Recovery of Edwardsiella tarda and Edwardsiella piscicida in freshwater fishes and assessment of their pathogenicity in Nile tilapia (Oreochromis niloticus)

Abstract

Edwardsiellosis, a bacterial disease affecting global aquaculture, is caused by several species in the genus Edwardsiella. This study examined cases of Edwardsiella infections in multiple fish hosts from intensive aquaculture farms in Thailand during 2021–2022. Twenty putative Edwardsiella isolates were identified, comprising eighteen E. tarda isolates from Nile tilapia Oreochromis niloticus (11 isolates), red tilapia Oreochromis sp. (6 isolates), and snakehead fish Channa striata (1 isolate). In addition, two E. piscicida isolates were identified from red tilapia and barramundi Lates calcarifer. Antimicrobial susceptibility testing using disc diffusion assays with eight classes of antibiotics revealed multiple antibiotic resistance (MAR) indices ranging from 0.22 to 0.71. Representative E. tarda and E. piscicida isolates were selected to assess their pathogenicity in juvenile Nile tilapia. The cumulative mortality of Nile tilapia intraperitoneally injected with E. tarda (10⁷ CFU/fish) was 60%, while injection of E. piscicida at the same dose resulted in 100% mortality within seven days post-challenge. Grossly, moribund fish displayed diffuse pinpoint white spots on the pale liver and spleen. Significant histopathological alterations were noted in the liver and spleen, characterized by tissue degeneration, cellular disassociation, necrosis, intracellular bacterial colonization, and granuloma formation. No appreciable difference in the clinical manifestations was observed in challenged fish among the two Edwardsiella species. In conclusion, the present study reports evidence of pathogenic E. tarda and E. piscicida infections in freshwater fishes, underscoring the importance of implementing effective disease prevention and control measures.