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

Crayfish farmers reported reduced tolerance of stress in specimens of Cherax quadricarinatus, which were formerly robust crayfish. Furthermore, one farmer reported a large reduction in yield with final harvest only equaling the stocking weight. Upon trapping, one-third of the crayfish regularly died overnight and a further one-third died on the sorting tray during sexing of juveniles (approximately 3 mo old). Histopathological examination revealed very light (1 or 2 cells per section) infections with Cherax giardiavirus and sometimes mild atrophy of hepatopancreatic cells. Gene probe analysis with a DIG-labeled spawner-isolated mortality virus (SMV) probe demonstrated extensive positive signals in nuclei of many tissues. The hepatopancreas, the midgut, glands associated with the midgut, the epithelium of seminal ducts and follicle cells surrounding oocytes gave the strongest positive signals. Nuclei of the heart, haemocytes, connective tissue and subcutis gave positive signals in some individuals. Although signals were intense and extensive, cytolysis of infected cells was very limited. The possibility of cross infections of SMV between prawns and freshwater crayfish is of international quarantine significance.