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Nunuk Listiyowati*, R.R. Sri Pudji Sinarni D, dan Alimuddin
Research Institute for Fish Breeding
Jalan Raya 2 Sukamandi Pantura
Patokbeusi, Subang 41263
West Java, Indonesia

Streptococcosis, a septicemic disease  that caused high mortality in nature and fish culture either in fresh and marinewater. S. agalactiae to be a significant cause of mortality and morbidity in tilapia. The objectives of the present study were to evaluate the mortality rate and clinical signs in Nirwana strain tilapia O. niloticus fry from streptococcosis survival broodstock that experimentally injected with S. agalactiae type non-hemolytic (N14G) 104 CFU/mL.

Fish fingerlings were produced by natural spawning in the freshwater fiber tank. The mature streptococcosis survival broodstock were mated with ratio sex 1:1/fiber tank. After 14 days of broodstock mating, larvae were harvested and reared in concrete tank measuring 2x2x1 m2 with a stocking density of 100 fish. The experimental fish were reared up to size 15-20g/fish. All of the fish, except negative control were challenged by injecting  0.1 mL/fish of the non-hemolytic S. agalactiae (104 CFU/mL of N14G and 107 CFU/mL of NK1) by intra-peritonial (IP).

Disease challenge test was carried out twice in the aquarium measuring 60x50x40 cm3 for two weeks (14 days/challenge), with density 10 fish/aquarium for the first challenge and 15  fish/aquarium  for the second challenge. Clinical signs of S. agalactiae infections may include anorexia, darkening of the skin, hemorrhages and necrotic lesions, exopthalmia, opacity, purulens and C-shaped body posturing.

The experimental result showed that the offspring had lower cumulative mortality (13.3; 26.7 and 30%) than the control (43.3%) for N14G infection. Following these result  the offspring that injected with NK1 also had lower cumulative mortality (64.4%) than the control (91.1%). It means that increasing streptococcosis resistance in tilapia can held with disease resistance selection.

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