Prevalence of Some Pathogenic Bacteria in Caged- Nile Tilapia (Oreochromis Niloticus) and their Possible Treatment

Abstract

Bacteria are the primary cause of fatal disease outbreaks in aquaculture. Nine fish cages located at three different sites (3 cages/site) in the north Rosetta branch of the Nile River have exhibited high mortality rates. A total of 220 moribund Oreochromis niloticus and fish feed and water samples were examined for pathogenic bacteria in this study. Fish infected with Vibrio parahaemolyticus were located at only site 1 (62.5% infection rate), and Streptococcus agalactiae was isolated from fish at sites 1 and 3 (25% and 37.5% infection rates, respectively). Fish infected with V. parahaemolyticus or S. agalactiae were coinfected with Aeromonas hydrophila. Further investigation revealed that V. parahaemolyticus infection at site 1 may occur via a fish feed that was contaminated with V. parahaemolyticus (the fish feed was containing improperly manufactured marine fish meal). The median lethal dose (LD50) 96h of A. hydrophila, V. parahaemolyticus, and S. agalactiae was 2.4 x 105, 1.9 x 105, and 5.2 x103 colony-forming unit / ml, respectively for O. niloticus (50 ± 2.5 g b.w.) at a water temperature of 25.1 °C ± 1.5 °C. In an indoor experiment, O. niloticus were injected with the LD50 of the isolated bacteria. Florfenicol was found to be superior to ciprofloxacin in treating A. hydrophila and V. parahaemolyticus infection (mortality 13.3 % and 16.7 %, respectively), and ciprofloxacin was found to be more efficient in treating S. agalactiae infection (mortality 13.3%). In conclusion, inappropriately manufactured marine fishmeal was the source of V. parahaemolyticus infection in caged fish. V. parahaemolyticus or S. agalactiae infection co-occurred with A. hydrophila in fish cages containing low-quality water (high unionized ammonia content).