We characterized 13 cultures of the enteric bacterium causing enteric septicemia of catfish by studying their biochemical reactions, deoxyribonucleic hybridizations, and deoxyribonucleic acid guanine-plus-cytosine contents. We confirmed that this bacterium is a new species, which is most closely related to Edwardsiella tarda of the family Enterobacteriaceae. Five strains of the bacterium causing enteric septicemia of catfish were 80% or more related to the type strain, SECFDL GA 77-52 (= CDC 1976-78 = ATCC 33202), in 60°C deoxyribonucleic acid homology reactions. Species level relatedness among the 13 strains which we studied was demonstrated by the more than 80% relatedness in 75°C reactions. The bacterium causing enteric septicemia of catfish was most closely related to E. tarda (56 to 62%) in 60°C reactions. The guanine-plus-cytosine was 53 mol%, as determined by buoyant density centrifugation. We propose the name Edwardsiella ictaluri sp. nov. for the bacterium causing enteric septicemia of catfish.Enteric septicemia of catfish (ESC) is a newly described bacterial disease primarily of cultured channel catfish (Ictalurus punctatus) (9). The causative agent of this disease is a gram-negative, rod-shaped, 0.5-by 1.25-pm, oxidase-negative, peritrichous, fermentative bacterium that has been isolated 26 different times from the kidney tissues of moribund catfish. Biochemically, this organism is most similar to Edwardsiella tarda but differs from E. tarda in several key diagnostic characteristics (1).ESC was fist detected in 1976. Although not always consistent, the clinical signs of this disease in I. punctatus are typical of the acute bacterial septicemias caused by E. tarda, Aeromonas hydrophila, and Pseudomonas fluorescens ( 5 , 11).ESC presently constitutes an economic threat to catfish farmers in Alabama, Georgia, and Mississippi. The pathogenicity of the ESC bacterium to species of fish other than catfish has not been demonstrated, although it has been isolated from channel catfish (I. punctatus), white catfish (Ictalurus catus), and brown bullhead (Ictalurus nebulosus). In this paper we characterize, classify, and name the ESC bacterium.

Francisella sp. is an emergent bacterial pathogen that causes acute to chronic disease in warm and cold water cultured and wild fish species. During the past 3 years, the bacterium has been detected in tilapia, Oreochromis niloticus, cultured in Costa Rica. Infected fish presented non-specific clinical signs, such as erratic swimming, anorexia, anaemia, exophthalmia and high mortality. Upon macroscopic and microscopic examination, several internal organs (mainly spleen and kidney) were enlarged and contained white nodules. Histological examination revealed the presence of multifocal granulomatous lesions, with the presence of numerous small, pleomorphic, cocco-bacilli. The bacteria were isolated from infected tilapia on selective media and grown on several media with and without antibiotics. Specific PCR primers to the Francisella genus were used to confirm the preliminary diagnoses. In comparison with several bacterial 16S rRNA sequences, our isolate was found to share 99% identity with other Fransicella spp. isolated from fish, and more than 97% identity to the human pathogen Francisella tularensis. Koch's postulates were fulfilled after experimental intraperitoneal and gill exposure challenges.

A new bacterial disease affecting primarily pond reared fingerling channel catfish, Ictalurus punctatus, is described from 17 instances reported from June 1976 through December 1978. Terramycin medicated feed controlled the disease when early diagnosis was possible. Terramycin resistant strains were not detected. The causitive organism was confirmed as a previously unidentified species of Edwardsiella by the Center for Disease Control Atlanta Georgia. The disease is referred to as Enteric Septicemia of Catfish (ESC). Optimum growth temperature is between 25 and 30 °C; this correlated with pond water temperatures during instances of mortality on fish farms. Koch's postulates were fulfilled using channel catfish fingerlings in 40-L aquaria. Key words: Fish disease, bacterial fish pathogen, enteric bacteria, channel catfish diseases

In 2004, cultured Nile tilapia Oreochromis niloticus in several Latin America farms began to succumb to a disease similar to the piscirickettsiosis-like syndrome previously reported in tilapia in Taiwan and the United States. Mortality increased during 2005; reductions in tilapia biomass ranged from 5% to 80% in individual ponds and averaged 50% overall. All ages of fish have been involved. Clinical signs include lethargy, loss of appetite, petechia, exophthalmia, and abnormal swimming behavior. Gross lesions have included splenomegaly, renomegaly, and numerous white nodules observed in the spleen, kidney, testes, heart, ovaries, and occasionally the liver. A previously unreported black granulomatous lesion was reported in up to 30% of the fillets. Histologically, granulomatous infiltrates were observed in the kidney, spleen, liver, testes, ovary, and choroid gland, and rarely in the brain and heart. A small pleomorphic bacterium was observed in Giemsa-stained blood smears and spleen imprints. The bacterium did not grow on standard microbiological media and has not been isolated in cell culture. We obtained a near-complete 16S ribosomal DNA sequence with high similarity to Francisella spp. sequences previously identified in tilapias Oreochromis spp. (Taiwan), Atlantic cod Gadus morhua (Norway), and three-line grunts Parapristipoma trilineatum (Japan).

We report the first cases of Edwardsiella ictaluri causing epizootics in laboratory populations of Zebrafish Danio rerio. Edwardsiella ictaluri is primarily recognized as a disease of catfish species and is known to cause an economically important bacterial disease of farm-raised catfish in the USA and abroad; however, it has been isolated on occasion from 10 other genera of nonictalurid fishes. We isolated E. ictaluri from moribund Zebrafish held in quarantine at two different universities in two states and from a research facility in a third state between February 23 and December 6, 2011. Edwardsiellosis in Zebrafish can be described as a severe systemic disease characterized by tissue necrosis and the presence of large numbers of extracellular and intracellular bacteria, often within macrophages. The kidneys (pronephros and mesonephros), spleen, nares, and forebrain were the most commonly and severely affected tissues. In outbreaks, mortality was acute and numerous fish died over a 1–2 week period. Mortality continued until the majority of the population was lost, at which time the remaining fish were euthanized. In addition to these cases, four cultures of bacteria isolated from Zebrafish by another diagnostic laboratory were submitted to the Louisiana Aquatic Diagnostic Laboratory for identification and were confirmed as E. ictaluri. In total, eight cultures of E. ictaluri from Zebrafish from Louisiana, Massachusetts, Pennsylvania, and Florida were identified. The isolates were confirmed as E. ictaluri by biochemical phenotype, API 20E (bioMérieux), and amplification and sequencing of a portion of the 16S rRNA gene. Edwardsiella ictaluri isolates from Zebrafish are believed to comprise a unique group and were differentiated from catfish isolates by exhibiting weaker motility, autoaggregation in broth, a different plasmid profile (two plasmids of 4.0 and 3.5 kb), a different API 20E code (4204000), and lack of lipopolysaccharide recognition with Mab Ed9.