Preliminary Study On The Early Mortality Syndrome Of Litopenaeus Vannamei

According to the outbreak of Early Mortality Syndrome(EMS) of Litopenaeus vannamei in a large area of China since2010, it has been caused significant economic losess to China’s marine shrimp farming. In result, we carried out a systematic investigation in6areas of Hainan province to collected the normal shrimps and the ones in sick of EMS which were cultured in5farms from September2011to February.2012. At the same time,we collected Litopenaeus vannamei which in normal growth with feed intake and infected with EMS after stocking30days. Then we detected the specific virus, observed the tissue section of hepatopancreas and midgut, tested the quantity of vibrio and cultivable bacteria in hepatopancreas of both normal and sick shrimp samples, selected two predominant strains of vibrio to carry out the artificial infection test and antibiotic sensitivity test for Litopenaeus vannamei. All above for researching the occurrence regularity, pathogen, prevention and control measures of the symptom.The survey showed that the high incidence of EMS was after stocking10to30days, and common features of the syndrome is that the hepatopancreas of shrimps in sick of EMS is abnormal enlargement with soft texture and its color is pale white or yellow, besides the color of its body was whitish and reddish. Usually, shrimp in sick of EMS swimming weakly and slowly over the surface or around slope of the ponds accompanied with loss of appetite, the stuffed degrees of stomach and intestines is low, but in severe cases its stomach and intestines were empty entirely accompanied with red. The condition of the disease spread very rapidly and high mortality. Usually, the shortest time from the discovery of a small number of sick shrimp to completely annihilated is2to3days. In addition, WSSV, TSV, IHHNV and other specific virus had not been detected in sick shrimps.The observation results of the tissue section of hepatopancreas and midgut showed that hepatopancreas of shrimps in sick of EMS is abnormal enlargement and pale yellow, the gland cells shrink accompanied by autolysis, intercellular space of hepatopancreas gets larger, the gland nucleus gets smaller. In addition, the cell wall of midgut thins and tissue in the cell wall sloughs off and scatters in the intestine.The results of detection of the vibrio and culturable bacteria density in normal and sick shrimps showed the average vibrio and culturable bacteria density of normal shrimp is4.69×105cfu·g-1and7.88×106cfu·g-1, and yellow vibrio strains is the predominant species. The average vibrio and culturable bacteria density of normal shrimp aquaculture water is6.3×103cfu·mL"1and9.95×105cfu·mL-1.The average vibrio and culturable bacteria density of sick shrimp is4.88×106cfu·g-1and2.42×107cfu·g-1. The average vibrio and culturable bacteria density of sick shrimp aquaculture water is2.23×104cfu·mL-1and6.79×105cfu·mL-1.The One-way ANOVA results showed that there are significant differences in vibrio species and density of hepatopancreas between normal and sick shrimps.The artificial infection test of two vibrio predominant species showed that the strains of HNDX-Y and HNDX-G can infect the digestive tract of the Litopenaeus vannamei by peroral infection and which can both in digestive organs. The green vibrio strains has stronger virulence than the yellow ones. The artificial infection showed green vibrio strains can infect shrimps with the typical symptoms of Early Mortality Syndrome and lead to the damage and necrosis of digestive organs. So the green vibrio strains may be the main pathogen of the disease of Early Mortality Syndrome. The results of physiological and biochemical identification showed that the strains of HNDX-Y. HNDX-G and the re-separation strains of HNDX-Y’. HNDX-G’after artificial infection test were consistent on physiological and biochemical characteristics and morphological features, and they belonged to the same strains each other. The strains of HNDX-Y、HNDX-G were Vibrio hanyi and V. Parahaemolyticus.Results of antibiotic sensitivity test showed that the strains of HNDX-Y antibiotic sensitivity percentage is89.2which is higher than the strains of HNDX-G(56.8%). The strains of HNDX-G is medium resistance, but the strains of HNDX-Y is resistent to Clindamycin, Cefotaxime, Penicillin, Cotrimoxazole and which is medium resistent to Aztreonam, Clarithromycin,, Midecamycin, in addition, it was highly sensitive to other30kinds of antimicrobial agents.