Electron microscopic observation on a non-occluded baculo-like virus in shrimps.

A severe disease of farm-raised shrimp, Penaeus chinensis has spread throughout Chinese coasts since 1993. Recently, a baculo-like virus has been diagnosed as the causative agent for this epidemic disease. The electron microscopic observation of the virus is described. Thin sections of hepatopancreatic and hypodermic tissue of diseased P. chinensis showed many rod-shaped, enveloped, baculo-like virions in hypertrophied nuclei of infected cells. The virion was filled with a highly electron dense core. No occlusion bodies have been found. Negative stained intact virions, purified from infected tissues by centrifugation on sucrose discontinuous gradients, demonstrated that the viral envelops had been broken, but the cylindrical nucleocapside could be observed clearly. The nucleocapsid of average 62 nm x 314 nm was composed of a helix system of capsomers, giving rise to an open stacked ring structure and repeating approximately every third ring. The number of repeating unit was 13 to 15. We propose that the virus described here could be designated as Non-Occluded Shrimp Virus (NOSV) or Penaeus chinensis Baculo-Like Virus (PcBLV).