An emerging pathogen found in farm-cultured fourfinger threadfin (Eleutheronema tetradactylum), Edwardsiella piscicida, its genetic profile, virulent genes, and pathogenicity

Abstract

The diseased fourfinger threadfin (Eleutheronema tetradactylum) exhibited clinical symptoms, such as hemorrhaging in the buccal area and skin. Gross pathological examination revealed hepatic hemorrhage, spleen enlargement, and brain congestion. Histopathological examination revealed multifocal necrosis and the presence of bacteria in various organs, including the heart, liver, spleen, and head kidney. Severe blood vessel congestion was also observed. All bacterial isolates obtained from the liver, spleen, head kidney, and brain were identified as gram-negative, motile, rod-shaped bacilli that displayed α-hemolysis. Utilizing a combination of the API 20 E system and species-specific primer-based polymerase chain reaction assays (EDtA, EDtD, and EDi primer sets), as well as sequencing of the 16S ribosomal DNA (16S rDNA) and iron-cofactor superoxide dismutase (sodB), the findings unequivocally established the identity of all isolates as E. piscicida. Examination of the E. piscicida isolates using transmission electron microscopy revealed the presence of flagella. Furthermore, genetic fingerprinting using enterobacterial repetitive intergenic consensus PCR (ERIC-PCR) demonstrated that the E. piscicida strains isolated from the fourfinger threadfin constituted a distinct genotype separate from the previously identified strains associated with Edwardsiellosis in different fish hosts. The pathogenicity of the representative E. piscicida isolates (E8–E12) was evaluated by subjecting the fourfinger threadfin to intraperitoneal injections. The gross lesions and histopathological alterations observed in experimentally infected fourfinger threadfins closely mirrored those observed in naturally infected fish. This study confirmed that E. piscicida-induced Edwardsiellosis could lead to disease outbreaks in farm-cultured fourfinger threadfin.