Mass mortalities of cultured kuruma shrimp Penaeus japonicus in Japan in 1993: Electron microscopic evidence of the causative virus

Mass mortalities presumably caused by an infectious disease of kuruma shrimp, Penaeus japonicus, occurred in many culture ponds in some western prefectures of Japan in 1993. The present study revealed the close resemblance in histological changes between spontaneously diseased and experimentally infected shrimp : degenerations of the cuticular epidermis, connective tissue and lymphoid organ. Electron microscopy revealed rod-shaped, enveloped, nonoccluded virus in nuclei in the cuticular epidermis of the stomach. The size of the nucleocapsid was 84±6×226±29nm. Empty capsid of 61nm in diameter and capsid originator of 52nm in diameter were also found in nucleoplasm. Although the virus shared certain morphological characteristics with various baculovirus species, it differed in that the nucleocapsid is not a true cylinder, which is also the case of the Polydnaviridae. The virus is temporarily named RV-PJ (rod-shaped nuclear virus of Penaeus japonicus) until its taxonomical position is ascertained by analyzing the structure of the genomic DNA.