Aetiological Study On Bacterial Diseases Of Cultured Sea Cucumber Apostichopus Japonicus In China

Mass mortalities of cultured sea cucumber Apostichopus japonicus had occurred frequently in the fishery farms of Shangdong Peninsula in recent years. According to our survey from 2002 to 2004, the mortalities were triggered by several diseases, which caused great economic losses. In order to find out the causative pathogens of these diseases and provide effective counter-measures, we firstly conducted epidemiological and etiological studies on the diseases of cultured sea cucumber along the coastal area of Shandong Province. As a result, several important bacterial diseases and a fungal disease had been discovered during larval and adult stages. In the present paper, a primary work was done to ascertain the etiology of these epidemic diseases namely 'syndrome of rotting edges', 'syndrome of off-plate' and 'syndrome of skin erosion'.The diseased specimens including lavae, juvenile and adults were mainly collected from the affected farms of A. japonicus in Shandong Province. The study on the causative pathogens involved the following steps: firstly, smears of the foci and susceptible organs were observed under a microscope to review the histopathological damage and microbiological situation; Secondly, isolation for the bacteria or fungi were carried out by using solid medium such as TSA, TCBS and HA, respectively; Thirdly, upon the isolation, the dominant strains were purified and artificial infection experiments were conducted to confirm their pathogenic ability to the diseases. Finally, morphological study and identification for the causative pathogens were subsequently performed, which including negative-staining, Gram staining, physiological and biochemical testing, as well as analysis of gene homology of 16S r RNA to the bacteria.The identification for the microbes revealed that the causative pathogens to the syndrome of rotting edges were Vibrio lentus, while for the syndrome of off-plate were Vibiro sp., which may be a new species near to Vibrio rumoence (homological comparability about 97.9%). As for the syndrome of skin erosion, the causative agent was Pseudoalteromonas nigrifaciens during juvinile stages, but V. splendidus for the adults. Based on the recent studies, we conclude that the Vibriosis are the main diseases in farmed sea cucumber A. japonicus. In addition, fungal diseases were also discoveredamong the cases of affected juvenile and adults, a primary study on the morphology to the fungi were also provided.In order to prevent and control these diseases, the possible sources of the pathogens were investigated. The results showed that Sargassum thunbergiland could carry high quantatity of bacteria (upto 107 CFU/g), and the bacterial species were always same to the causative pathogens corresponding to the disease, as such S. thunbergiland was assumed to be the source of the causative pathogens. Meanwhile, drug sensitive tests to the pathogens had also been conducted; some effective drugs were selected and consequently applied to the farms where the diseases occurred.