Pathogenic bacterium identification and histopathology of septicemia of juvenile southern catfish, Silurus meridionalis Chen

Abstract

The study focused on bacteriological and histopathological examination of septicemia in juvenile southern catfish,Silurus meridionalis,when the disease took place in the catfish farm in Chongqing.There were 15%-40% of cultured southern catfish suffering from the disease and the mortality rate was 45%-80%.The diseased individuals exhibited traits of hemorrhagic septicemia,and showed symptoms of emaciation,sluggishness,eye exophthalmia,and fin hemorrhages.The viscera symptoms of the diseased fish included ascites,liver swelling,and clear histopathological alterations in spleen and kidney.Histopathological examination revealed hydropic swelling of hepatocytes,degeneration and necrosis of renal tubular epithelia in kidney,tumidity in lymphocytes in spleen.One strain of a bacterial species was isolated from liver,kidney and spleen of diseased southern catfish with typical septicemia symptom,and was named SCL1.The challenge experiments were carried out with healthy juvenile southern catfish by means of injection with live SCL1 at the concentration of 1.0×109 CFU/mL.All individuals of the catfish injected with SCL1 died in a week,and the symptom of the challenged fish was similar to those of the naturally diseased fish,and the LD 50 of SCL1 was calculated as 2.94×106 CFU/mL in the 22 g body weight southern catfish,while the catfish injected with 0.65% sterile solution in control group had no signs in 7 d post challenge.Therefore,SCL1 was the pathogenic bacterium strain,which was gram negative,rod-shaped,and the same as Aeromonas hydrophila in physiological and biochemical indexes except for H2S production,and was further proved to be A.hydrophila by means of its 16S rDNA sequence,which was 99.5% identical with A.hydrophila.SCL1 and was sensitive to oxytetracycline,streptomycin,ofloxacin,norfloxacin,kanamycin,and gentamycin,but resistant to penicillin sodium,midecamycin,roxithromycin,trimethoprim,and acetylspiramycin.This paper will be helpful to disease control and health management during Southern catfish farming.