Ostreid herpesvirus 1 detection and relationship with Crassostrea gigas spat mortality in France between 1998 and 2006

Abstract

Since its molecular characterisation, Ostreid herpesvirus 1 (OsHV-1) has been regularly detected in Crassostrea gigas in France. Although its pathogenicity was demonstrated on larval stages, its involvement during mortality outbreaks at the juvenile stage was highly suspected but not evidenced. To investigate mortality outbreaks, the French National Network for Surveillance and Monitoring of Mollusc Health (REPAMO) carried out two surveys in juvenile C. gigas. The first survey lasted from 1998 to 2006 and was an epidemiological inquiry occurring when oyster farmers reported mortality outbreaks. The second survey, a longitudinal one, was set up in 1998 to complete the network observations on OsHV-1. Data analysis showed a specific pattern of mortality outbreaks associated with OsHV-1 detection. Ostreid herpesvirus 1 detection mainly appeared during the summer, suggesting the influence of the seawater temperature on its occurrence. It mostly presented a patchy distribution in the field in contrast to the nursery. Significant relationship between OsHV-1 detection and spat mortality was found, preferentially in sheltered and closed environments. The longitudinal survey confirmed most of the network observations. Although subsequent works particularly epidemiological surveys would be useful to confirm the causal link between the detection of OsHV-1 and the mortality outbreaks in juvenile C. gigas, the role of OsHV-1 in oyster mortality is progressing.

Antiviral immunity in marine molluscs

“…Less is known regarding the activity of AMPs against molluscan viruses. Investigations into the antiviral activity of molluscan AMPs and tissue homogenates is hampered by the lack of continuous cell lines from marine molluscs (Yoshino et al, 2013) and the fact that OsHV-1 cannot be cultured in primary cell cultures from bivalves (details in Garcia et al, 2011). Numerous studies have therefore utilized a heterologous model involving HSV-1 and African green monkey kidney (Vero) cells to identify antiviral compounds in tissue homogenates from commercially important bivalves (Carriel-Gomes et al, 2006;Defer et al, 2009;Green et al, 2014c;Olicard et al, 2005a, b;Segarra et al, 2014a;Zeng et al, 2008) and gastropods (Dang et al, 2011;Zanjani et al, 2014).…”

“…The reference (OsHV-1 ref) and variant (OsHV-1 var.) genotypes were associated with sporadic mortality events of Crassostrea gigas larvae and spat (oysters less than 1 year old) (Garcia et al, 2011). From 2008, Crassostrea gigas mortalities greatly increased on the French coast and spread to other European countries (Renault et al, 2012).…”

“…Very few studies have investigated the antiviral responses of molluscs (reviewed by Loker et al, 2004). Routine techniques used in virology and immunobiology are complicated by the absence of continuous cell lines for marine molluscs (Yoshino et al, 2013) and the fact that marine viruses cannot be propagated in the freshwater pond snail (Biomphalaria glabrata) embryonic cell line (Bge) or in molluscan primary cell cultures (further details in Garcia et al, 2011). Therefore, knowledge gaps remain regarding the antiviral responses of molluscs.…”

Oyster mortality

“…The reference type OsHV-1 was implicated in larvae mortalities and was known to interfere with spat mortalities Friedman et al, 2005;Burge et al, 2006;EFSA, 2010;Garcia et al, 2011). Assuming a role of OsHV-1 in oyster mortality is both plausible and coherent with existing biological knowledge and can be fully explained using well-established scientific concepts of biology and virology.…”

“…For this study, the surveillance strategy was essentially based on mandatory notification of observed mortality by shellfish farmers (passive surveillance) to the Competent Authority (European Directive 2006/88/EC). For the surveillance of oyster mortality, the French coastline was divided into 123 areas, defined according to hydrodynamic criteria, sedimentological criteria and mollusc presence and if a mortality notification is carried out in one of these areas, an investigation is performed and, if necessary, a sample is collected for laboratory tests (Francois et al, 2011).…”