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 normal mortality rate in 12 to 15 g prawns from growout ponds. The farmers named this problem mid-crop mortality syndrome (MCMS). Intramuscular injection of filtered (450 nm), cell-free extracts of moribund prawns from these ponds killed healthy prawns between 5 to 30 d post-injection. A 20 nm virus was visualized by electron microscopy from a 1.4 g ml-1 band recovered from caesium chloride gradients of extracts from the moribund prawns. DNA was extracted from this band, restriction enzyme digested and ligated into pGEM7zf(+) vector. A digoxigenin-labelled polymerase chain reaction (PCR)-generated, gene probe was subsequently prepared by amplifying an inserted sequence (approximately 2 kb) of one selected clone specific for the virus. Specimens of the moribund prawns stained positively by in situ DNA hybridization in endodermal tissues, including the apical ends of hepatopancreatic tubules, the midgut and hindgut caecae, the midgut, and the hindgut folds. In prawns that showed haemocytic enteritis, some haemocytes in the affected midgut showed limited staining. The positively-staining cells showed no cytolysis. In prawns injected with cell-free viral extracts, additional tissues were positive by probe analysis, including strong staining in the male reproductive tract, specifically in the terminal ampoule and the medial vas deferens. Limited staining also occurred in the ovary and in both the stromal matrix and spheroid cells of the lymphoid organ. It was evident that the infection 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 the naturally and experimental MCMS prawns. Altogether, the evidence suggested that the MCMS agent was a parvo-like virus very similar or identical to SMV.