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)

This study evaluated the effect of daily fluctuations between optimum (27 degrees C) and high water temperature (33 degrees 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 SID50 of either virulent WSSV Thai-1 or less virulent WSSV Viet or mock inoculated. Temperature was kept either continuously at 27 degrees C or switched from 27 degrees C to 33 degrees C at 12 h post inoculation (hpi) and maintained at that temperature for 6 h (6 h 33 degrees C), 12 h (12 h 33 degrees C), 18 h (18 h 33 degrees C) or 24 h (24 h 33 degrees C) per day. Temperature was then lowered and maintained at 27 degrees 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 degrees 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 degrees C (24 h 33 degrees C), mortalities were 0% with WSSV Thai-1 and 5% or 10% with WSSV Viet. With exposure to 33 degrees C for 6 or 12 h per day (6 h 33 degrees C or 12 h 33 degrees 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 degrees 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 degrees C for 18 h per day. At constant 27 degrees C and at 33 degrees C for 6 h or 12 h per day, dead shrimp were WSSV positive and euthanized shrimp were WSSV negative. At 33 degrees 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 degrees C with Thai-1 were WSSV positive. The present study showed that daily temperature fluctuations have negative (6 h 33 degrees C with WSSV Viet) or positive effects (6 h 33 degrees C with WSSV Thai-1 and 12 h or 18 h 33 degrees C with both strains) on disease and mortality of shrimp infected with WSSV. (C) 2007 Elsevier B.V. All rights reserved.