Vibrio parahaemolyticus is a Gram-negative bacterium commonly found in coastal areas and estuaries. Rising ocean temperatures in recent years have contributed to its global spread. Vibrio parahaemolyticus can infect fish, shrimp, shellfish and other aquaculture animals, especially shrimp. It causes acute hepatopancreatic necrosis disease (AHPND), a condition previously known as early death syndrome (EMS). So far the prevention and control of parahaemolyticus vibrio disease is mainly through the use of antibiotics, but the large and long-term use of antibiotics not only causes serious harm to the aquaculture environment, but also makes the resistance of bacteria continue to strengthen. Phytogenics has been widely used in aquaculture because of their antibacterial activity and less pollution to water environment. The feeding and disease challenge trial was conducted to evaluate the impact and efficacy of PhytoAqua based on plant extracts on the growth and health of Litopenaeus vannamei.
Four treatment groups were designed, among which, the control group was standard feed; the trial group 1 was standard feed with 0.2% PhytoAqua; the trial group 2 was standard feed with 0.4% PhytoAqua; the negative control group was standard feed with 0.0015% florfenicol which purity is above 98%. First, Litpenaeus vannamei was temporarily raised in a 400L tank (200L water) for 4 days. Then 360 shrimps with body weight of (2.3±0.07)g were randomly divided into 12 buckets with 3 replicates in each treatment group and 30 shrimps in each replicate. The water in the bucket was about 100L and then the trial was conducted for 52 days. During the trial, the water was natural seawater after gauze filtration and disinfection, and the water temperature was 29~31℃. The water was changed 4/5 per day, the feed was 4 times per day, the daily feed was 5% ~ 7% of the shrimp body weight, based on the actual food intake, siphon once a day, and clean the residual feed and feces at the bottom of the bucket. After 52 days of the feeding trial, the remaining shrimp after sampling were challenged with Vibrio parahaemolutes (AHPND strain). Shrimp were soaked in a red bucket containing 108CFU/ml of Vibrio parahaemolutes for 2h, then water was added to the red bucket, diluted 3 times. Shrimp mortality was then monitored for 72 hours.
As can be seen from Table 1, there was no significant difference in survival rate between all treatment groups, all of which were over 90%. The final weight, weight gain rate and specific growth rate showed a similar trend, and the administration of 0.4% PhytoAqua was significantly higher than that of control group and florfenicol group (P<0.05), indicating that PhytoAqua can promote the growth of Litopenaeus vannamei. According to the results of challenge test in table 2, shrimp fed either with 0.2% PhytoAqua (36.01%) or 0.4% PhytoAqua (28.36%) had lower mortality than the control group (57.31%) and 0.0015% florfenicol group (57.11%) after 72 hours.