Impact of daily fluctuations of optimum (27 °C) and high water temperature (33 °C) on Penaeus vannamei juveniles infected with white spot syndrome virus (WSSV)

Abstract

This study evaluated the effect of daily fluctuations between optimum (27 °C) and high water temperature (33 °C) on the clinical and virological outcome of a WSSV infection in Penaeus vannamei juveniles. Shrimp were inoculated intramuscularly with a dose of 10,000 SID 50 of either virulent WSSV Thai-1 or less virulent WSSV Viet or mock inoculated. Temperature was kept either continuously at 27 °C or switched from 27 °C to 33 °C at 12 h post inoculation (hpi) and maintained at that temperature for 6 h (6 h 33 °C), 12 h (12 h 33 °C), 18 h (18 h 33 °C) or 24 h (24 h 33 °C) per day. Temperature was then lowered and maintained at 27 °C for the remaining hours of the day. The experiments ran for 132 hpi with WSSV Thai-1 and 168–192 hpi with WSSV Viet. WSSV infections were demonstrated by indirect immunofluorescence (IIF). Shrimp kept continuously at 27 °C, started to show clinical signs at 24 hpi and mortalities started at 36 hpi with both strains. Cumulative mortalities reached 100% at 60 hpi with WSSV Thai-1 and at 168–192 hpi with WSSV Viet in two experiments. At constant 33 °C (24 h 33 °C), mortalities were 0% with WSSV Thai-1 and 5% or 10% with WSSV Viet. With exposure to 33 °C for 6 or 12 h per day (6 h 33 °C or 12 h 33 °C), cumulative mortalities reached 90–100% at 72–96 hpi with WSSV Thai-1. With WSSV Viet cumulative mortalities reached 100% at 84–96 hpi or 50–95% at 96–108 hpi with 33 °C for 6 h or 12 h per day. Mortalities were clearly reduced with WSSV Thai-1 (0–40%) and WSSV Viet (5–15%) at 33 °C for 18 h per day. At constant 27 °C and at 33 °C for 6 h or 12 h per day, dead shrimp were WSSV positive and euthanized shrimp were WSSV negative. At 33 °C for 18 h or 24 h per day, the dead and euthanized shrimp were WSSV negative except the dead shrimp at 18 h 33 °C with Thai-1 were WSSV positive. The present study showed that daily temperature fluctuations have negative (6 h 33 °C with WSSV Viet) or positive effects (6 h 33 °C with WSSV Thai-1 and 12 h or 18 h 33 °C with both strains) on disease and mortality of shrimp infected with WSSV.