VIBRIO-SPLENDIDUS BIOVAR-II AS THE CAUSATIVE AGENT OF BACILLARY NECROSIS OF JAPANESE OYSTER CRASSOSTREA-GIGAS LARVAE

Recurrent outbreaks of a disease leading to mass mortalities in an oys ter (Crassostrea gigas) hatchery located in western Japan were investi gated. The disease occurred regularly in 2- to 8-d-old larvae and has been experimentally controlled in the hatchery by treating the larval rearing water with streptomycin, without ascertaining the etiological agent. The signs of the disease and the course of infection resembled bacillary necrosis reported in oysters and other bivalve molluscs in t he USA and Europe. Quantitative and qualitative examinations of the ba cterial flora of hatchery samples including source water, broodstock, larval feed and larvae revealed a very high total bacterial load and p resumptive vibrios in diseased larvae. Further, the bacterial profile revealed that Vibrio spp. constituted approximately 60 to 95% of the b acteria isolated from infected larvae and most isolates were identifie d as V. splendidus biovar II and V. harveyi, suggesting their possible role in the disease. However, experimental challenges proved the path ogenicity of V.splendidus II. Several isolates of V. splendidus II fro m infected larvae were highly pathogenic, producing 100 % mortality at levels of 10(5) cfu ml(-1) in 24 h, while isolates from other sources demonstrated a low degree of virulence. Detection of V. splendidus II from broodstock, especially in the gonad of a few breeders, suggests the probability that broodstock could be the source and route of trans mission of this pathogen.