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 se edling production at public sea-farming centers in west Japan during 1 994 and 1995. Moribund shrimp exhibited no apparent external signs exc ept small obscure white spots on the body, and were proved to be free from any causative bacteria, fungi and parasites. Infection trials rep roduced the disease in both healthy juvenile M. ensis and Penaeus japo nicus. The disease was characterized by nuclear hypertrophy of the nec rotic cells in various organs and tissues originated from meso- and ec toderm, such as cuticular epidermis and hematopoietic tissue. Envelope d ovoid-shaped virus particles were observed in such hypertrophied nuc lei in sectioned preparations and in the hemolymph in negatively stain ed preparations. Penaeid rod-shaped DNA virus (PRDV)-specific DNA frag ments were detected by PCR method. These results indicate that the mas s mortalities were caused by penaeid acute viremia (PAV).