We compared Edwardsiella ictaluri from striped catfish in Vietnam with US channel catfish isolates. Biochemical analyses and sequencing of the 16S rRNA gene confirmed that the Vietnamese isolates were E. ictaluri. Comparison using rep-PCR fingerprinting demonstrated no significant differences between the isolates, but plasmid analysis indicated that the Vietnamese isolates grouped into 4 plasmid profiles, each different from the typical pEI1 and pEI2 plasmid profile found in the US isolates. Sequencing plasmids representative of the 4 profiles indicated that all contained derivatives of the E. ictaluri plasmid pEI1, whereas only 1 contained a plasmid derivative of the E. ictaluri plasmid pEI2. The pEI2 encoded type III secretion effector, EseI, and its chaperone, EscD, were found to be present on the chromosome in isolates lacking a pEI2 derivative. In addition, 1 isolate carried a 5023 bp plasmid that does not have homology to either pEI1 or pEI2. Furthermore, Vietnamese isolates were PCR positive for the type III and type VI secretion system genes esrC and evpC, respectively, and the urease enzyme, but were PCR-negative for the putative type IV secretion system gene virD4. A monoclonal antibody against the lipopolysaccharide of E. ictaluri ATCC 33202 did not react with the Asian isolates or with the more recent US isolates. Antibiotic resistance patterns were variable and did not correlate to the presence of any particular plasmid profile. Finally, the Vietnamese isolates were avirulent and had a significantly reduced capacity for intracellular replication within head-kidney-derived channel catfish macrophages.

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In the present study, samples of pond water, sediment and farmed species were collected at 12 fish and shrimp farms in Malaysia, Thailand and Vietnam to determine the biodiversity and environmental distribution of chloramphenicol-resistant (CmR) mesophilic heterotrophs in Southeast Asian aquaculture sites. Following isolation on Iso-Sensitest agar supplemented with 35mug ml(-1) Cm and dereplication using (GTG)(5)-PCR fingerprinting, 557 genotypically unique CmR strains were subjected to polyphasic identification. The 557 mesophilic heterotrophic CmR isolates represented 18 different genera largely dominated by the genera Escherichia (40.2%), Pseudomonas (11.7%), Acinetobacter (11.1%), Klebsiella (7.5%) and Bacillus (5.9%). A total of 439 CmR isolates were further assigned to 31 described species or species groups, mainly including organisms that have been associated with various human opportunistic infections such as Escherichia coli (n=219), Pseudomonas putida (n=47), Klebsiella pneumoniae (n=38) and Acinetobacter baumannii (n=23). Strains of Escherichia, and most notably, of E. coli, were the only common group of CmR heterotrophs irrespective of country, sample type or farm type. Together with other predominant but less widespread groups such as acinetobacters and pseudomonads, the results of this biodiversity study suggest that E. coli can be regarded as a potential indicator of Cm resistance in Southeast Asian aquaculture environments.

Outbreaks of white spot syndrome virus (WSSV) in shrimp culture and the relationship between the virus and virulence are not well understood. Here, we provide evidence showing that WSSV mixed-genotype infections correlate with lower outbreak incidence and that disease outbreaks correlate with single-genotype infections. We tested 573 shrimp samples from 81 shrimp ponds in the Mekong delta with outbreak or non-outbreak status. The variable number tandem repeat (VNTR) loci of WSSV were used as molecular markers for the characterization of single-and mixed-genotype infections. The overall prevalence of mixed-genotype WSSV infections was 25.7 %. Non-outbreak ponds had a significantly higher frequency of mixed-genotype infections than outbreak ponds for all VNTR loci, both at the individual shrimp as well as at the pond level. The genetic composition of WSSV populations appears to correlate with the health status of shrimp culture in ponds. The causal relationship between genotypic diversity and disease outbreaks can now be experimentally approached.

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