Mass Mortalities in the Production of Juvenile Greasyback Shrimp, Metapenaeus ensis, Caused by Penaeid Acute Viremia (PAV)

Very high mortalities of juvenile Metapenaeus ensis occurred in the seedling production at public sea-farming centers in west Japan during 1994 and 1995. Moribund shrimp exhibited no apparent external signs except small obscure white spots on the body, and were proved to be free from any causative bacteria, fungi and parasites. Infection trials reproduced the disease in both healthy juvenile M. ensis and Penaeus japonicus. The disease was characterized by nuclear hypertrophy of the necrotic cells in various organs and tissues originated from meso- and ectoderm, such as cuticular epidermis and hematopoietic tissue. Enveloped ovoid-shaped virus particles were observed in such hypertrophied nuclei in sectioned preparations and in the hemolymph in negatively stained preparations. Penaeid rod-shaped DNA virus (PRDV) specific DNA fragments were detected by PCR method. These results indicate that the mass mortalities were caused by penaeid acute viremia (PAV).