Monitoring ostreid herpesvirus-1 (OsHV-1) outbreaks in juvenile Pacific oysters Crassostrea gigas along the west coast of the USA

Ostreid herpesvirus-1 (OsHV-1) is a threat to the global production of Pacific oysters Crassostrea gigas, often resulting in nearly complete mortality in affected stocks. A sentinel monitoring program was conducted between June and October 2020, to characterize OsHV-1 outbreaks in Pacific oysters along the west coast of the USA. Deployment of sentinel oysters at 5 commercial growing locations, coupled with frequent sampling, allowed measurement of the spatial and temporal occurrence of OsHV-1 outbreaks as well as the viral load and pathogenesis of OsHV-1 during C. gigas mortality events. In addition, 2 divergent oyster families were deployed at sites that have historically tested positive for OsHV-1 to measure the effect of oyster genotype on the severity of OsHV-1-induced mortality in the field. Mortality events at California test sites were associated with elevated levels of OsHV-1 in oyster tissue. OsHV-1 was not detected in oysters at Oregon and Washington test sites. In Tomales Bay, California, high variation among replicate culture units was observed in cumulative field survival and peak viral load. A negative relationship was observed between peak OsHV-1 load in oyster tissues and shell height at the time of peak viral load, suggesting larger seed may be less vulnerable during periods of OsHV-1 infection risk. Cumulative survival over the duration of the growing season in Tomales Bay was related to peak viral load and differed by family. These results corroborate previous findings suggesting selective breeding may effectively increase survival of oyster families during OsHV-1 outbreaks along the US west coast.