which can fail to distinguish A. hydrphila and A. dhakensis. Virulence gene analysis showed that A. dhakensis generally harbors more virulence genes than A. hydrophila. Both species share key virulence factors, including the heat-stable cytotonic enterotoxin and the T6SS secretion system, suggesting their critical role in pathogenicity.

All E. ictaluri strains causing BNP belonged to ST-26, pointing to a common sequence type in the Mekong Delta. However, pangenome and virulence gene analyses revealed variations within ST-26, identifying two main groups: one with T3SS-encoding genes and one without. Three distinct gene profiles were also found. Phylogenetic analysis showed that E. ictaluri strains in Vietnam are genetically distinct from those in other countries, suggesting local outbreaks stem from inadequate biosecurity, not new clades. Transmission is likely driven by shared resources like water, equipment, and fingerlings. Managing BNP outbreaks requires comprehensive risk analysis and source tracking.

THE FUTURE 
Overall, our findings highlight the need for reliable diagnostics and updated diagnostic protocols. The study emphasises the necessity for epidemiological cut-off values, the establishment of clinical antibiotic resistance breakpoints for florfenicol and oxytetracycline, and publicly available treatment guidelines for BNP and MAS in striped catfish.