The index case of white tail disease (WTD) is presented in adult broodstock prawns Macrobrachium rosenbergii from the Flinders River in western Queensland, Australia, in mid-2007. Histological examination revealed extensive myonecrosis with massive infiltration of myonuclei and some haemocytes. Juveniles from the same broodstock but not from 3 other families displayed white muscle lesions. Low-grade chronic mortalities approaching 100% over 1 yr occurred. Reverse transcriptase polymerase chain reactions (RT-PCR) were attempted for both M. rosenbergii nodavirus (MrNV) with 2 sets of primers and for the satellite virus, extrasmall virus (XSV). All 3 PCRs generated amplicons of the expected sizes. Basic local alignment search tool (BLAST) analyses of the 3 consensus sequences identified a 91% match with MrNV viral capsid protein gene, 96% match with MrNV RNA-directed RNA polymerase gene, and a 99% match with M. rosenbergii XSV capsid protein gene. The clinical signs, histopathological lesions and RT-PCR amplicons could be reproduced in M. rosenbergii inoculated with cell-free extracts fulfilling River's postulates. We conclude that this is an endemic strain of MrNV as the sequences are dissimilar to strains of MrNV circulating around Asia and the Americas. This case only poorly meets the Office International des Epizooties (OIE) case definition for WTD due to the age of the prawns involved and the nature of the inclusion bodies. Perhaps the OIE case definition needs broadening.

Macrobrachium rosenbergii nodavirus (MrNV) that causes white tail disease (WTD) is an emerging disease that contributes to serious production losses in Macrobrachium hatcheries worldwide. Mosquito cell lines (C6/36) have been reported to support the growth of MrNV and used to observe the cytopathic effects (CPE) in infected cells. This study determined the susceptibility of C6/36 mosquito cells to the Australian isolate of MrNV in order to use fewer animals in further investigations. Different staining methods were used to observe MrNV viral activity in C6/36 cells. Typical cytopathic effects such as vacuolation and viral inclusion bodies were observed in infected C6/36 cells with H&E and Giemsa staining. With acridine orange, it was easier to detect presumptive MrNV messenger ribonucleic acid in the infected cells. Using neutral red staining to measure mitochondrial activity showed light absorption of infected cells maximized at day 4 (O.D. = 0.6) but was significantly lower (chi-square = 41.265, df = 1, P < 0.05) than control groups (O.D. = 2) which maximized at day 12. Using trypan blue staining to count the number of cells with disrupted cell membranes, the maximum number of presumptively dead cells at day 8 (4 × 10(5)  cells) in infected treatments was higher than the control treatment at day 10 (1.8 × 10(5)  cells). However, TaqMan real-time PCR did not confirm the replication of MrNV in the cells over 14 days. The mean viral copies and mean cycle times of positive samples were stable at 2.07 × 10(4) and 24.12, respectively. Limited evidence of viral replication was observed during four serial passages. This study determined the mortality of the C6/36 cell line to the Australian isolate of MrNV but suggests limited patent replication was occurring. Trying different cell lines or adapting the virus to the C6/36 cells may be necessary to successfully replicate Australian MrNV in cell lines.

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