Effects of acute change in salinity and moulting on the infection of white leg shrimp (Penaeus vannamei) with white spot syndrome virus upon immersion challenge.

In the field, moulting and salinity drop in the water due to excessive rainfall have been mentioned to be risk factors for WSSV outbreaks. Therefore, in this study, the effect of an acute change in environmental salinity and shedding of the old cuticle shell on the susceptibility of Penaeus vannamei to WSSV was evaluated by immersion challenge. For testing the effect of abrupt salinity stress, early premoult shrimp that were acclimated to 35 g L-1 were subjected to salinities of 50 g L-1 , 35 g L-1 , 20 g L-1 , 10 g L-1 and 7 g L-1 or 5 g L-1 and simultaneously exposed to 105.5  SID50 mL-1 of WSSV for 5 h, after which the salinity was brought back to 35 g L-1 . Shrimp that were transferred from 35 g L-1 to 50 g L-1 , 35 g L-1 and 20 g L-1 did not become infected with WSSV. Shrimp became infected with WSSV after an acute salinity drop from 35 g L-1 to 10 g L-1 and lower. The mortality in shrimp, subjected to a salinity change to 10 g L-1 , 7 g L-1 and 5 g L-1 , was 6.7%, 46.7% and 53.3%, respectively (P < 0.05). For testing the effect of moulting, shrimp in early premoult, moulting and post-moult were immersed in sea water containing 105.5  SID50 mL-1 of WSSV. The resulting mortality due to WSSV infection in shrimp inoculated during early premoult (0%), ecdysis (53.3%) and post-moult (26.72%) demonstrated that a significant difference exists in susceptibility of shrimp during the short moulting process (P < 0.05). The findings of this study indicate that during a drop in environmental salinity lower than 10 g L-1 and ecdysis, shrimp are at risk for a WSSV infection. These findings have important implications for WSSV control measures.