Effect of different salinities (5, 20, 37 ppt) at a low Temperature (22 oC) on White spot syndrome virus infected shrimp

Whitespot syndrome virus (WSSV) continues to bring havoc to the shrimp industry. After more than 3 decades, there is still no guaranteed treatment against WSSV infection. Reported treatments give inconsistent results whichc ouldb e due to the interaction of the risk factors.Previous studies identified low temperature (22oC) and low salinity (5ppt) as WSSV risk factors. This study aimed to determine the effect of different salinities at low water temperature on WSSV infection in Penaeus monodon. Shrimps (n=300; average body weight=4g) were divided into 2 groups: one, the infected group, fed to satiation with WSSV positive shrimp carcass and the other, the control group, fed with commercial pellet. A day after the feeding treatment, the two groups were stocked in separate experimental tanks, at 12ind/tank, with different salinities (5, 20, 37ppt) and cultured at 22oC. Shrimp in both groups were fed daily with commercial pellet at 1% of shrimp biomass. No water change was implemented until termination at 144 hours post infection (hpi). Prior to stocking in separate tanks, 10 shrimp were sampled from each group for WSSV detection using nested PCR and QPCR. One shrimp, prefereably dead/weak was taken daily from each tank for WSSV analysis using QPCR. Water microflora, pH, and temperature were monitored daily. After the treatment, infected shrimps were WSSV positive; QPCR showed 3 copies/mg gill tissue. Analysis showed no significant difference in shrimp mortality and viral load among the infected group. WSSV load of those maintained at 20 ppt was 5.11 copies/mg gill; 37ppt with 5.04 copies/mg gill and 5 ppt with 4.55 copies/mg gill. Highest mortality was observed in infected shrimp maintained at 37ppt (50%), followed by those maintained at 20 ppt(44%); lowest at 5 ppt (33%). After the treatment, control shrimp were WSSV negative. Interestingly, at 144hpi, WSSV was detected in the control group. Among the control group, significantly higher viral load and mortality were observed in control shrimp maintained at 5 ppt. Viral load of control shrimp maintained at 5 ppt was 2.88 copies/mg gill; 1.26 copies/mg gills at 20 ppt, and 0.3 copies/mg gills at 37ppt. Mortalities were observed in the control group maintained at 5 ppt (19%) but not in those at 20 and 37ppt. Mortality was negatively correlate withsalinity and positively with the infection, water pH, and hours post infection; viral load was positively correlated with mortality, infection, and hours post infection. Uninfected shrimp became infected with WSSV at higher rates at lower salinities, but the source of infection was not detected. Mortality is affected by the viral load which is affected by the duration the virus has been in the shrimp body. At salinities of 5, 20 and 37ppt, infection may result in mortality if the viral load is >2.88 copies/mg gill. Shrimp with low viral load of