Experiments on the effect of temperature on white spot syndrome virus infection in Litopenaeus vannamei shrimp

Abstract

White Spot Disease (WSD) is an aggressive and devastating viral disease caused by the White Spot Syndrome Virus (WSSV). This highly pathogenic and widespread disease, present throughout Asia and the Americas, can cause up to 100% mortality within 3-7 days after infection. It is annually responsible for huge ecological and economical losses in the main producing countries and forms as such one of the greatest threats for the further sustainable development of shrimp aquaculture. Previous research showed that manipulation of physical factors gave promising results: manipulation of the environmental factors such as temperature produced the most interesting and promising results. For this thesis three experiments were performed, all in which pacific white shrimp (Litopenaeus vannamei) were intramuscularly inoculated with a well-defined viral dose (30 and/or 10000 SID50) and exposed to high water temperature via standardised protocols. The first experiment looked at the efficacy of elevated temperature for protecting shrimp against WSSV. Practically, four temperature treatments in which an elevated temperature (33 °C) was either applied before virus inoculation, after the inoculation, both before and after inoculation, and in the fourth treatment a low temperature (27ºC) was used throughout the test. In the second series of experiments the protective value of high temperature after an initial period of viral replication was evaluated. Water temperature was raised from 27ºC to 33ºC at 0, 12 or 24 hours post WSSV inoculation. Maintaining and controlling such high water temperatures for longer periods of time is of course very unpractical in field conditions and probably economically unfeasible, so the third experiment evaluated the effectiveness of shorter cyclic exposure periods to high water temperature. Hence, the shrimp were exposed to daily temperature cycles (33ºC/27ºC) with 6, 12 and 18 hours of high water temperature, during five consecutive days. Experiment 1 demonstrated a total blocking of disease progression when hyperthermia was applied immediately post inoculation. The protection was very effective even with a high viral dose (10000 SID50). The second experiment, at a low viral dose (30 SID50), showed that high temperature to some extent also worked therapeutic in that previously 24 hours of virus replication could be allowed. At a high infection dose (10000 SID50) the level of protection was however not so effective. In Experiment 3, only a minimum of 18 hours at 33°C resulted in a significant lower I mortality with the infected shrimp. The results from all the experiments clearly show the potential of high water temperature for preventing mortality in WSSV infected shrimp.