QX disease was detected in Sydney Rock Oysters (SRO) for the first time in Port Stephens, NSW's second-largest oyster producing estuary, in August 2021. In areas where QX occurs, stock losses of up to 90% can be observed in a single outbreak. QX, caused by the parasite Marteilia sydneyi, is a seasonal disease that has a defined period in which infection occurs, usually in late summer. Further, the impacts of QX vary spatially within an estuary, with areas prone to low salinity and with a smaller sediment size typically the worst affected.
In larger estuaries, such as the Georges River, it has been possible to manage the impacts of QX disease through the use of selectively bred QX resistant oysters that are deployed at times and locations which maximise their survival. However, this varies between estuaries and defining the "window" of infection and the spatial extent to which the disease is occurring is vital to ongoing farming of SROs in affected areas.
QX differs from some other diseases, in that it is not transmitted directly from oyster to oyster, but requires an intermediate host. Knowing what other hosts the parasite infects and where they occur is also important in understanding and managing the disease.
As QX is seasonal and is currently active in Port Stephens, now is the time to be sampling oyster populations and to look for which other species in the Port are harbouring the parasite. Undertaking this sampling and processing now will provide vital information for managing oyster stocks in coming seasons. Given the comparatively slow growth rate of SROs many farmers will not be able to sustain losses at their current rate and it is critical we have this information before August and farmers begin to source and deploy stocks for their next crop.