The Aetiology And Histopathologic Study Of "Rotten Skin" Disease On Chinese Giant Salamander (Andrias Davidianus)

In September 2009, a "rotten skin" disease with main symptoms of skin ulcers occurred in a artificially farms in Sichuan. The main pathologic changes showed a lot of depigmented, white, irregular, variably sized areas on the body surface, ulcers on the four limbs and hyperaemia, hemorrhage on internal organs. The morbidity achieved 80% and the mortality achieved as high as 95%. Heavy economic losses had been caused. Therefore, in order to make clear the aetiological agent and research the pathogenesis, etiological, pathological, preventive and curative study was conducted.The examination of viral and parasitic pathogens was conducted and then the infection of these two types was ruled out. Pathogenic bacteria from pathologic tissues of diseased and dying chinese giant salamander were also examined and isolated. Each one predominant bacteria strain was isolated from each chinese giant salamander's festering tissues. Each one bacteria strain was also isolated from each chinese giant salamander's diseased liver and kidney. Identifications of the 60 isolated strains including the morphological, physiological and biochemical characteristics were conducted. It showed all the strains were the same bacteria. One strain (YA090911) was selected to conduct 16S rDNA gene detection. Then, it's sequence was submitted to GenBank(No. GU735964) and phylogenetic tree construction was conducted. Based on the above research results, the selected strain was identified as Aeromonas veronii. It had strong pathogenicity to giant salamander and channel catfish, as well as mice. Respectively, the LD50 was 5.4×106 CFU/n,3.9×105CFU/n and 2.0×106 CFU/n. The antibiotic susceptibility test revealed that the strain was highly sensitive to 6 agents, sensitive to 2 agents, and resistant to 19 agents. Highly sensitive drugs were selected to cure the giant salamander, the disease was effectively controlled. Based on the all results, the bacteria Aeromonas veronii was the pathogens.The enzymatic activity and hemolytic activity of extracellular products(ECP) of Aeromonas veronii isolated from the giant salamander were determined by agar diffusion method. The enterotoxigenic activity, cytotoxicity and pathogenicity were also studied. The results showed that the extracellular products had the ability to produce many kinds of enzymes, just like gelatinase, protease, amylase, lipase and so on. At the same time, the hemolytic activity, enterotoxigenic activity and cytotoxicity were found. The extracellular products also had strong pathogenicity to mice and channel catfish.The pathologic changes of the muscle, liver, kidney, lung, intestinal tract and kidney from the giant salamander which infected with Aeromonas veronii were observed by the pathological section and the HE staining technique. In result, the oval cells proliferation appeared in interlobular organizations and portal area. General granular and blisters degeneration could be observed in hepatocytes. A large number of lymphocytes infiltrated into the lobule. Part of the hepatocytes was dissolved. Suppurative necrosis and even ulcers appeared in renal mucous epithelium. Interstitial cells infiltrated into the kidney tubules. Fibrosis appeared in some glomerulus. Congestion and bleeding in heart and vacuolar degeneration even limits vague in myocardial fibers was observed. Granular and ballooning degeneration occured in pancreatic cells.A little cytoplasm was vesicular. Sometimes the hyperemia could been observed in lung. Ratio imbalance occurred between white pulp and red pulp in spleen. Necrosis in parenchymal cells was obvious. Nuclei was dissolved or broken. Swelling and even disintegration appeared in cytoplasm. Intestinal villus was dissolved and dropped. Serious shedding and necrosis occurred in mucous epithelium. Goblet cells were increased. Dilatation and congestion could be observed in small blood vessels of mucous. Muscle fibers were necrotic just like wax. Muscle nuclei were pyknotic, trachychromatic and even cytoclastic. The results showed that Aeromonas veronii could give rise to the serious pathological changes in many tissues or organs of giant salamander and cause the death to the animal.