Isolation And Indentification Of Two Pathogenic Bacteria And Associated Histopathology In Cultured Black Rockfish (Sebastes Schlegelii)

Sebastes schlegelii is one of the important economic fishes of sea cage culture inthe North China. It is very popular with fish farmers and consumers because its speed ofgrowth, cold tolerance, delicious and nutritious meat. Hower, with the development ofaquaculture, the poor management and the limit of disease control, the various diseasesespecially bacteical diseases have been come into been frequently and tempestuously.That leads to much economic loss to the aquatic farmers.In August2011, the skin ulcer disease occurred among cultured S.schlegelii in acage culture farm in Qingdao, which caused cumulative mortality rate30%. Thesymptom exhibited hemorrhagic ulcer on the body, abnormal accumulation of asceticfluid in the body cavity, liver hyperaemia, swollen kidney and spleen; In July2011, anepizootic occurred among cultured S.schlegelii in a cage culture farm in Yantai, whichcaused cumulative mortality rate30%-40%. The symptom exhibited hemorrhagic ulceron the head and fin, liver hyperaemia, yellowish green fluid in the intestine. Two strainswere isolated from the diseased Sebastes schlegelii of the two places. They are labeledSS-1and SS-3. The morphological, physiological&biochemical characteristics,16SrRNA sequence analysis, and the drug sensitivity of the bacterium were researched, alsothe histopathological changes were observed.The strain SS-1was isolated from the ulcer skin, liver, kidney and spleen of thefish. Artificial infection test with intramuscular injection method proved that bacteriumstrain SS-1was the causative pathogen associated with the disease. The strain SS-1wasgran negative, rod bacteria, polar flagellum. Based on the morphological, physiological&biochemical characteristics, and16S rRNA sequence analysis, the causativebacterium SS-1was identified as Listonella anguillarum. The results of drug sensitivitytest showed that Ceftazidine, Fleroxacin, Nalidixicacid, Novobiocin were mosteffective against the pathogenic bacterium.The histopathological result showed that there were edema and hyperemia in thesecondary gill lamellae, hyperplasia and necrosis of epithelial cells and mucous cellscaused the fusion with the adjacent secondary gill lamella; the muscle fibers were necrotic, the blood cells were infiltrated into the muscle fibers; fatty degeneration andthe necrosis were found in the liver; the spleen was necrotic with brown plaque;epithelial cells of the renal tubule were necrotic and fused. Red blood cells weredeclining in renal intertitum; the epithelial cells of the intestine were exfoliated, themuscle of the intestine was congested and inflamed.The strain SS-3was isolated from the ulcer skin, liver and intestine of the diseasedfish. Artificial infection test with intramuscular injection method proved that bacteriumstrain SS-3was the causative pathogen associated with the disease. The strain SS-3wasgran negative, rod bacteria, polar flagellum. Based on the morphological, physiological&biochemical characteristics, and16S rRNA sequence analysis, the causativebacterium SS-3was identified as Vibrio ichthyoenteri. The results of drug sensitivitytest showed that Penicillin, Rocephin, Fleroxacin, Clarithromycin, Amikacin,Minocycline were most effective against the pathogenic bacterium.The fish had significant histopathological changes in the tissues and organs. Thesecondary gill lamellae were necrotic, hyperplasia of epithelial cells and mucous cellscaused the fusion with the adjacent secondary gill lamella; the capillaries were dilatancyand congested, the muscle fibers were breakdown, the muscle cells were necrotic, evencan caused the nodule; There were swell and vacuolation in the liver, the hepaticsinusoid and capillaries were congested; the spleen was congested and hemorrhaged, thecells were necrotic which had no prominent nucleoli; in the kidney there were renaltubular atrophy, the cavity was disappeared; the intestinal villus is atrophy, the epithelialcells were necrotic, the intestinal wall was congested, also there were inflammatorycells infiltration.Both of the bacteria could cause severe infections in several tissues and organs ofthe fish and caused a high mortality rate, according to the result of bacteria isolated andhistopathological changes observed.This paper reported for the first time that Listonella anguillarum and Vibrioichthyoenteri caused the disease of Sebastes schlegelii in China and the sensitivity toantibiotic medicines and histopathological changes were also researched, which willprovide reference in fish health management and disease control.