Pathogenicity of Vibrio splendidus biovar II, the causative bacterium of bacillary necrosis of Japanese oyster larvae

The potential pathogenicity of strains of Vibrio splendidus biovar II, which were isolated from bacillary necrosis of triploid larvae of Pacific oyster Crassostrea gigas in a hatchery in western Japan, was investigated. The course of experimental infection with virulent strains of V. splendidus biovar II was very rapid in 5-day-old veliger larvae with disease signs apparent within 6 to 12 h after exposure of larvae at doses of 10^4 to 10^6 CFU/ml, and mortalities up to 100% were recorded in 24 h at 10^5 and 10^6 CFU/ml. Although all the tested stages of larvae were experimentally infected by virulent strains of V. splendidus biovar II, a later stage (17 days post-hatching) was less susceptible than the earlier stages of development. Diploid and triploid larvae were almost equally susceptible to this pathogen. Extracellular products and intracellular components of the strains were lethal to larvae, but their lethality did not correlate with the virulence of live cultures. These results suggest that the ability to elaborate toxins is not the only virulence factor in the pathogenicity of V. splendidus biovar II. The ability of this pathogen to bring about significant mortalities in oyster larvae at densities of 10^4 CFU/ml and its long survival in seawater make this pathogen a potential threat to larval oyster productions in hatchery systems