Hyperthermia reduces viral load of white spot syndrome virus in Penaeus vannamei

We have previously reported that white spot syndrome virus-infected Penaeus van- namei (also called Litopenaeus vannamei) maintained at 32°C show higher survival rates and a sig- nificant increase in number of apoptotic cells when compared to infected shrimp kept at 26°C. As apoptosis plays an important part in the antiviral response of invertebrates, we hypothesized that this process would reduce WSSV replication, allowing the shrimp to control the disease and survive. To test this hypothesis, shrimp were orally infected and maintained at either 26°C (Group 1) or 32°C (Group 2), DNA was extracted from haemolymph collected at various times from 6 to 216 h post- infection, and the number of viral units was quantified by real time PCR using SYBR Green. In parallel, histological examination was carried out to confirm the WSSV infection and to rule out con- comitant diseases. Linear regression of real time PCR units (rtPCRU) of WSSV from Group 1 showed a significant increase with time post-infection (r 2 = 0.7383; p < 0.001). Conversely, there was no increase in rtPCRU with time post-infection in Group 2 (r 2 = 0.142), indicating that hyperthermia inhibited, either directly or indirectly, viral replication. In addition, comparison between the groups showed no difference in WSSV rtPCRU up to 48 h post-infection. After 72 h, shrimp from Group 1 had a significantly higher viral rtPCRU (ANOVA, p < 0.001). We conclude that hyperthermia-associated WSSV rtPCRU reduction could reflect either an increase in the shrimp antiviral response, or a direct negative effect on viral replication, or both.