Persistence of viral load in shrimp that survived WSSV infection.

We established a cohabitation model to study shrimp survival after a white spot syndrome virus (WSSV) outbreak. Naïve shrimp were reared individually in plastic boxes immersed in a tank with ten free-roaming shrimp injected each with 1000 copies of purified WSSV. A WSSV outbreak commenced from day four (elevated mortality levels), which lasted for about 10 days. When no further mortalities occurred, surviving shrimp were collected for observation. Survival levels of the cohabitating shrimp were between 5.3 % and 15.9 % from independent infection trials. Determination of viral loads by qPCR and RT-PCR demonstrated 10,000-fold higher viral copy numbers in the moribund shrimp than in the survivors. Western blot analysis using an anti-VP28 antibody confirmed PCR results that high VP28 expression occurs in moribund shrimp, but no signals were detected in the survivors. Histological examination depicted eosinophilic inclusion bodies with hypertrophied (swollen) nuclei and marginated, slightly basophilic, chromatin in the moribund shrimp, but not in the survivors. These data suggest that the surviving shrimp are resilient and posses a mechanism to curtail viremia. Expression levels of selected antimicrobial factors - ALF3, ALF6, PmCrustin1, PmPenaeidin3 and PmPenaeidin5 were compared between moribund and survivor shrimp. PmCrustin1 and ALF3 expression were substantially higher in the moribund shrimp than those of survivors. Interestingly, expression levels of PmCrustin1 were correlated positively with viral loads. Our data provides new insight into WSSV resilience in Penaeus monondon.