Abstract
Since 2008, mass mortalities of 1-yr-old Crassostrea gigas associated with the ostreid herpesvirus OsHV-1 μVar have occurred along all the coasts of France when seawater temperature reaches 16 to 17°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°C for 40 d and then at 20.6°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°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°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°C. Our results suggest that a long-term period (40 d) at low temperature (13°C) may offer a method of mitigating mortalities in oysters that have been exposed to an infective environment.
Highlights
Summer mortalities of oysters have been reported for decades in many countries throughout the world, but no strict or single pathological causal factor has been found
Our study suggests that healthy oysters put in cohabitation with oysters that have high levels of OsHV-I DNA μVar and vibrios became infected by these pathogens, as previously reported in other experiments with ostreid herpesvirus 1 (OsHV-1) (Schikorski et al 2011b) or vibriosis (De Decker & Saulnier 2011)
The limitation of our method is that we cannot ascertain whether the observed effect of temperature on survival of exposed oysters and oysters that had been kept in cohabitation with them is associated with a sole pathogen, such as OsHV-1 μVar, because other pathogens may have been contracted
Summary
Summer mortalities of oysters have been reported for decades in many countries throughout the world, but no strict or single pathological causal factor has been found (reviewed in Samain & McCombie 2008). These abnormal mortalities (affecting > 30% of the cultivated population) occur during summer months, when oysters are in sexual maturation. (
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