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Title: EFFECTS OF SUBLETHAL DISSOLVED OXYGEN STRESS ON BLOOD GLUCOSE AND SUSCEPTIBILITY TO STREPTOCOCCUS AGALACTIAE IN NILE TILAPIA OREOCHROMIS NILOTICUS

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

Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2003
Publication Date: 3/1/2004
Citation: Evans, J.J., Shoemaker, C.A., Klesius, P.H. 2004. Effects of sublethal dissolved oxygen stress on blood glucose and susceptibility to Streptococcus agalactiae in nile tilapia Oreochromis niloticus. Journal of Aquatic Animal Health. 15:202-208.

Interpretive Summary: Streptococcal disease, caused by Streptococcus agalactiae, has occurred in both cultured and wild fish. Recently, we reported that S. agalactiae was responsible for an epizootic involving wild Klunzinger's mullet, Liza klunzingeri, in Kuwait Bay, Arabian Gulf. Among the conditions that were suspected of favoring this epizootic was depletion of dissolved oxygen (DO) to sublethal concentrations and algal abundance. Blood glucose has been used extensively to measure stress responses in fish. The stress effects of sublethal DO exposures on blood glucose levels and changes in susceptibility to S. agalactiae infection in Nile tilapia, Oreochromis niloticus, were determined using the OneTouch ® Ultra blood glucose monitoring system. One hundred fish were monitored for changes in blood glucose concentrations before, during and after exposure to low DO for 24 h. In additional experiments, fish were exposed to optimal DO or sublethal DO. After exposure, fish were administered 95 or 750 cells of S. agalactiae (infected) or tryptic soy broth (TSB) (uninfected) by injection. Blood glucose was found to significantly increase in response to sublethal DO exposure. Significantly higher mean blood glucose levels were also found in both infected and uninfected fish following sublethal DO exposure. Fish exposed to sublethal DO had significantly higher mortality rates as compared to fish exposed to optimal DO after infection with S. agalactiae. None of the fish exposed to optimal DO died due to streptococcal infection after challenge with either dose. Increases in susceptibility and impairment disease resistance of tilapia to S. agalactiae may be the result of the stress response to the sublethal DO exposure. This simple technique may be useful in monitoring the stress response in the field during future epizootics that result from low DO, algal blooms that cause low DO and/or infectious agents.

Technical Abstract: The stress effects of sublethal dissolved oxygen (DO) exposures on blood glucose levels and changes in susceptibility to Streptococcus agalactiae infection in Nile tilapia, Oreochromis niloticus, were determined using the OneTouch < Ultra blood glucose monitoring system. Fish were monitored for temporal changes in blood glucose concentrations before, during and after exposure to < 1 mg/L DO for 24 h. In additional experiments, fish were exposed to optimal DO or sublethal DO. After exposure, fish were administered 9.5 x 10x1 CFU or 7.5 x 10x2 CFU of S. agalactiae (infected) or tryptic soy broth (TSB) (uninfected) by intraperitoneal (IP) injection. Blood glucose (30.7 +/- 1.31 mg/dL) was found to significantly (P >0.001) increase (121 +/- 7.41 mg/dL) in response to sublethal DO exposure. Significantly higher mean blood glucose levels were also found in both infected and uninfected fish following sublethal DO exposure. Fish exposed to sublethal DO had significantly higher mortality rates of 27 and 80% as compared to fish exposed to optimal DO after infection with S. agalactiae at 9.5 x 10x1 CFU or 7.5 x 10x2 CFU, respectively. None of the fish exposed to optimal DO died due to streptococcal infection after challenge with either dose. Increases in susceptibility and impairment of innate resistance of tilapia to S. agalactiae may be the result of the stress response to the sublethal DO exposure.