Temperature influence on pathogen transmission and subsequent mortalities in juvenile Pacific oysters Crassostrea gigas

Since 2008, mass mortalities of 1-yr-old Crassostrea gigas associated with the ostreid herpesvirus OsHV-1 mu Var have occurred along all the coasts of France when seawater temperature reaches 16 to 17 degrees C. The present study aimed to characterize the effect of temperature on oyster survival in combination with OsHV-1 DNA quantification by standard real-time PCR and total vibrio population levels in oyster tissues. To examine the effect of seawater temperature on disease transmission and related mortality of oysters, cohabitation experiments were conducted between healthy naive oysters and oysters previously exposed to field conditions in areas where mortalities were occurring. Oysters initially maintained in controlled conditions (free of mortality and negative for OsHV-1), and then transferred to an area where high mortalities were occurring among farmed stocks, became infected with OsHV-1 and exhibited high loads of vibrios followed by significant mortalities. When previously exposed oysters were maintained indoors at 13.0 degrees C for 40 d and then at 20.6 degrees C, they exhibited no mortality, were negative for OsHV-1 detection, and did not transmit the disease to healthy oysters. Survival of previously exposed oysters maintained indoors at 8 temperatures ranging from 13.4 to 29.0 degrees C varied from 25 to 48% and was negatively correlated with holding temperature. Concomitantly, survival of naive cohabiting animals (62 to 98%) decreased with increasing seawater temperature until a plateau was reached between 16.2 and 21.9 degrees C, and increased at higher temperatures. Therefore, the optimal temperature range for disease transmission from field-exposed to naive animals was between 16.2 and 21.9 degrees C. Our results suggest that a long-term period (40 d) at low temperature (13 degrees C) may offer a method of mitigating mortalities in oysters that have been exposed to an infective environment.