Pathology And Pathogen Distribution Study Of Streptococus Agalactiae Natural Infected In Schizothorax Prenanti

Streptococcus agalactiae (Group B streptococcus) has emerged as an important pathogen that affects humans and animals, including aquatic species,which has become a threat against the health of humans and animals.Since Hoshina et al. reported the first outbreak of streptococcosis in rainbow trout (Oncorhynchus mykiss)cultured in Japan, streptococcosis has become a serious bacterial disease in wild and cultured fish species,and it gave heavy beat to global aquiculture productions.In 2012-2013, a serious infectious disease characterized by exophthalmia, haenorrhage and neurological symptoms has been emerged in the Schizothorax prenanti farms in Sichuan Province. A Gram-positive, chain-forming coccus was founded in liver and kidney smear examination. Two strains of Gram-positive, chain-forming coccus were isolated from the diseased fish which are consistent with the smaer examination. The colonies were white, circular, smooth-surface, regular and pin-head sized after incubation at 28℃ for 48 h on BHI. It was primarily identified to be Streptococcus agalactiae according to the morphology, physiological and biochemical characteristics. Furthermore, the 16s rRNA gene sequencing analysis by BALST in GenBank indicated that both two isolates showed high levels of similarity to S. agalactiae (more than 96.5%). A phylogenetic tree was constructed by comparing with the 16S rDNA sequences of both two isolates (GenBank accession number KF773744 and KF761304) and other relative bacteria species in the GenBank database. In the phylogenetic tree the two isolates and other S. agalactiae strains constituted a branch. In addition, the two isolates were positive in a specific PCR detection of Streptococcus agalactiae based on cfb gene. Based on the phenotypic characteristics, phylogenetic analysis of the 16S rRNA gene and specific PCR detection, both isolates were identified as S. agalactiae. The sensitivity results showed that both the 2 strains were sensitive toPenicillin, Cefalexin,Ciprofloxacin,Cefradine,Rifampin,azithromycin,Vancomycin,Deoxycycli ne, Amikacin,Florfenicol,Chloramphenicol,Tetracycline,resisant to Amoxicillin,Sulfonamides and Aminoglycosides, such as Gentamicin, Tenemycin, Kanamycin, Streptomycin, while they were sensitive diversities to Ampicillin, Lomefloxacin, Norfloxacin, Ambramycin and Oxytetracycline.Samples of heart, liver, spleen, stomach, intestine and gills were aseptically collected from naturally infected fish in par formaldehyde (4%) for 24-48h. Using paraffin section for the diagnosis of histopathology changes. On the pathological histology,the tissues and organs of all fishes shows congestion, hemorrhage, necrosis and inflammation, expecially in liver,kidney,spleen and brain.Vacuolar degeneration and necrosis of hepatocytes,focal necrosis and hemorrhage occurred in liver,lymphocytes depletion, hemorrhage and macrophages infiltration showed in the spleen; Necrosis and degeneration of renal tubular epithelial cells, hemorrhage appeared in kidney, macrophages and neutrophils infiltration; Hyperemia and hemorrhage of meninges showed in the brain with neutrophils infiltration,vacuolus showed in matrix, neurons swollen; Myocardial degeneration, hemorrhage, epicardial, loose, edema, thickening and a large number of macrophages and lymphocytes infiltration; Eye choroid hemorrhage, hemosiderin generation,anterior chamber volume accumulation within red blood cells.Electron microscopical examination founded that bacteria could invade hepatocytes, renal cells and neurons, causing damages to destroy of mitochondria and endoplasmic reticulum.