It is unknown how POMS spread from Sydney Harbour/Botany Bay to the Hawkesbury River, or whether in fact it did spread from these locations. The spread of OsHV-1 between major oyster growing regions in Europe has involved movement of infected oysters by oyster farmers. There are no known oyster movements into the Hawkesbury River within a relevant time that can account for the current outbreak of POMS. Movement of oysters within the industry is a major potential risk factor but one that can be managed well with the biosecurity and testing protocols that are already in place in Australia.
Other possible means of spread (including a new introduction of infection from overseas) include discharge of shipping ballast water, movement of virus with ocean currents and transfers of virus on the hulls of boats which are fouled with molluscs. These types of events are not predictable and probably not manageable.
Within an estuary or region the key steps that can be taken to reduce risk include accurate record keeping of all oyster movements and rapid reporting and investigation of all mortality events. These actions improve the chances of early discovery of an outbreak, tracing the source of infection and reducing of the risk of further spread.