Marine red hybrid tilapia is a type of tilapia fish that is ideal for both fresh and marine water farming. This fish is popular among both producers and consumers due to its hardy culture natures and its white and red appearances. Due to the demand, the production of this marine hybrid tilapia needs to be increased. However, hybrid tilapia has been facing disease outbreak caused by Vibrio parahaemolyticus resulting in mortality in the aquaculture farm. V. parahaemolyticus is a pathogenic bacterium that inhabit temperate and tropical estuarine, marine and coastal environments.
Therefore, the objective of this study is to determine the pathogenicity and LD50 of V. parahaemolyticus in marine red hybrid tilapia. A total of one hundred marine red hybrid tilapia with the average weight of 5.5 ± 0.5g were used in this experiment. Four different concentrations of V. parahaemolyticus used are 1×109 cfu/mL, 1×108 cfu/mL, 1×107 cfu/mL and 1×106 cfu/mL and were injected intraperitoneally including phosphate buffer saline or PBS solution as control. Observation of clinical signs and mortality of fish were carried out for 14 days to determine the LD50 and pathogenicity. The earliest clinical sign was observed at concentration of 1×109 cfu/mL as early as 4 hours post infection with red eye and passive swimming behaviour as in Figure 1. Other clinical signs that were observed throughout the experiment including haemorrhagic body parts such as operculum, fins and tails, congested kidney, elongated spleen and also patchy liver. The first mortality was recorded at 4 hours post infection in the tank where fish were injected with 1×109 cfu/mL and it also accumulated the highest cumulative mortality which was recorded at 100% mortality rate. No mortality was observed from the control tank. From the experiment, the value of LD50 was calculated at 1.2977x108 cfu/ml (Figure 2). In conclusion, based on the results of pathogenicity and LD50, V. parahaemolyticus are pathogenic in marine hybrid Tilapia and could cause mortality.