The effect of raising water temperature to 33°C in <i>Penaeus vannamei</i> juveniles at different stages of infection with white spot syndrome virus (WSSV)

This study investigated the effect of high water temperature (33°C) at different stages of infection with a highly virulent and low virulent white spot syndrome virus strain (WSSV Thai-1 and WSSV Viet) in Penaeus vannamei juveniles. Shrimp were inoculated intramuscularly with either a high dose (HD) or low dose (LD). Water temperature was kept either at continuously 27°C or switched from 27°C to 33°C at 0, 12 or 24 h post inoculation (hpi) for both strains and in addition at 48 or 96 hpi for WSSV Viet. The increased temperature 33°C was maintained till the end of the experiments (120–144 hpi with WSSV Thai-1 and 240 hpi with WSSV Viet). To determine the infection status at the moment of temperature increase, five shrimp that were kept continuously at 27 °C were euthanized at 0, 12, 24, 48 and 96 hpi with each dose of two strains. WSSV infections (viral antigen VP28) in dead and euthanized shrimp were demonstrated by indirect immunofluorescence.Shrimp inoculated with HD or LD of WSSV Thai-1 and kept continuously at 27°C till euthanasia were 100% viral antigen positive from 12 (HD) or 24 hpi (LD). Shrimp inoculated with WSSV Viet were 100% positive from 24 (HD) and 48 hpi (LD). Shrimp kept at 27°C, showed clinical signs from 24 (HD) or 24–36 hpi (LD) with both strains. Cumulative mortalities reached 100% with WSSV Thai-1 at 60 (HD) or 84–144 hpi (LD) and with WSSV Viet 100% at 216 hpi (HD) or 90% at 240 hpi (LD). Switch of temperature to 33°C from 0, 12 or 24 hpi was effective in reducing mortality of shrimp inoculated with the LD of both strains and with the HD of WSSV Viet. The switch to 33°C from 24 hpi with the Thai strain (HD) and from 48 and 96 hpi with the Viet strain (LD or HD) had no effect or even accelerated the mortality rate (80–100%). All shrimp were viral antigen positive at death and euthanasia (one shrimp LD WSSV Viet) when kept continuously at 27°C. All dead and euthanized shrimp kept at 33°C from 0 or 12 hpi were viral antigen negative. With 33°C from 24, 48 or 96 hpi, all dead shrimp were viral antigen positive and euthanized shrimp were negative.This study showed that 33°C is effective to prevent disease, reduce mortality and block WSSV replication, but only in the early stages of infection