LAUSR

Abstract To understand the transmission of Ostreid herpesvirus-1 microvariant(s) (OsHV-1) in seawater, it is necessary to understand the temporal pattern of occurrence of the virus in seawater in relation to demonstrated infection events in oysters. The aims of the current study were: (1) to investigate the temporal sequence of detection of OsHV-1 in seawater and Crassostrea gigas spat in an upweller system, (2) to assess whether filtration or ageing of seawater reduced the detection of OsHV-1 in seawater and spat, and (3) to assess whether retentate or membranes from filters that were installed to enable continuous sampling of particles in seawater could be used for OsHV-1 surveillance and to investigate OsHV-1 particulate attachment. This study is the first to detect OsHV-1 DNA in 5 μm retentates and membranes used to continuously filter natural seawater. OsHV-1 DNA was detected in seawater before it was detected in oysters in 65.4% of the 26 detection events examined across 6 trials in an OsHV-1 endemic estuary. The time of sampling (morning or afternoon) did not affect the detection of OsHV-1 DNA in seawater. Seawater treatment by filtration or ageing did not reduce the frequency of detection of OsHV-1 DNA in seawater. In contrast, the odds of detecting OsHV-1 DNA in the tissues of oysters kept in aged seawater and some filtered seawater treatments were reduced (odds ratios: 0.16–0.36) in comparison to the control. Although OsHV-1 DNA was detected in the retentate from 5 μm filtration, it is unlikely these samples will be useful for investigating OsHV-1 particle attachment due to low viral signals. Similarly, OsHV-1 DNA was detected on the 5 μm filter membranes, however further investigation is required to ascertain their usefulness in assessing OsHV-1 particle attachment. Despite this and with further development, 5 μm filter retentates and filter membranes may prove to be useful samples for the surveillance of OsHV-1 in seawater.