Abstract

Tilapia is one of the most important cultured fish species worldwide. With a broad tolerance to different salinities and temperatures, they can be cultured in fresh, brackish or marine water.^1,4 Infectious diseases of cultured tilapia (Oreochromis sp.), especially those caused by bacterial pathogens, such as Francisella noatunensis subsp. orientalis (Fno)(syn. Francisella asiatica), pose a substantial constraint to the potential of aquaculture.^2,3 Fno is a gram-negative, facultative intracellular emergent fish pathogen and the causative agent of francisellosis in warm water fish.^2,3 This project focused on the health status of tilapia niloticainfected with Fno at different water temperatures and salt concentrations. Fno was cultured at 25 and 30°C in a modified Mueller Hinton (MMH) broth medium or in a MMH supplemented with 30 ppt of NaCl (MMH + Salt). Immersion challenges of tilapia niloticafingerlings with 10⁸ CFU/mL of Fno at water temperatures of 25 and 30°C in both marine and fresh water were conducted for 14 days. Moribund and dead fish were collected daily, and at the completion of the study bacterial burdens in the spleens of the surviving fish were analyzed. Results obtained from this project demonstrate that temperature significantly influences the development of francisellosis in tilapia. Fish maintained at a water temperature of 25°C developed francisellosis and had considerably higher mortality events and bacterial loads in the spleen of surviving fish, compared to the control, and to the fish maintained at a water temperature of 30°C. Additionally, similar mortality and morbidity events were found in fish maintained in fresh and marine water; thus salinity in the water does not seem to be a constraint in the development of francisellosis infections in tilapia.

Acknowledgements

The authors wish to thank Dr. Barrington Brown of the St. Kitts & Nevis Aquaculture Pilot Project & Environmental Research. The authors also thank Ross University School of Veterinary Medicine research program.

References

1.  Alston DE. Global outlook for tilapia culture with emphasis in Latin America. World Aqua. 2009;40:36–40.

2.  Birbeck TH, Feist SW, Verner-Jeffrey DW. Francisella infections in fish and shellfish. J Fish Dis. 2011;34:173–187.

3.  Colquhoun DJ, Duodu S. Francisella infections in farmed and wild aquatic organisms. Vet Res. 2011;42:47.

4.  Hoque MA, Awal A, Kuri AC, Sarker S. Effect of salinity on the oxygen consumption of tilapia fingerlings. Fisheries. 2011;41:15-16.