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Bacillary necrosis in hatcheries of Ostrea edulis in Spain

Abstract:

Two hatcheries of flat oyster (Ostrea edulis) located in the North-West of Spain, which were affected by massive larval mortalities during summer months, were monitored in order to look for the causative organism(s) and the source(s) of the pathogens. Qualitative and quantitative analyses of the bacterial flora, conducted in different parts of the hatcheries, revealed that the total bacterial count (TVC) and the presumptive vibrios (PVC) increased during the spring, following the rise of temperature, up to the start of the outbreaks. Very high TVC levels were found in the phytoplanktonic food of broodstock and larvae; these bacterial populations were composed mainly of Flavobacterium and Pseudomonas but no vibrios were detected. Whereas the number of vibrios in the incoming water was effectively reduced by filtration and U.V. light, Vibrio strains were detected as part of the normal flora in the broodstock and larval tanks favoured by the conditions of nutrients and temperature provided in the hatcheries. The fact that vibrios were present in the gonads of hatchery-conditioned broodstock but not in wild oysters seems to indicate that these opportunistic pathogens can invade and proliferate in the oyster tissues, and subsequently infect the larvae. The agents responsible for the vibriosis outbreaks in both hatcheries resembled Vibrio tubiashii which produces lethal exotoxins for larvae and the typical symptoms of bacillary necrosis. Chloramphenicol was the most effective drug for controlling the disease. © 1987.