PATHOGENICITY OF VIBRIO-SPLENDIDUS BIOVAR-II, THE CAUSATIVE BACTERIUMOF 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 Pac ific oyster Crassostrea gigas in a hatchery in western Japan, was inve stigated. The course of experimental infection with virulent strains o f V. splendidus biovar II was very rapid in 5-day-old veliger larvae w ith disease signs apparent within 6 to 12 h after exposure of larvae a t doses of 10(4) to 10(6) CFU/ml, and mortalities up to 100% were reco rded 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. sple ndidus biovar II, a later stage (17 days post-hatching) was less susce ptible than the earlier stages of development. Diploid and triploid la rvae were almost equally susceptible to this pathogen. Extracellular p roducts and intracellular components of the strains were lethal to lar vae, but their lethality did not correlate with the virulence of live cultures. These results suggest that the ability to elaborate toxins i s not the only virulence factor in the pathogenicity of V. splendidus biovar II. The ability of this pathogen to bring about significant mor talities in oyster larvae at densities of 10(4) CFU/ml and its long su rvival in seawater make this, pathogen a potential threat to larval oy ster productions in hatchery systems.