Putative spawner-isolated mortality virus associated with mid-crop mortality syndrome in farmed Penaeus monodon from northern Australia

Beginning in 1994, farms in northern Australia experienced a higher than no rmal mortality rate in 12 to 15 g prawns from growout ponds. The farmers na med this problem mid-crop mortality syndrome (MCMS). Intramuscular injectio n of filtered (450 nm), cell-free extracts of moribund prawns from these po nds killed healthy prawns between 5 to 30 d post-injection. A 20 nm virus w as visualized by electron microscopy from a 1.4 g ml(-1) band recovered fro m caesium chloride gradients of extracts from the moribund prawns. DNA was extracted from this band, restriction enzyme digested and Ligated into pGEM 7zf(+) vector. A digoxigenin-labelled polymerase chain reaction (PCR)genera ted, gene probe was subsequently prepared by amplifying an inserted sequenc e (approximately 2 kb) of one selected clone specific for the virus. Specim ens of the moribund prawns stained positively by in situ DNA hybridization in endodermal tissues, including the apical ends of hepatopancreatic tubule s, the midgut and hindgut caecae, the midgut, and the hindgut folds. In pra wns that showed haemocytic enteritis, some haemocytes in the affected midgu t showed Limited staining. The positively-staining cells showed no cytolysi s. In prawns injected with cell-free viral extracts, additional tissues wer e positive by probe analysis, including strong staining in the male reprodu ctive tract, specifically in the terminal ampoule and the medial vas defere ns. Limited staining also occurred in the ovary and in both the stromal mat rix and spheroid cells of the lymphoid organ. It was evident that the infec tion was enteric by natural pathways and systemic by injection. Historical specimens of Penaeus monodon experimentally infected with spawner-isolated mortality virus (SMV) were probe-positive in exactly the same pattern as th e naturally and experimental MCMS prawns. Altogether, the evidence suggeste d that the MCMS agent was a parvo-Like virus very similar or identical to S MV.