ST251 and are major emerging pathogens of striped catfish in Vietnam.

INTRODUCTION

Aeromonads are ubiquitous in aquatic environments and several species are opportunistic pathogens of fish. Disease losses caused by motile species, particularly , can be challenging in intensive aquaculture, such as at striped catfish () farms in Vietnam. Outbreaks require antibiotic treatments, but their application is undesirable due to risks posed by resistance. Vaccines are an attractive prophylactic and they must protect against the prevalent strains responsible for ongoing outbreaks.

METHODS

This present study aimed to characterize strains associated with mortalities in striped catfish culture in the Mekong Delta by a polyphasic genotyping approach, with a view to developing more effective vaccines.

RESULTS

During 2013-2019, 345 presumptive spp. isolates were collected at farms in eight provinces. Repetitive element sequence-based PCR, multi-locus sequence typing and whole-genome sequencing revealed most of the suspected 202 isolates to belong to ST656 ( = 151), which corresponds to the closely-related species , with a lesser proportion belonging to ST251 ( = 51), a hypervirulent lineage (vAh) of already causing concern in global aquaculture. The ST656 and vAh ST251 isolates from outbreaks possessed unique gene sets compared to published and vAh ST251 genomes, including antibiotic-resistance genes. The sharing of resistance determinants to sulphonamides () and trimethoprim () suggests similar selection pressures acting on ST656 and vAh ST251 lineages. The earliest isolate (a vAh ST251 from 2013) lacked most resistance genes, suggesting relatively recent acquisition and selection, and this underscores the need to reduce antibiotics use where possible to prolong their effectiveness. A novel PCR assay was designed and validated to distinguish and vAh ST251 strains.

DISCUSSION

This present study highlights for the first time , a zoonotic species that can cause fatal human infection, to be an emerging pathogen in aquaculture in Vietnam, with widespread distribution in recent outbreaks of motile septicaemia in striped catfish. It also confirms vAh ST251 to have been present in the Mekong Delta since at least 2013. Appropriate isolates of and vAh should be included in vaccines to prevent outbreaks and reduce the threat posed by antibiotic resistance.