Studies On The Pathogenic Mechanism Of Hepatopancreas Albinism In Chinese Mitten Crab (Eriocheir Sinensis) And Its Control Technology

In recent years, hepatopancreas albinism in Chinese mitten crab (Eriocheir sinensis) has occurred as a kind of severe fulminant disease that leads to substantial economic loss to local culturists in Suzhou regions. In order to ascertain the cause of hepatopancreas albinism symptoms and its pathogenic mechanism, explore its control strategies, detailed investigations on epidemiological survey, pathogen detection, histological examination, nutritional feeding models and integrated control strategies were undertaken in this study. The main results were presented as follows:1. The pathogenic factors of albinism in hepatopancreas of Chinese mitten crabIn recent years, a kind of disease in hepatopancreas of Chinese mitten crab, called "BAIGAOZHENG Symptom" by local culturists, occurred frequently in the intensive fanning areas of east Taihu Lake and other regions and led to huge economic loss to local culturists.In order to probe the possible pathogens for albinism in hepatopancreas, isolation procedures were performed, and three bacterial strains from Aeromonas, Pseudomonas, Vibrion, respectively, were isolated from the hepatopancreas and gills. The challenge experiments with these bacterial strains reliably excluded possibility of the involvement of bacteria in the hepatopancreas albinism. Electronic microscope observation on diseased tissues also did not find any virus associated with it.In the form of visiting individually with culturists and of collecting questionnaires, a systematic survey was conducted to investigate the occurrence of the disease. The items surveyed included the sources of 2-year old mitten crab larvae, feed casting patterns, coverage of aquatic plants, the amount of feed input, bed mud status, recapture rates, and commercial crab production. The results revealed that there was a marked gender difference in the incidence rate of albinism in hepatopancreas, and male crabs were significantly more prone to develop hepatopancreas albinism than the females. By examining the diseased male crabs for any abnormalities, the diagnostic symptoms or signs included that the smooth and intact body surface, yellowish carapace, dark purple hepatic region inside the breastplate, pale yellow or white hepatopancreas. Moreover, as the disease progressed, the hydrothorax in the thoracic cavity, loosen and shrunk muscles, rotten hepatopancreas, and blackened branchial filaments were commonly observed.These results, together with the absence of parasites in the diseased crabs, implied that "BAIGAOZHENG Symptom" disease was not caused by pathogenic microorganisms. Investigation results indicated that improper feed-casting brought malnutrition to the Chinese mitten crab, and made water environment worse. Therefore, the occurrence of "BAIGAOZHENG Symptom" disease, which could be regarded as a nutritional disease, has close relation to feeding amount of feeds, the ratio of animal and plant based feeds, and the quality of water environments. Based on the main symptoms and pathogenic factors, it is suggested that this disease be nominated scientifically as hepatopancreas albinism.2. The histopathological changes in tissues of Chinese mitten crab with hepatopancreas albinismHepatopancreas albinism is one of the most serious diseases of Chinese mitten crab. Observations on the lesions in tissues and cells of Chinese mitten crab with hepatopancreas albinism were carried out by using the routine paraffin section preparation method and by a high-resolution transmission electron microscope technique.The structural changes in varying degrees were observed in the hepatopancreas, gills, and muscles of diseased crabs. The pathological characteristics mainly appeared as follows: firstly, liver cells showed swelling, degeneration, and even necrosis, within which the fatty granules greatly decreased, the swollen mitochondria lost their cristae and formed vacuoles, and the endoplasmic reticulum and lysosome degraded into vesicles. Secondly, the gills appeared to be expanded, and the microvilli arranged sparsely and abnormally in gill epithelial cells. Some mitochondria cristae were disappeared, and regressed into membranous structures. Also, bacteria particles could be found in the necrotic gill epithelial cells and liver cells. Thirdly, in the muscle cells, nuclei showed pyknosis, muscular fibers appeared to be relaxed, swollen or split, and sarcoplasmic reticulum disaggregated into vesicles. It is suggested that severe structural changes in hepatopancreas, gills, and muscles impaired the physiological functions of crabs, thereby crabs with hepatopancreas albinism did not resist to long-distance transportation.In addition, the relaxation of muscles and the dramatic reduction in fat content of hepatopancreas caused poor quality of commercial crabs. Ultrastructural observations showed that no any virus and other biological agents were found in tissues or cells of diseased crabs, which indicated that hepatopancreas albinism was caused by non-living agents. 3. Optimization of models for feeding Chinese mitten crabThe orthogonal design method of table L₉(3⁴) was adopted to construct nine different feeding models for Chinese mitten crabs based on four factors, such as the coverage of aquatic plants, the input of live snail, the ratio of animal based feed to total feed and different feed casting patterns, and their three levels. Growth, quality, incidence of hepatopancreas albinism, non-specific immunity, and water quality index were used as evaluation criteria to screen the optimal feeding models during the course of raising the Chinese mitten crab. 540 two-year old Chinese mitten crabs with initial body weight of 9.67±0.47 g were fed in 27 separate pens (6 m×5 m×2.5 m) in the raising areas of east Taihu Lake for 240 days following these feeding models with three replicates. The results showed that different feeding models exerted significant effects on its growth and hepatopancreas albinism incidence. Among these feeding models, model IV, produced the best raising results as shown by the best quality, lowest incidence of hepatopancreas albinism, highest non-specific immunity, and best water quality. the effects of the four factors on growth, quality, albinism incidence, non-specific immunity, and water quality were listed in the order of importance as: the coverage of aquatic plants> the ratio of animal based feed to total feed>the feed casting patterns> the input of live snail.A statistical regression analysis was conducted based on the four factors, together with the above parameters. The results confirmed that the coverage of aquatic plants, the ratio of animal based feed to total feed, and the feed casting patterns had close relations to the albinism incidence and growth of Chinese mitten crabs. It is suggested that the feeding model suitable for the raising of Chinese mitten crabs be characterized as follows: the coverage of aquatic plants was 73.95%, the animal based feed accounted for 56.15%-62.22% of total, the feed was given during whole growth and developmental stages and in the amount of 7.5%-5.0% of the crab weight, the amount of live snail input was 5578.05 kg·(hm²)^-1.