Effect of water temperature on mortality of Pacific oysters Crassostrea gigas associated with microvariant ostreid herpesvirus 1 (OsHV-1 µVar)

The ostreid herpesvirus 1 microvariant (OsHV-1 µVar) causes mass mortality of Pacific oysters Crassostrea gigas. Water temperature can directly influence the incidence of disease or correlate with seasonal changes in the environment and oyster physiology that modify the susceptibility of the oysters to disease. The effect of water temperature was evaluated in con- trolled laboratory conditions by intramuscular injection of OsHV-1 µVar after acclimation of 8 mo old spat and 17 mo old adult oysters at 4 different temperatures (14, 18, 22 and 26°C). Mortality was 84 and 77% at 26 and 22°C, respectively, compared to 23% at 18°C and nil at 14°C. There was a statistically significant interaction between the dose of OsHV-1 µVar and water temperature. At 18°C, mortality occurred exclusively at a dose of 10 6 OsHV-1 µVar genome copies per oyster whereas at the higher temperatures, oysters challenged with 10 3 copies per oyster also died. Mor- tality did not occur at 14°C and OsHV-1 µVar was detected in tissues of only 1% of the oysters after 14 d. When accounting for temperature and dose, spat (8 mo) were 2.7 times more likely to die than adults (17 mo). Our study confirms a direct effect of water temperature on infection and dis- ease caused by OsHV-1 µVar. We identified a threshold water temperature of between 14 and 18°C below which productive infection does not occur and the requirement for a higher dose of OsHV-1 µVar to initiate infection at 18°C than at 22°C. These results have implications for predict- ing and managing disease outbreaks caused by OsHV-1 µVar.