Molecular and Histopathological Detection of Vibrio harveyi Associated with Mortality in Asian Sea Bass Cultured in Sri Lanka

Abstract

Asian sea bass (Lates calcarifer), commonly known as barramundi, is an economically important food fish inhabiting coastal and brackish waters. In Sri Lanka, demand for commercial and recreational fishing has increased due to its mild flavor and culinary versatility. However, disease outbreaks pose significant challenges to aquaculture production, particularly in cage culture systems. This study investigated the causative agent of mass mortality events in Asian sea bass farms in Trincomalee, Sri Lanka. Fish exhibiting clinical signs, including skin ulcers, fin rot, exophthalmia, and lethargy, were collected for examination. Samples from the kidney, liver, and spleen were cultured on Tryptic Soy Agar (TSA) and Thiosulfate Citrate Bile Salts Sucrose (TCBS) agar at 37 °C for 24-48 hr. Isolates produced small to medium, round, convex colonies on TSA and green colonies on TCBS. Biochemical characterization revealed Gram-negative, oxidase-positive, catalase-positive, motile rods that fermented glucose without gas production. Molecular identification using 16S rRNA polymerase chain reaction confirmed the pathogen as Vibrio harveyi with 99.44% similarity to strain CP014038.2. The sequence was deposited in GenBank (accession number OR351233). Histopathological examination revealed tubular injury, necrotic epithelial cells, and macrophage-lymphocyte infiltration in liver and kidney tissues. This study confirms that Vibrio harveyi is strongly associated with Vibriosis, causing mass mortality in Asian sea bass cage culture in Trincomalee. These findings provide essential baseline data for developing effective disease prevention and control strategies in Sri Lankan aquaculture systems.