The Study Of Pathogen, Pathology And Prevention On Turbot With White Stool Disease

As an important farmed specie in Europe,turbot,Scophthalmus maximus, is popular with itshigh nutritional and economic value. Since introduced into China in1992, the rapid rise ofits aquaculture in coastal areas of northern China has brought us great economic and socialbenefit. In Hebei Province, turbot breeding is mainly located in Qinhuangdao, however,there also has a lot of saline wasteland suitable for the construction of turbot-farmingfactory in some other areas like Cangzhou and Tangshan,and needs to be developed.Relatively speaking, there is great room for the development.On the other hand, with therapid expansion of the farming scale,continuous improvement of the intensification and thedeteriorating farming ecological,the issue of germplasm degradation, chaotic managementas well as serious disease became frequent, especially bacterial diseases, which havealready been the bottlenecks restricting the healthy development of turbot farmingindustry.According to the survey of six turbot farms in Hebei and neighboring areas alongthe coast, we found that white stool disease has become the most common and mostserious factor causing large economic losses in turbot breeding process there, especiallyduring the five months from every June to October. This disease has great infectivity with ahigh mortality. Without effective measures conducted, the daily mortality will reach0.5%~1.0%, and the total will be above30%. To ensure the healthy and stable developmentof turbot aquaculture and to achieve the technique of early diagnosis and develop effectiveprevention and treatment of this disease, it is particularly important to isolate pathogensand study the pathogenic mechanism on typical fish suffered with white stool disease.Based on the survey about the prevalence of turbot caused by white stool disease in ourprovince,we systematically described its propagation characteristics,symptoms and thedegree of harm. Through isolating and analyzing the pathogens from typically sick turbotin different farms on various periods, we obtained8purified colonies. With conducting theexperiments of manually infecting the healthy turbot with injection method, the resultshowed that the strain with the highest dominance could cause healthy turbot totallyappearing the same symptom with those who suffered from white stool disease in thenatural farming state. And we got a large number of that bacteria again, which confirmedthat the strain was the pathogen for the turbot white stool disease in our province. At the same time,it conformed that with a high concentration, this pathogen will have obviouslypathogenic and lethal effects on turbot.On the other hand, with the injection of gradientconcentration, we calculated that LD50=3.91×105cfu/ml when we injected the turbotweighting (7.5±2)g. Flounder pathogenic infections experimental results showed that:acertain concentration of the bacteria could also cause the flounder suffering from thisdisease and it might cause death. Combining the method of conventional physiological andbiochemical characteristics analysis with the16SrDNA gene sequence analysis, weidentified the isolated pathogenic bacteria and concluded that it was a wild-typeEdwardsiella tarda. With the study of tissue pathology on liver, kidney, gill and so on fromsick turbot, we found that diseased fish intestinal tissue had obvious pathological changes,which was its main role playing in organ.In addition, the Edwardsiella tarda could infect avariety of other visceral organs, resulting in hemolysis and other pathological changes.Taking the result of the pathogens isolated into account, it confirms that the infections ofthe bacteria on turbot are systemic.Susceptibility test results said that gallic along with some other herbs had a certain degreeof effect on this Edwardsiella tarda strain, and the MIC, MBC were12.5mg/ml,25mg/ml.Antibiotics such as levofloxacin showed an obvious effec, its MIC, MBC were0.125μg/ml,0.25μg/ml.In addition,this bacteria has a strong sensitivity to most of otherantibiotics such as meropenem. Production practices were also able to confirm a series ofantibiotics had a good preventive effect on turbot white stool disease.According to a largenumber of tests in vitro antibacterial and production practice, based on the sensitive drugscreening, we got the ideal drug formulations,and summed up the integrated controlmeasures of the disease,and confirmed that the results have a good function on the turbotwhite stool disease prevention.To some extent, it succeeds to meet the urgent need ofproduction.