Natural infection of the redclaw crayfish Cherax quadricarinatus with presumptive spawner-isolated mortality virus

Crayfish farmers reported reduced tolerance of stress in specimens of Chera x quadricarinatus, which were formerly robust crayfish. Furthermore, one fa rmer reported a large reduction in yield with final harvest only equaling t he stocking weight. Upon trapping, one-third of the crayfish regularly died overnight and a further one-third died on the sorting tray during sexing o f juveniles (similar to 3 mo old). Histopathological examination revealed v ery light (1 or 2 cells per section) infections with Cherax giardiavirus an d sometimes mild atrophy of hepatopancreatic cells. Gene probe analysis wit h a DIG-labeled spawner-isolated mortality virus (SMV) probe demonstrated e xtensive positive signals in nuclei of many tissues. The hepatopancreas, th e midgut, glands associated with the midgut, the epithelium of seminal duct s and follicle cells surrounding oocytes gave the strongest positive signal s. Nuclei of the heart, haemocytes, connective tissue and subcutis gave pos itive signals in some individuals. Although signals were intense and extens ive, cytolysis of infected cells was very limited. The possibility of cross infections of SMV between prawns and freshwater crayfish is of internation al quarantine significance.