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

•OsHV-1 type interpreted as the variant μVar detected in 29 Irish bays•Microvariant of OsHV-1 μvar was detected in 6 of the 29 OsHV-1μvar infected bays.•Examination of FFPE archive samples found variant interpreted as the variant μVar•Only 1 specimen consistent with the OsHV-1 reference type was identified in 2009.

The variant μVar 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 μVar 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 μVar 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 μVar was confirmed by PCR and sequence analysis.In 2009, 16 bays had confirmed mortality events associated with the variant interpreted as the variant μVar, 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 μVar of OsHV-1.Although the variant μVar 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 μVar (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.