An investigation of ostreid herpes virus microvariants found in Crassostrea gigas oyster producing bays in Ireland

The variant mu Var of OsHV-1 has been identified as the causative agent of an emerging disease in Europe affecting Crassostrea gigas oysters which has caused significant mortalities in C. gigas oyster stocks in France, Ireland, the UK, Spain, the Netherlands and Italy. Microvariants presenting some differences to the variant mu Var have also been associated with mortalities in Australia and New Zealand. This study examined 30 Crassostrea gigas production bays in Ireland for the presence of Ostreid herpesvirus-1 (OsHV-1 and variants) between 2003 and 2012. A retrospective study conducted on archive Formalin Fixed Paraffin Embedded(FFPE) C. gigas material associated with mortality outbreaks between 2003 and 2008, found no evidence of the OsHV-1 reference type. Molecular analysis using a locked nucleic acid (LNA) real-time PCR specific for the identification of variants presenting the deletion in the microsatellite locus upstream of the ORF4 present in the variant mu Var of OsHV-1 found one FFPE sample to be positive from a cohort of C. gigas oysters collected in 2007. Following reports of significant mortalities in three bays in 2008, the variant of OsHV-1, interpreted as the variant mu Var was confirmed by PCR and sequence analysis. In 2009, 16 bays had confirmed mortality events associated with the variant interpreted as the variant mu Var, and in a separate site the OsHV-1 reference type was also confirmed. By 2012, 29 bays around the Irish coastline were confirmed positive for the presence of the variant interpreted as the variant mu Var of OsHV-1. Although the variant mu Var was found to be the predominant genotype detected in 29 bays of the 30 bays in the study, co-existence with another genotype close to the variant mu Var (with guanidine insertion) was also observed in 6 of these 29 bays. Only one sample consistent with the OsHV-1 reference type (HQ842610) was identified in one oyster producing bay in Ireland in 2009. (C) 2015 Elsevier B.V. All rights reserved.