Ultrastructure and morphogenesis of White Spot Syndrome Baculovirus (WSSV)

Since 1993 non-occluded baculoviruses, associated with a syndrome with high mortalities, have been reported in cultured penaeid shrimp from Asia and the Indo-Pacific region. Infections are typically accompanied by the presence of white spots on the cuticle. Numerous names were given to the virus(es) in early reports on the disease, but the syndrome is increasingly known as White Spot Syndrome (WSS) and its viral agent(s) as White Spot Syndrome Baculovirus (WSSV). The WSS virion is a stocky rod-shaped particle with an apical envelope extension. The nucleocapsid is cylindrical with asymmetric ends, and has a superficial segmented appearance. The pattern of degradation confirms that the nucleocapsid is a cylinder formed by stacks of rings, which are in turn composed of 2 rows of regularly spaced subunits. WSSV replication takes place in the nucleus and is first indicated by chromatin margination and nuclear hypertrophy. Viral morphogenesis begins by the formation of membranes de novo in the nucleoplasm and by the elaboration of segmented, empty, long tubules. These tubules break into fragments to form naked empty nucleocapsids. After that, membranes envelop the capsids leaving an open extremity. The nucleoproteins, which have a filamentous appearance, enter the capsid through this open end. When the core is completely formed, the envelope narrows at the open end and forms the apical tail of the mature virion.