Virulence status, viral accommodation and structural protein profiles of white spot syndrome virus isolates in farmed Penaeus monodon from the southeast coast of India

The objective of this study was to investigate the reason for variation in the virulence of white spot syndrome virus (WSSV) from different shrimp farms in the Southeast coast of India. Six isolates of WSSV from farms experiencing outbreaks (virulent WSSV; vWSSV) and three isolates of WSSV from farms that had infected shrimps but no outbreaks (non-virulent WSSV; nvWSSV) were collected from different farms in the Southeast coast of India. The sampled animals were all positive for WSSV by first-step PCR. The viral isolates were compared using histopathology, electron microscopy, SDS-PAGE analysis of viral structural proteins, an in vivo infectivity experiment and sequence comparison of major structural protein VP28; there were no differences between isolates in these analyses. A significant observation was that the haemolymph protein profile of nvWSSV-infected shrimps showed three extra polypeptide bands at 41, 33 and 24 kDa that were not found in the haemolymph protein profile of vWSSV-infected shrimps. The data obtained in this study suggest that the observed difference in the virulence of WSSV may not be due to any change in the virus, rather it could be due to the shrimp defence system producing certain factors that help it to accommodate the virus without causing any mortality.