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United States Department of Agriculture

Agricultural Research Service

Title: CHARACTERIZATION AND EPIZOOTIOLOGY OF AN EMERGING FISH PATHOGEN, GROUP B STREPTOCOCCUS AGALACTIAE.

Authors
item Evans, Joyce
item Klesius, Phillip
item Shoemaker, Craig

Submitted to: European Association of Fish Pathologists
Publication Type: Abstract Only
Publication Acceptance Date: June 4, 2003
Publication Date: September 21, 2003
Citation: EVANS, J.J., KLESIUS, P.H., SHOEMAKER, C.A. CHARACTERIZATION AND EPIZOOTIOLOGY OF AN EMERGING FISH PATHOGEN, GROUP B STREPTOCOCCUS AGALACTIAE. BOOK OF ABSTRACTS THE EUROPEAN ASSOCIATION OF FISH PATHOLOGISTS 11TH INTERNATIONAL CONFERENCE "DISEASE OF FISH AND SHELLFISH". 2003.

Technical Abstract: Catalase negative, ß hemolytic Gram positive cocci, and serogroup B isolates from wild mullet from a Kuwait epizootic and cultured seabream were identified as Streptococcus agalactiae by conventional microbiological techniques and microbial identification systems (API 32 Rapid Strep, FAME and BIOLOG). A phenotypic comparison using FAME and BIOLG data, revealed that seabream and mullet isolates shared a very high degree of similarity, indicating that only one strain was responsible for the epizootic. Nile tilapia, Oreochromis niloticus, experimentally inoculated with these isolates demonstrated that S. agalactiae was the bacterial pathogen responsible for the epizootic. Antibiotic sensitivity testing revealed that the S. agalactiae isolates were resistant to gentamicin and sensitive to amoxcillin, ciprofloxacin, rifampin, choramphenicol, romet, and ampicillin. Environmentally stressful conditions (sublethal dissolved oxygen (DO), high temperature, and elevated nutrient concentrations) were suspected as contributory factors in weakening the resistance of these fish against S. agalactiae and predisposing the fish to infection. To explore the stressful effects of sublethal DO on the susceptibility of fish to S. agalactiae infection, tilapia were experimentally exposed to optimal DO or sublethal DO (< 1 mg/l DO) concentrations, subsequently challenged with S. agalactiae and blood glucose levels determined using the OneTouch® Ultra blood glucose monitoring system. Blood glucose levels were measured before sublethal DO exposure, 24 h after exposure and then 24 h after challenge with either 95 CFU or 750 CFU of S. agalactiae (infected) or tryptic soy broth (TSB) (uninfected) by intraperitoneal (IP) injection. Blood glucose was found to significantly (P <0.001) increase (121 ± 7.41 mg/dl) in response to sublethal DO exposure. Fish exposed to sublethal DO after infection with S. agalactiae at 95 CFU or 750 CFU had significantly higher mortality rates of 27 and 80%, respectively, as compared to fish exposed to optimal DO. None of the fish exposed to optimal DO died due to streptococcal infection after challenge with either dose. These results indicate that DO stress may have been a major contributing factor that increased the susceptibility of fish to S. agalactiae in the Kuwait epizootic.

Last Modified: 10/31/2014