Prior exposure to Ostreid herpesvirus 1 (OsHV-1) at 18 °C is associated with improved survival of juvenile Pacific oysters (Crassostrea gigas) following challenge at 22 °C

The high mortality and economic loss in farmed Pacific oysters (Crassostrea gigas) caused by Ostreid herpesvirus 1 (OsHV-1) has spurred research into strategies to mitigate the impact of the disease. Water temperature strongly influences the outcome of exposure with 14 degrees C not being permissive for an epidemic, infection occurring with limited mortality at 18 degrees C if there is a high viral dose, and full disease expression occurring at 22 degrees C. Observations in the field suggest improved survival to subsequent exposures in cohorts of oysters that have survived an OsHV-1 outbreak, but this assessment is complicated by the increasing age of the oysters, different viral exposures, different environmental conditions and the removal of naturally susceptible individuals. A laboratory infection model was used to evaluate the effect on survival of two temporally-spaced challenges with OsHV-1 under different water temperature regimes. Triploid Pacific oyster spat were recruited from commercial farm stocks at 6 months of age and exposed to OsHV-1 by intramuscular injection at 18 degrees C or 22 degrees C and then re-exposed at either 18 degrees C or 22 degrees C. Mortality did not occur in oysters exposed at 18 degrees C or when the temperature was increased to 22 degrees C at 14 days post challenge. Oysters challenged with OsHV-1 at 22 degrees C were protected if pre-exposed to OsHV-1 at 18 degrees C (Hazard ratio: 0.22, 95% CI: 0.06-0.80). The present study suggests that non-lethal exposure to OsHV-1 can reduce the mortality on subsequent exposure to the virus, but further experiments are required to investigate the duration of the protective response and its effect in different environments. Such protection may help describe the dynamics of seasonally recurrent disease epidemics and provide a novel approach to disease management.