Ostreid herpes virus 1 infection in families of the Pacific oyster, Crassostrea gigas, during a summer mortality outbreak: Differences in viral DNA detection and quantification using real-time PCR

Abstract

Ostreid herpes virus 1 (OsHV-1) infections, notably reported in Europe and the USA, are closely associated with significant mortalities of the Pacific oyster, Crassostrea gigas, especially during its early stages of life. In summer 2006, we monitored mortality by strict daily verification of three full-sib families of oysters reared under common conditions. We quantified OsHV-1 using real-time PCR in dead and living individuals during and after a mortality event. Mortality events were severe and brief, but significantly different between tested families (cumulative mortality ranging from 1.2 to 49%). Real-time PCR assays revealed different viral DNA loads in dead individuals from different families (P<0.001). Moreover, the mean level of infection among families was correlated with mortality (P<0.05). Living oysters showed a significantly lower amount of viral DNA compared with dead ones. This is the first experiment showing the daily changes of individual OsHV-1 DNA load during a mortality outbreak. Our results also support the previously reported high genetic basis underlying the variance of resistance of Pacific oyster to summer mortality, suggesting that there might be a possibility to improve resistance to OsHV-1 by selective breeding.

Experimental infections of Pacific oyster Crassostrea gigas using the Australian ostreid herpesvirus-1 (OsHV-1) µVar strain

“…This result is also in accordance with previous studies using the IM injection method and conducted on 7 to 12 mo old Pacific oysters (Schikorski et al 2011a, Segarra et al 2014. No mortality was observed at 24 hpi, which might be due to the time needed for viral replication in oyster tissues leading to irreversible cell damage and oyster mortality, as suggested by Sauvage et al (2009). Segarra et al (2014) detected the presence of OsHV-1 mRNAs in spat tissues as early as 2, 4 and 18 hpi, suggesting that the virus starts replicating in the early hours of the infection (although the first mortalities were not observed until 42 hpi).…”

Identification and characterisation of an ostreid herpesvirus-1 microvariant (OsHV-1 µ-var) in Crassostrea gigas (Pacific oysters) in Australia

“…While field observations and experimental transmission studies have demonstrated that the presence of herpesvirus and a water temperature >16°C are sufficient for viral transmission and disease induction (Hine et al 1998, Pernet et al 2012), other stress factors are believed to play a role during severe mortality events in Crassostrea gigas, often referred to as 'summer mortality' (Gagnaire et al 2006, Sauvage et al 2009). These stressors include toxins, biotoxins from certain species of phytoplankton (Sauvage et al 2009, Botner et al 2010, pathogenic Vibrio spp.…”

“…These stressors include toxins, biotoxins from certain species of phytoplankton (Sauvage et al 2009, Botner et al 2010, pathogenic Vibrio spp. (Lacoste et al 2001, Le Roux et al 2002, Li et al 2009, Botner et al 2010, Pernet et al 2012) and host metabolic factors related to spawning (Gagnaire et al 2006, Li et al 2009).…”

“…Several studies conducted in France (Le Deuff et al 1996, Sauvage et al 2009), Ireland (Malham et al 2009, Cotter et al 2010) and the United States (Friedman et al 2005, Burge et al 2006) strongly suggest a role for environmental stress in OsHV outbreaks. Specifically, OsHV outbreaks appear to be related to both increases in water temperature (>16°C) and intensive farming (Pernet et al 2012).…”

“…Mortality events in which OsHV has been definitively identified as the causative agent have previously been confined in the northern hemisphere, with outbreaks in Europe (da Silva et al 2008, Sauvage et al 2009, Segarra et al 2010, the United States (Friedman et al 2005, Burge et al 2006 and Mexico (Vásquez-Yeomans et al 2010). Herpes-like virus particles have been observed in moribund Crassostrea gigas and Ostrea chilensis in New Zealand (Hine et al 1992(Hine et al , 1998, in Bonamia-infected O. angasi (Hine & Thorne 1997) and in hatcheryreared Saccostrea glomerata larvae experiencing mortalities (M. Hine pers.…”