Differential Mortality and High Viral Load in Naive Pacific Oyster Families Exposed to OsHV-1 Suggests Tolerance Rather than Resistance to Infection.

M Victoria Agnew,Arun K Dhar,Benjamin Morga,Peter Kirkland,Colleen A Burge,Christopher Langdon,Blaine Schoolfield,Carolyn S Friedman,Konstantin Divilov,Lionel Degremont,Brett Dumbauld

doi:10.3390/pathogens9121057

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, which is similar to the OsHV-1 reference, found in Tomales Bay, CA. The goals of this study were to investigate the resistance of C. gigas juveniles 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. The survival probability of oysters exposed to the French (FRA) or Australian (AUS) µvar was significantly lower (43% and 71%, respectively) than to the reference variant and controls (96%). No oyster family demonstrated resistance to all three OsHV-1 variants, and many surviving oysters contained high copy numbers of viral DNA (mean ~3.53 × 108). These results indicate that the introduction of OsHV-1 µvars could have substantial effects on US Pacific oyster aquaculture if truly resistant lines are not achieved, and highlight the need to consider resistance to infection in addition to survival as traits in breeding programs to reduce the risk of the spread of OsHV-1 variants.

Highlights

Aquaculture provides a substantial contribution to food production throughout the world.Farming of aquatic species contributed over 80 million tons of food in 2017, resulting in a revenue of Pathogens 2020, 9, 1057; doi:10.3390/pathogens9121057 www.mdpi.com/journal/pathogens~USD 230 billion globally [1]
We examined the resistance of 20 oyster families to three variants of Ostreid herpesvirus-1 (OsHV-1): a French μvar, an Australian μvar, and a non-μvar from Tomales Bay, California
Our findings highlight the importance of the inclusion of qPCR viral loads as part of breeding programs focused on OsHV-1 resistance to identify tolerant oyster families, as increased survival does not always indicate resistance

Summary

Introduction

Aquaculture provides a substantial contribution to food production throughout the world. ~USD 230 billion globally [1]. In this lucrative market, the health of aquaculture species is vital to the success of the industry; many of these species are threatened by disease. Distinct periods of increase in marine disease have been reported for multiple species over the years, including mollusks [5,6], which make up ~21% of aquaculture production [1]. Understanding and managing the risk of disease for primary economic aquaculture species becomes very important for the future success of the aquaculture industry [10]

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