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 mu Var) 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 controlled laboratory conditions by intramuscular injection of OsHV-1 mu Var after acclimation of 8 mo old spat and 17 mo old adult oysters at 4 different temperatures (14, 18, 22 and 26 degrees C). Mortality was 84 and 77% at 26 and 22 degrees C, respectively, compared to 23% at 18 degrees C and nil at 14 degrees C. There was a statistically significant interaction between the dose of OsHV-1 mu Var and water temperature. At 18 degrees C, mortality occurred exclusively at a dose of 10(6) OsHV-1 mu Var genome copies per oyster whereas at the higher temperatures, oysters challenged with 10(3) copies per oyster also died. Mortality did not occur at 14 degrees C and OsHV-1 mu Var 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 disease caused by OsHV-1 mu Var. We identified a threshold water temperature of between 14 and 18 degrees C below which productive infection does not occur and the requirement for a higher dose of OsHV-1 mu Var to initiate infection at 18 degrees C than at 22 degrees C. These results have implications for predicting and managing disease outbreaks caused by OsHV-1 mu Var.