The in‐vitro antibacterial effects of organic salts,chemical disinfectants and antibiotics against pathogens of black disease in fairy shrimp of Thailand

The antibacterial effects of organic salts, chemical disinfectants and antibiotics were evaluated on cultures of Aeromonas hydrophila C03, Aeromonas sobria C26, A. sobria C29, Aeromonas caviae C24 and Acinetobacter sp. SH‐94B, the pathogens that cause black disease found in fairy shrimps (Streptocephalus sirindhornae Sanoamuang et al. (2000) and Branchinella thailandensis Sanoamuang, Saengphan & Murugan) of Thailand. The minimal inhibitory concentrations (MICs) of organic salts (sodium chloride and potassium chloride) and antibiotics (oxytetracycline dihydrate, streptomycin sulphate, kanamycin monosulphate, chloramphenicol and ampicillin) were determined using the agar‐dilution method. The effect of chemical disinfectants (sodium hypochlorite and chlorine dioxide) was evaluated by exposing bacteria to different concentrations of these chemicals for different periods of time. Interestingly, all strains were intrinsically resistant to 0.25–3% sodium chloride and potassium chloride. The effect of sodium hypochlorite was greater than that of chlorine dioxide, and 5–20 μg mL^?1 of sodium hypochlorite was sufficient to inhibit the growth of these bacteria, but the exposure time varied, depending on the bacterial species. Of the antibiotics tested, chloramphenicol and oxytetracycline dihydrate completely inhibited the selected strains. Chloramphenicol showed the highest antibacterial effect against all pathogenic species – the MIC and minimal bactericidal concentration (MBC) ranged from 0.8 to 3.1 μg mL^?1 from 3.1 to 6.25 μg mL^?1, respectively. To achieve control of black disease during cultivation of fairy shrimp, data derived from this study can be used as a basis for further toxicity tests in vivo.     

Comparative Susceptibility of Channel Catfish,Ictalurus punctatus; Blue Catfish,Ictalurus furcatus; and Channel (♀) × Blue (♂) Hybrid Catfish to Edwardsiella piscicida,Edwardsiella tarda,and Edwardsiella anguillarum

Members of the genus Edwardsiella are important pathogens of cultured and wild fish globally. Recent investigations into the phenotypic and genotypic variation of Edwardsiella tarda have led to the segregation of E. tarda into three distinct taxa: E. tarda, Edwardsiella piscicida, and Edwardsiella anguillarum. In catfish aquaculture in the southeastern USA, E. piscicida has been more commonly associated with disease than E. tarda or E. anguillarum, and recent research has demonstrated E. piscicida to be more pathogenic in channel catfish than E. tarda or E. anguillarum. Anecdotal reports from industry suggest an increased prevalence of E. piscicida associated with the culture of channel (♀) × blue (♂) hybrid catfish. This work investigated the comparative susceptibility of channel catfish, blue catfish, and their hybrid cross to molecularly confirmed isolates of E. tarda, E. piscicida, and E. anguillarum. There was significantly higher mortality in hybrid catfish compared to channel catfish following intracoelomic injection of E. piscicida. To our knowledge, E. piscicida is the first bacterial pathogen to demonstrate increased pathogenicity in hybrid catfish compared to channel catfish.     

Response of Bacterial Biofilms in Recirculating Aquaculture Systems to Various Sanitizers

ABSTRACT

Pathogenic microorganisms may be incorporated into biofilms found in aquaculture systems, causing recurring exposure to potential disease agents. Aerobic plate counts, the presence of Enterobacteriaceae, and the presence of Escherichia coli, modified to express a green fluorescent protein (GFP E. coli), was used to evaluate the effectiveness of various sanitizers in decreasing bacterial incorporation into newly generated biofilms in recirculating aquaculture systems. Disks of Buna-N rubber, polyvinyl chloride (PVC), chlorinated PVC, glass, fiberglass, and stainless steel were placed in aquariums stocked with Nile tilapia, Oreochromis niloticus. The effectiveness of water, an alkaline cleanser, sodium hypochlorite, a quaternary ammonium compound, or peracetic acid as a sanitizer was evaluated on each substrate by enumerating total plate counts, GFP E. coli, and Enterobacteriaceae. Sodium hypochlorite and peracetic acid were the most effective sanitizers, with an overall percentage reduction of GFP E. coli of approximately 2 logs₁₀. The quaternary ammonium compound was moderately effective, 1 log₁₀, against the target organisms. Water demonstrated a 2 log₁₀ reduction of the total plate count, suggesting that some mechanical cleaning was achieved. The type of material used as substrate for the biofilm had no significant (P?>?0.05) effect on the effectiveness of the sanitizers.     

Insights into the virulence‐related genes of Edwardsiella tarda isolated from turbot in Europe: genetic homogeneity and evidence for vibrioferrin production

Edwardsiella tarda has long been known as a pathogen that causes severe economic losses in aquaculture industry. Insights gained on E. tarda pathogenesis may prove useful in the development of new methods for the treatment of infections as well as preventive measures against future outbreaks. In this report, we have established the correlation between the presence of virulence genes, related with three aspects typically involved in bacterial pathogenesis (chondroitinase activity, quorum sensing and siderophore‐mediated ferric uptake systems), in the genome of E. tarda strains isolated from turbot in Europe and their phenotypic traits. A total of 8 genes were tested by PCR for their presence in 73 E. tarda isolates. High homogeneity was observed in the presence/absence pattern of all the strains. Positive results in the amplification of virulence‐related genes were correlated with the detection of chondroitinase activity in agar plates, in vivo AHL production during fish infection and determination of type of siderophore produced by E. tarda. To the best of our knowledge, this is the first study carried out with European strains on potential virulence factors. Furthermore, we demonstrated for the first time that E. tarda produces the siderophore vibrioferrin.     

Comparative genomic insights into the taxonomy of Edwardsiella tarda isolated from different hosts: Marine,freshwater and migratory fish

Edwardsiella tarda, a Gram‐negative member of the family Enterobacteriaceae, has been isolated from many animal species worldwide, especially fish species. Its broad host range indicates the diversity in taxonomy, which attracted the attention of many researchers. Here, we added genome of E. tarda strain isolated from freshwater fish to comparative genomics study for the first time. We sequenced and assembled the genome of E. tarda ASE201307 which was isolated from freshwater Asian swamp eel. ASE201307 genome contained a single circular chromosome of 3.68M with G+C 57.09% content. Comparative genomics including SNP calling, synteny block, Core/Pan genes analysis and phylogeny analysis was conducted among ASE201307 and other Edwardsiella strains isolated from different fish species. Results of SNP analysis and synteny block demonstrated the close relative of ASE201307, FL95.01 and DT which were all isolated from freshwater fish. In further analysis heat map of dispensable genes and phylogenetic tree, all E. tarda strains were divided into two groups. One was isolated from freshwater fish and the other was isolated from marine/migratory fish. Based on all studies above, we proposed the living environment of hosts as a new taxonomic character and divided E. tarda isolated from diseased fish into freshwater group and marine/migratory group.     

Occurrence of multiple antibiotic resistance and genotypic characterization in Edwardsiella tarda isolated from cage‐cultured hybrid red tilapia (Oreochromis sp.) in the Ping River,Northern Thailand

Edwardsiella tarda is a pathogen that causes edwardsiellosis in aquatic animals. The emergence of multiple antibiotic‐resistant strains makes antibiotic treatment difficult. This study aimed to investigate the antibiotic susceptibility patterns and the genotypic characterization of E. tarda isolated from cage‐cultured red tilapia in Thailand. A total of 30 isolates were identified as E. tarda using biochemical and molecular analysis. The disc diffusion method for testing antibiotic susceptibility showed all the isolates were resistant to colistin sulphate and oxolinic acid. High levels of resistance to amoxicillin, ampicillin, ceftazidime, oxytetracycline and sulphamethoxazole/trimethoprim were observed as well. The multiple antibiotic resistance index ranged from 0.25 to 0.92, indicating that these isolates had been exposed to high risk sources of contamination where antibiotics were commonly used. All the isolates carried the bla_TEM gene based on polymerase chain reaction (PCR). The tetA and sul3 genes were detected in 90% (27/30) and 26.7% (8/30) of the isolates respectively. Nine different genetic groups of isolates were obtained using enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC‐PCR). A correlation between genetic types and multiple antibiotic‐resistant patterns was found. These results highlight the potential risks of multiple antibiotic‐resistant isolates for humans and the environment.     

Comparative analysis of the phenotypic characteristics of high- and low-virulent strains of Edwardsiella tarda

Edwardsiella tarda is a causative agent of edwardsiellosis in freshwater and marine fish. Extracellular enzymic, haemolytic, hydrophobic and serum resistance activities, haemagglutination, autoagglutination and siderophores of high‐ and low‐ virulent E. tarda strains were examined. The results revealed different haemagglutination, autoagglutination, haemolytic, hydrophobic and serum resistance activities in different strains. Analysis of extracellular proteins (ECPs) and outer membrane proteins (OMPs) demonstrated several major, low molecular weight, virulent‐strain‐specific proteins, which could be virulence‐related. Based on the database search with MALDI‐TOF MS data, the closest homologies of the three protein bands Ed1, Ed2 and Ed3 were phosphotransferase enzyme family protein, nitrite reductase [NAD(P)H], large subunit and ATP‐dependent Lon protease, respectively. A comparison of pathogenicity of purified lipopolysaccharide (LPS) and lipid A from virulent and avirulent strains demonstrated that LPS was one of the virulence factors of the E. tarda isolates, and lipid A was a biologically active determinant of LPS.     

Isolation and identification of fish pathogen Edwardsiella tarda from mariculture in China

The causative agent was isolated from diseased turbots (Scophthalmus maximus) stricken by a high‐mortality outbreak of bacterial septicaemia occurring in a mariculture farm in Yantai, a northern coastal city of China. Seven pure isolates, namely EH‐15, EH‐103, EH‐107, EH‐202, EH‐203, EH‐305 and EH‐306, belonged to Edwardsiella tarda. The phenotypic features of the cultures were analysed extensively. Three of the isolates showed high 16S rDNA sequence similarities with E. tarda sequence (GenBank accession no. EF467289). However, unlike the E. tarda ATCC 15947, all the isolates, except EH‐15, contained a novel large plasmid sized about 23.7 kb. Furthermore, pathogenicity of the isolates was addressed by experimental challenges with fish models. The isolates exhibited strong virulence to swordtail fish with LD₅₀ ranging between 3.8 × 10³ and 3.8 × 10⁵ CFU g^?1, and EH‐202 displaying the lowest LD₅₀ value among them. Antibiotic susceptibilities of E. tarda isolates were assayed. Compared with E. tarda ATCC 15947, the isolates displayed strong resistance to chloramphenicol, and the probable dominant chloramphenicol resistance determinant was cat III. Depicting the main biological properties of turbot‐borne E. tarda strains in China, the study provided useful information for further unveiling their pathogenic mechanisms.     

Edwardsiellosis in farmed turbot,Scophthalmus maximus (L.), associated with an unusual variant of Edwardsiella tarda: a clinical,aetiological and histopathological study

During 2005 and 2010, a survey of edwardsiellosis on eight turbot, Scophthalmus maximus (L.), farms was conducted in China. This report presents the detailed results of the study on this disease. Diseased turbot displayed two distinct types of gross signs: black discoloration of the dorsal skin on the posterior portion of the body; and red cutaneous foci on the ventral side. Internally, the most pronounced clinical signs in all fish examined were enlarged kidneys. The causal agent of the disease was finally proved to be one species of bacterium that was identified as Edwardsiella tarda by physiological and biochemical tests, API 32E and 16S ribosomal RNA sequence analysis. It is noteworthy that unlike the commonly described E. tarda strains, the isolates in this study were non‐motile strains without flagella. A histopathological study revealed that E. tarda infection was systemic in turbot and that kidney showed the most significant pathological changes, including acute focal necrosis, an influx of macrophages and formation of granuloma. The most common histopathological characteristics of this disease are the proliferation of macrophage in various organs and formation of granuloma. In addition, this article also gave background information on the disease and presented the results of virulence tests with the E. tarda strain identified in this study.     

Genotypic and phenotypic characterization of Edwardsiella isolates from different fish species and geographical areas in Asia: Implications for vaccine development

The genus Edwardsiella is one of the major causes of fish diseases globally. Herein, we examined 37 isolates from ten different fish species from India, South Korea and Taiwan to gain insight into their phenotypic and genotypic properties, of which 30 were characterized as E. tarda with phenotypic homology estimated at 85.71% based on API‐20E biochemical tests. Genotyping using 16S rRNA put all isolates together with E. anguillarum, E. hoshinae, E. tarda, E. piscicida and E. ictaluri reference strains in a monophyletic group. In contrast, the gyrB phylogenetic tree clearly separated E. ictaluri, E. tarda and E. hoshinae reference strains from our isolates and put our isolates into two groups with group I being homologous with the E. anguillarum reference strain while group II was homologous with the E. piscicida reference strain. Hence, our findings point to E. piscicida and E. anguillarum as species infecting different fish species in Asia. Homology of the ompW protein suggested that strains with broad protective coverage could be identified as vaccine candidates. This study underscores the importance of combining genotyping with phenotyping for valid species classification. In addition, it accentuates the importance of phylogenetic comparison of bacterial antigens for identification of potential vaccine candidates.     

Intra- and interspecific phenotypic characteristics of fish-pathogenic Edwardsiella ictaluri and E. tarda

Intra‐ and interspecific characteristics of fish‐pathogenic Edwardsiella ictaluri, and E. tarda were determined by numerical analysis of gel electrophoresed protein profiles, fatty acid methyl esters (FAMEs) and immunoblotting. The 18 E. ictaluri isolates revealed a high degree of homogeneity (70% similarity or higher) in their protein profiles and 95% similarity in their FAME, while the nine E. tarda isolates revealed 30% similarity in their protein profiles and 95% similarity in their FAME. Immunoblots probed for antigenic epitopes with goat antiserum produced against E. ictaluri and E. tarda, respectively, revealed that E. ictaluri were more homogeneous compared with the E. tarda isolates. Overall, there was a considerable degree of relatedness between the two species. Our findings suggest that phenotypically E. ictaluri represents a clonal bacterial population structure compared with the less monomorphic E. tarda.     

Identification and Detection of the Pathogenic Bacteria Responsible for Swollen Abdomen Disease in Cultured Turbot,Scophthalmus maximus,and Flounder,Paralichthys olivaceus

Swollen abdomen disease (SAD) seriously threatens the aquaculture of turbots and flounders. Two dominant bacterial strains, FS1 and FS2, were isolated from the livers and kidneys of fish with diagnosed SAD. Applications of biochemical analyses, sequence analyses of 16S ribosomal RNA gene and heat shock protein 60 gene revealed two distinct pathogenic bacterial species, Edwardsiella tarda and Vibrio alginolyticus. These two hypothesized SAD‐causing pathogens were validated by challenge trials on flounder, Paralichthys olivaceus. Challenges with E. tarda and V. alginolyticus demonstrated lethal dose 50 (LD₅₀) values at 1.51 × 10⁵ colony‐forming units (CFU) and 1.05 × 10⁵ CFU, respectively. To develop a rapid SAD diagnosis method in flounders and turbots, a multiplex polymerase chain reaction (PCR) assay method was developed to simultaneously detect E. tarda and V. alginolyticus. Our multiplex PCR assay successfully detected as low as 10⁵ CFU/mL E. tarda and V. alginolyticus in flounders and turbots. No other common fish pathogens were detected with the multiplex PCR, suggesting a high specificity of this assay. The multiplex PCR assay developed in this study showed a great reliability in detecting SAD‐causing bacterial pathogens. Further optimization of this assay may contribute to the development of a novel SAD diagnosis tool in aquaculture.     

Effects of dietary inclusion of yacon,ginger and blueberry on growth,body composition and challenge test of juvenile rockfish (Sebastes schlegeli) against Edwardsiella tarda

Effects of dietary inclusion of yacon, Polymnia sonchifolia (YC), ginger, Zingiber officinale (GG), and blueberry, Vaccinium ashei (BB), on growth, body composition and challenge test of rockfish against Edwardsiella tarda compared to ethoxyquin were investigated. Three hundred and sixty fish were randomly distributed into 12 flow‐through tanks. Four experimental diets were prepared: the control diet (Con) with 0.1 g/kg ethoxyquin, and YC, GG and BB diets. Each diet was assigned to triplicate tanks of fish and hand‐fed for 8 weeks. Externally normal fish after fourth and eighth weeks of feeding trial were infected with Edwardsiella tarda for challenge test. Weight gain and specific growth rate (SGR) of fish fed the YC diet were greater than those of fish fed all other diets. Feed efficiency, protein efficiency ratio and protein retention of fish fed the YC diet were higher than those of fish fed all other diets. In the both fourth and eighth weeks of infection trials, mortality of fish fed the Con diet was higher than that of fish fed all other diets. In conclusion, dietary inclusion of YC, GG and BB increased weight gain and SGR of fish. YC, GG and BB for 4 and 8 weeks lowered mortality of fish at occurrence of E. tarda.     

Effect of temperature and salinity on virulence of Edwardsiella tarda to Japanese flounder, Paralichthys olivaceus (Temminck et Schlegel)

Experiments were designed to determine the effects of temperature and salinity on the virulence of Edwardsiella tarda to Japanese flounder, Paralichthys olivaceus. In the temperature experiment, a two‐factor design was conducted to evaluate the effects of both pathogen incubation temperature and fish cultivation temperature on pathogen virulence. E. tarda was incubated at 15, 20, 25 and 30±1°C, and the fish (mean weight: 10 g) were reared at 15, 20 and 25±1°C respectively. The fish reared at different temperatures were infected with the E. tarda incubated at different temperatures. The results of a 4‐day LD₅₀ test showed that temperature significantly affected the virulence of E. tarda (P<0.01) and the interaction between the two factors was also significant (P<0.01). For fish reared at 15°C the virulence of E. tarda was the highest at 25°C of pathogen incubation, followed by 20, 15 and 30°C. When the fish rearing temperature was raised to 20 and 25°C, the virulence of E. tarda incubated at all temperatures increased. Isolation testing demonstrated results similar to those of LD₅₀. The higher rearing temperature increased the proliferation rate of the pathogen in fish. In the salinity experiment, the incubation salinity of E. tarda was at 0, 10, 20 and 30 g L^?1, respectively, and the fish with mean weight of 50 g were cultured in natural seawater of 30 g L^?1. The results of one‐way anova in 4‐day LD₅₀ test showed that incubation salinity significantly affected virulence. Virulence was lower when the salinity of the incubation medium was at 0 and 30 g L^?1, higher at 10 and 20 g L^?1. The results of isolation test were in accordance with those of LD₅₀. At 20 g L^?1E. tarda had a faster proliferation rate than that at 10 g L^?1.     

Systemic Edwardsiella tarda infection in a Western African lungfish (Protopterus annectens) with cytologic observation of heterophil projections

This report describes a case of systemic bacterial infection caused by Edwardsiella tarda in a Western African lungfish (Protopterus annectens) exposed to poor environmental and husbandry conditions. The fish presented with a large, external ulcerative lesion and died 2 weeks after developing anorexia. Histological evaluation revealed multifocal areas of necrosis and heterophilic and histiocytic inflammation throughout multiple tissues. Gram stain identified small numbers of intra‐ and extracellular monomorphic Gram‐negative 1 to 2 μm rod‐shaped bacilli. Cytology of lung granuloma, kidney and testes imprints identified heterophilic inflammation with phagocytosis of small monomorphic bacilli and some heterophils exhibiting cytoplasmic projections indicative of heterophil extracellular traps (HETs). Initial phenotypic analysis of isolates from coelomic fluid cultures identified E. tarda. Subsequent molecular analysis of spleen, liver and intestine DNA using an E. tarda‐specific endpoint PCR assay targeting the bacterial fimbrial subunit yielded a 115 bp band. Sequencing and BLASTN search revealed the sequence was identical (76/76) to E. tarda strain FL95‐01 (GenBank acc. CP011359 ) and displayed 93% sequence identity (66/71) to Edwardsiella hoshinae strain ATCC 35051 (GenBank acc. CP011359 ). This is the first report of systemic edwardsiellosis in a lungfish with concurrent cytologically identified structures suggestive of HETs.     

An evaluation of three potential methods for preventing the spread of larval Lepeophtheirus salmonis (Krøyer, 1837)

Within the State of Maine, only a portion of the farm sites experience sea lice (Lepeophtheirus salmonis) infections on an annual basis. There has been concern about the further spread of lice by farming activities to unaffected areas despite routine disinfection of equipment with sodium hypochlorite or iodophors. We examined the effects of Povidone‐iodine, sodium hypochlorite and desiccation on egg strings of L. salmonis and the potential of these methods for preventing hatching of nauplii or development to the copepodid stage. L. salmonis egg strings were exposed to one of eight treatments: 200 ppm of sodium hypochlorite or Povidone‐iodine solutions for 1 min, 500 ppm of either disinfectant for one minute or 10 min, or desiccation for either 4 or 24 h. The egg strings were then incubated and the hatched copepods were kept for 6 days in filtered natural seawater at 32 ppt salinity at 12°C. Desiccation for 4 or 24 h were the only methods that prevented L. salmonis nauplii from hatching or developing to the infective copepodid stage. Common disinfection procedures against pathogenic bacteria and virus were not found to be effective against L. salmonis eggs.     

The plasmid profile of Edwardsiellaictaluri*

Abstract. The plasmid content of 18 isolates of Edwardsiella ictaluri found in channel catfish was analysed by agarose gel electrophoresis. Each isolate contained an identical set of plasmids. This set consisted of two prominent plasmids of 5.2 and 4.4 kilobase pairs, and three smaller and less apparent plasmids. One plasmid (the 5.2 kilobase plasmid) was radiolabelled and used to probe a blot of DNA from all isolates. All plasmids were hybridized but no chromosomal DNA was labelled. These plasmids must share some homologous regions. DNAs from other bacteria, including the related Edwardsiella tarda, were also probed and no hybridization was seen. It is suggested that a plasmid probe may be a sensitive, specific probe for detection of E. ictaluri in channel catfish     

Edwardsiella tarda isolated in integrated fish farming

The use of pig manure in fish farming reduces the environmental pollution from this material and gives it an economic application. However, the microbiological implications of such a process are not completely understood. Five hundred and forty tilapia Oreochromis niloticus L. from an integrated fish farm (using pig excrements as food) were collected and evaluated microbiologically for the presence of Edwardsiella tarda. Samples of the external surface (skin, gills and fin), intestines and muscle were analysed. E. tarda was isolated from 93 (17.2%) of the external surface samples, 77 (14.3%) of the muscle samples and 61 (11.2%) of the intestine samples. These rates were compared using the chi‐squared test, which showed that the skin samples had higher contamination than the others. Selenite–cystine (SC) broth was better than Rappaport–Vassiliadis (RV) enrichment broth for the microbiological assay. Salmonella–Shigella (SS) agar presented superior performance to that of Hektoen enteric (HE) medium. The combination SC and SS showed the best efficiency for E. tarda detection, isolating the bacterium in 107 (6.6%) out of 1620 samples studied.     

Effects of Yeast Oligosaccharide Diet Supplements on Growth and Disease Resistance in Juvenile Nile Tilapia,Oreochromis niloticus

Commercially available yeast and yeast subcomponents consisting mainly of β-glucan or oligosaccharide feed additives were added to diets of juvenile (12–18g) Nile tilapia (Oreochromis niloticus) at rates recommended by suppliers. Three experiments were conducted following a basic protocol with varied rates of supplementation, duration of feeding, and stocking densities. Experimental diets were fed twice daily to apparent satiation for a period of two or four weeks, at the end of which feed consumption and weight gain were measured. Following the experimental feeding period, serum components, including protein and immunoglobulin concentrations, as well as lysozyme and complement activities, were measured. A disease challenge was conducted with pathogenic isolates of Streptococcus iniae or Edwardsiella tarda. Weight gains were not significantly different in fish fed the supplemented diets when compared to the control diet. There were significant differences in fed intake within individual experiments; however, this effect was not consistent in all three experiments. Overall feed efficiency was not significantly affected by diet. There were no differences in serum components of fish sampled at two or four weeks. Fish fed the experimental diets did not have lower mortality or morbidity after disease challenge compared to fish fed the control diets. Specific antibody against S. iniae or E. tarda measured by ELISA did not reveal differences in the fish surviving the challenge. We conclude that the incorporation of these commercial yeast component products into the diet of juvenile Nile tilapia at these rates and for these feeding periods had no effect on growth, serum components, antibody responses, or survival following S. iniae or E. tarda infection.     

Edwardsiella tarda infection in Korean catfish,Silurus asotus,in a Korean fish farm

Mass mortality of Korean catfish, Silurus asotus, occurred in a culture farm situated in Jeollabukdo Province, Korea. The cumulative mortality rates reached up to 5% of the total fish in the farm per day. In clinical signs, the affected fish showed abdominal distension, vent protrusion, enteritis, liver congestion and abscess‐like lesions in enlarged spleen and kidney. Histopathologically, in the liver, hepatocytes lost fat and underwent atrophy or necrosis. The spleen showed necrotized splenocytes and a haemorrhagic pulp. In the kidney, glomerular destruction, degeneration of renal tubular epithelial cell and haemorrhage were observed. However, necrotic muscular lesions were not observed. A pure bacterial isolate was obtained from the liver, spleen and kidney lesions of affected fish. Experimental infection of normal catfish with the isolate resulted in the development of clinical signs similar to those seen on the farm. The isolates were identified as Edwardsiella tarda through biochemical tests (99.4%) and analysis of bacterial genes (16S rDNA) sequences (98%). The bacteria possessed two virulent genes: sodB and katB genes. These results suggest that E. tarda can act as a pathogen of farmed catfish. This is the first report showing that E. tarda caused mortality in cultured Korean catfish.