A profound effect of hyperthermia on survival of Litopenaeus vannamei juveniles infected with white spot syndrome virus

This study was conducted to examine the effect of increasing seawater temperature on White Spot Syndrome Virus (WSSV) infection in juvenile Pacific White shrimp (Litopenaeus vannamei). Infection by WSSV was achieved using two methods, intramuscular injection and per os (oral) administration. Forty injected and 20 per os infected animals were kept in heated tanks at 32.3 ± 0.8 C, and the same number of WSSV infected animals were maintained in tanks at ambient temperature (25.8 ± 0.7 C). Despite the route of exposure, there were no survivors among the animals kept at ambient temperature; whereas, in heated tanks the survival of the WSSV infected juvenile shrimp was always above 80%, suggesting the existence of a beneficial effect from hyperthermia that mitigated the progression of WSSV disease. Moreover, this beneficial effect was not attributable to viral inactivation. Infected animals kept at 32 C had histologically detectable lymphoid organ spheroids suggestive of a chronic viral infection but were PCR negative (hemolymph) for WSSV. These findings might be related to low viral replication in WSSV-infected shrimp held at the higher environmental temperature. When the WSSV-infected shrimp were transferred from 32 C to ambient temperature, the mortality from WSSV ensued and was always 100%. Although the mechanism related to the benefacial effect of heating was not determined, our results indicate that increasing the water temperature modifies dramatically the natural history of the WSSV disease and the survival curves of WSSV-infected juvenile Pacific White shrimp.