Mortalities of oyster seed of Crassostrea gigas associated with ostreid herpes virus OsHV-1 mu Var have been observed in many oyster producing countries since 2008. The objective of this study was to further investigate whether low temperature treatments can offer a viable option to mitigate oyster mortalities. An experiment was set-up to further evaluate the effect of low temperature treatments (10 and 13 degrees C vs. 21 degrees C) and their duration (6 d to 83 d) on the survival of oysters previously infected with OsHV-1 mu Var by means of exposure to field conditions in areas where mortalities were occurring. Average survival of oysters infected with OsHV1-mu Var was 71% after 83 d at low temperatures compared to only 23% in controls maintained at 21 degrees C. During cold-exposure, levels of OsHV-1 DNA in oyster tissues gradually decreased, down to nearly the detection limit after 24 d. However, when cold-acclimated oysters were suddenly exposed at 21 degrees C in the laboratory, they exhibited high levels of mortality along with an enhancement of OsHV-1 DNA concentration in their tissues. Therefore, OsHV-1 persists in oysters even at low temperature and is reactivated during subsequent thermal elevation to 21 degrees C. Low temperature treatments did not improve overall survival of oyster seed infected with OsHV-1. These results suggest that moving infected oysters to a cooler area only delays mortality and may increase the risk of infection in neighbouring stocks when rising temperatures become permissive for viral replication. Statement of relevance: This study is relevant for building disease control measures.