Effects of Sublethal Dissolved Oxygen Stress on Blood Glucose and Susceptibility to Streptococcus agalactiae in Nile Tilapia Oreochromis niloticus

The stress effects of sublethal dissolved oxygen (DO) exposures on blood glucose levels and susceptibility to Streptococcus agalactiae infection in Nile tilapia Oreochromis niloticus were determined with the One Touch Ultra blood glucose monitoring system. Fish were monitored for temporal changes in blood glucose concentration before, during, and after exposure to up to 1 mg DO per liter for 24 h. Blood glucose was found to increase significantly (P < 0.001) in response to sublethal DO exposure. In additional experiments, fish were exposed to acceptable or sublethal DO. After exposure, fish were administered 9.5 × 101 or 7.5 × 102 colony-forming units (CFU) of S. agalactiae (infected) or tryptic soy broth (uninfected) by intraperitoneal injection. Significantly higher mean blood glucose levels were found in both infected and uninfected fish following sublethal DO exposure. None of the fish exposed to acceptable DO died due to streptococcal infection after challenge with either dose. Fish exposed to sublethal DO had significantly higher mortality rates (27% and 80%) after infection with S. agalactiae at 9.5 × 101 or 7.5 × 102 CFU than fish exposed to acceptable DO (0%). Increases in the susceptibility of Nile tilapia to S. agalactiae may be the result of the stress response to the sublethal DO exposure.