Long-term temporal and spatial patterns of Ostreid herpesvirus 1 (OsHV-1) infection and mortality in sentinel Pacific oyster spat (Crassostrea gigas) inform farm management

Disease due to Ostreid herpes virus - 1 (OsHV-1) is a major constraint to farming the Pacific oyster Crassotrea gigas in Europe and Australasia, requiring new management strategies. The objectives of this study were to determine the seasonal limits of transmission of OsHV-1, the spatial distribution of the virus, the seasonal water temperatures associated with OsHV-1 transmission and disease and the suitability of active surveillance for OsHV-1 using sentinel oysters. Spat were deployed and sampled regularly between 2012 and 2017 at 15 sites in two large estuaries and the times of OsHV-1 exposure and mortality were identified at weekly to monthly intervals. Mortality due to OsHV-1 was observed each summer season but was significantly more likely in the Georges than the Hawkesbury River estuary. Mortality was most widespread and frequent between December and April and was not observed prior to late October or after May. However, subclinical infections with OsHV-1 that did not progress to mortality were detected from early October until late June. There was a significant reduction in mortality due to OsHV-1 as well as subclinical infection with OsHV-1 between 2012-13 and 2016-17, and viral loads in infected oysters also appeared to decline after 2012-13. Mortality due to OsHV-1 was observed in sentinel spat at 14 of 15 sites but not at every site in every season. Different outcomes were sometimes seen between spat in paired baskets that were 1 to 100 m apart at the same site, attributable to differential exposure to OsHV-1 (clustering). Fifteen different batches of spat were used and mortality due to OsHV-1 occurred in all but three; both triploid and diploid spat were affected. In 2016-17, there was a trend of better survival in selected spat with potential genetic resistance to OsHV-1 compared to unselected spat. There were no biologically significant differences in the growth of spat that subsequently died due to OsHV-1 infection and those that survived. Mortality due to OsHV-1 commenced when the mean water temperature rose above approximately 20 degrees C in spring, which is 4-5 degrees C warmer than the initiation threshold in Europe. These consistent epidemiological patterns of infection and mortality will enable strategic placement of susceptible oysters in these estuaries at certain locations and within specific time windows with predicted low risk of mortality due to OsHV1; water temperature monitoring may be used to predict this risk. Use of sentinel oysters for surveillance for OsHV-1 can be effective, but the clustering of exposure at all scales renders this more reliable at estuary level rather than at bay or site level and it requires an intensive sampling program. Non-specific mortality of spat was relatively common, therefore tests for OsHV-1 should be applied in field trials to distinguish mortality due to OsHV-1 from other causes of mortality.