Contrasting outcomes of Vibrio harveyi pathogenicity in gilthead seabream, Sparus aurata and European seabass, Dicentrachus labrax

Vibrio harveyi has been reported as the dominant heterotrophic bacterial species in western Mediterranean coastal areas during warm seasons, and is recognized as an economically significant pathogen for the aquaculture industry. The present work aimed to evaluate the pathogenicity of a V. harveyi strain isolated from ascitic fluid collected from cultured gilthead seabream and then used in a challenge experiment involving the two most important fish species in Mediterranean aquaculture: gilthead seabream, Sparus aurata and European seabass, Dicentrarchus labrax. The ascitic fluid from diseased juvenile seabreams, previously vaccinated against Photobacterium damselae and Vibrio anguillarum, was extracted and bacteria cultivated for isolation and characterization. Additionally, different tissues were sampled for histological evaluation and description. Significant histopathological responses were observed in hepatic and mucosal tissues. One of the strains isolated from ascitic fluid, IRTA 17-43, was selected for a bacterial challenge. Additionally, the attenuation of virulence through sequential passage of the strain on solid media was also assessed. In parallel, a co-habitation trial was performed in order to evaluate the possible transfer of the bacteria between injected and healthy individuals. Pathogenicity trials in gilthead seabream resulted in only 25% mortality when injected with 107 CFU mL−1, whereas, for European seabass, a mortality of 95% was recorded, with clear signs of vibriosis. When passed sequentially on solid media, the strain IRTA-17-43 showed a decrease of 35% in cumulative mortality for European seabass. No apparent transmission of the pathogen occurred during the co-habitation trial for both species. In conclusion, although few external signs of V. harveyi are observed in vaccinated carriers, internal effects of the infection were clear and severe. Although no horizontal transfer of infection was observed, the risk of occurrence between carriers and immunosuppressed individuals or between different species should be considered. This further validates that the establishment of a good health management system within fish farms is of major importance in order to avoid the onset of disease outbreaks.