Epidemiological And Histopathological Studies Of Main Diseases Of Cultured Sea Cucumber Apostichopus Japonicus And Its Parasitic Etiology

Sea cucumber farming has been developed rapidly along the northern coast of China at the begin(?)ing of 21^th century. The total culture area of sea cucumber has reached 33, 000 ha and the production valued RMB 10 billion, which has become an very important industry. However, the rapid expansion and intensification of sea cucumber farming has led to the occurrence of various diseases, causing serious economic losses and becoming one of the limiting factors in the sustainable development of this industry.The epidemiological study was carried out on the diseases of cultured sea cucumber Apostichopus japonicus in China for the first time, in which several non-reported diseases have been discovered. Syndrome of rotting edges, ulceration of stomach and off-plate syndrome of young juveniles were considered as the main diseases in nursery stages, whereas the skin ulceration syndrome was the most important disease during the out-door cultivation. In addition, several kinds of pathogens were found, including bacteria, fungi and parasites.Syndrome of rotting edges occurred during the auricularia stages from June to July. The mortality could reach up to 90%. The clinical signs of infected larvae were recognized as the presence of darkening edges in the early infection. Then, tissue proliferation, black discoloration and ulceration were displayed, and finally the whole body was disorganized and disappeared. The histopathological analysis revealed that the pathogen mainly infected the epithelium of auricularia larvae, resulting in the infiltration of inflammatory cells and the proliferation of the tissues in the lesions.Symptom of stomach ulceration often occurred at the post auricularia larvae stage in June and July, and the mortality could reach 90%. Under a microscope, the gross signs included the proliferation of stomach wall, stomach atrophy and ulceration, as well as larvae dying. The affected larvae appeared weak, anorexic, and having lower metamorphosis rate. Histopathological analysis revealed that the causative pathogenmainly causes the damage and ulceration on the stomach wall.Off-plate syndrome often occurred during the pentactula and the young juvenile stages. It was the most severe disease in the post hatchery stages. The transmission of this disease is rapid and could result in high mortality, even up to 100% in several days. The typical signs of these infected larvae included the disability to attach on the substrates. The wet mount examination demonstrated that the body changed to be white and opaque. In addition, large amount of bacteria and organic dirtiness were found on the skin where brown stains were often seen. In such cases, the spicules can be found on the bottom of the infected tanks as they drop from the dissolved carcasses. Histopathological study showed that the susceptible tissue were the epithelium of spicules, tube-foot, tentacls and the epithelium of abdomen.Skin ulceration syndrome was the most common disease during nursery and out-door culture stages. The infections could affect both young juveniles and adults. Its prevalence peaked during low temperature from December to April. In the early infection, the diseased sea cucumbers appeared swollen mouth, stop feeding, evisceration and weak activity. Finally, the skin was eroded, following with deep ulceration, thus turned to be whitish blue. Histopathological study illustrated that the intestine, tentacles, tube-foot and the skin were susceptible to the infection.In addition, the present study carried out preliminary work on the fungal and parasitic diseases. Two parasites were found during this study. One was identified as Boveia cf. labialis, another was named Apostichopus japonicus platyhelminth. The morphological studies of these parasites were carried out by means of smear microscopy, staining with the silver carbonate method and Chatton-Lwoff silver impregnation techniques as well as scanning electron microscopy. Infusorian Boveia cf. labialis mostly distributed along the inner wall of the respiratory trees. Pathological sections revealed large number of infusorians attached to the inner wall of the respiratory trees, while none found in other tissues and organs. The epithelial tissues were damaged, shown as cell destruction, epithelial cells scattered and finally resulting in an iiregular epithelial line. The parasite platyhelminth was observed in the skin and subcutis and even deep tissues of sea cucumber, thus we named it as A. yapomcws-platyhelminth temporarily. The fungal and parasitic diseases of sea cucumber were firstly reported in China in this present paper, and they were verified to be pathogenic etiology.The paper preliminarily established the characters of main diseases of cultured sea cucumber through epidemiological and histopathological studies, which would provide the essential knowledge in disease diagnoses, prevention and treatment, in order to conduct healthy practice during the management.