Differential mortality and high viral load in naive Pacific oyster families exposed to OsHV-1 suggests tolerance rather than resistance to infection

Authors: AGNEW, M. VICTORIA - University Of Maryland; FRIEDMAN, CAROLYN - University Of Washington; LANGDON, CHRIS - Oregon State University; KONSTANTIN, DIVILOV - Oregon State University; SCHOOFIELD, BLAINE - Oregon State University; MORGA, BENJAMIN - French Research Institute For The Expolotation Of The Sea (IFREMER); DEGREMONT, LIONEL - French Research Institute For The Expolotation Of The Sea (IFREMER); DHAR, ARUN - University Of Arizona; KIRKLAND, PETER - Nsw Department Of Primary Industries; Dumbauld, Brett; BURGE, COLLEEN - University Of Maryland

Submitted to: Journal of Pathogens
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2020
Publication Date: 12/17/2020
Citation: Agnew, M., Friedman, C.S., Langdon, C., Konstantin, D., Schoofield, B., Morga, B., Degremont, L., Dhar, A.K., Kirkland, P., Dumbauld, B.R., Burge, C.A. 2020. Differential mortality and high viral load in naive Pacific oyster families exposed to OsHV-1 suggests tolerance rather than resistance to infection. Journal of Pathogens. 9(12):1057. https://doi.org/10.3390/pathogens9121057.
DOI: https://doi.org/10.3390/pathogens9121057

Interpretive Summary: Pacific oysters are one of the most productive aquaculture species in the world. The oyster aquaculture industry is threatened by the spread of the viral disease Ostreid herpesvirus-1 (OsHV-1) and its microvariants (known as µvars) which have caused mass mortalities in many areas of the world. While breeding programs elsewhere have been successful in developing disease resistant oysters and reducing losses, an OsHV-1 resistant oyster line does not yet exist in the United States. It is unknown how OsHV-1 µvars will affect US oyster populations compared to an OSHV-1 reference strain that has been present only in a narrow range of locations in northern California for three decades. As part of an effort to develop resistant US west coast oyster stocks, researchers in this study examined how several families of oysters responded to three variants of OsHV-1: the California reference OsHV-1, an Australian µvar, and a French µvar. This is the first study to directly compare the effects of these three viral strains. Oyster survival was significantly lower when exposed to the French and Australian µvars (43% and 72%, respectively) than to the reference strain which did not differ from controls. No oyster family demonstrated resistance to all three strains, and many surviving oysters contained very high levels of the virus. These results indicate that the introduction of OsHV-1 µvars could have devastating effects on US west coast populations, and also highlight the need to quantify infection in addition to survival as a response trait used to develop disease resistant stocks and protect against spread of OsHV-1 variants.

Technical Abstract: Pacific oysters, Crassostrea gigas, are one of the most productive aquaculture species in the world. However, they are threatened by the spread of Ostreid herpesvirus-1 (OsHV-1) and its microvariants (collectively “µvars”), which cause mass mortalities in all life stages of Pacific oysters globally. Breeding programs have been successful in reducing mortality due to OsHV-1 variants following viral outbreaks; however, an OsHV-1 resistant oyster line does not yet exist in the United States (US), and it is unknown how OsHV-1 µvars will affect US oyster populations compared to the current variant, similar to OsHV-1 reference found in Tomales Bay, CA. The goals of this study were to investigate the resistance of C. gigas spat produced by the Molluscan Broodstock Program (MBP) to three variants of OsHV-1: a California reference OsHV-1, an Australian µvar, and a French µvar. This is the first study to directly compare OsHV-1 µvars to a non-µvar. Survival probability of oysters exposed to the FRA or AUS µvar was significantly lower (43% and 72%, respectively) than to the non-µvar and controls (95%, p<0.05). No oyster family demonstrated resistance to all three OsHV-1 variants, and many surviving oysters contained high copy numbers of viral DNA (average 3.33x108). These results indicate that the introduction of OsHV-1 µvars could have devastating effects on US Pacific oyster aquaculture, and highlight the need to consider resistance to infection in addition to just survival as traits in breeding programs to reduce risk of spread of OsHV-1 variants.