It is frequently reported that cultivated holothuroids can suffer from a disease affecting their integument. We report here on a disease of juvenile Holothuria scabra, the widely marketed edible sea cucumber, reared in the Aqua-Lab hatchery of Toliara, Madagascar. This disease. which has been called skin ulceration disease, is very contagious and results from a severe bacterial infection that causes death within 3 days. The first sign of the infection is a white spot that appears on the integument of individuals, close to the cloacal aperture. The spot extends quickly onto the whole integument leading to the death of individuals. Microscopic (histology, scanning and transmission electron microscopies) and biomolecular (denaturing gradient gel electrophoresis (DGGE) and sequencing) techniques have been used to describe lesions to investigate the infecting microbial communities. The lesions consist in a zone where the epidermis is totally destroyed and where collagen fibres and ossicles are exposed to the external medium. This zone is surrounded by a border line where degrading epidermis is mixed with the connective tissue. Lesions include three bacterial morphotypes: rod-Shaped bacteria. rough ovoid bacteria, and smooth ovoid bacteria. The last morphotype is the only one found on the ossicles and is assumed to be responsible for their degradation. Three species of bacteria have been put in evidence in the lesions thanks to biomolecular analyses: Vibrio sp., Bacteroides sp., and an alpha-Proteobacterium. Infection assays of healthy holothuroids have been performed from lesions and from bacterial cultures but the causative agent has not been identified. It is suggested that combined events or agents, including bacteria, are required to induce the disease. |
