Since summer 2008, high mortality rates of young Pacific oysters Crassostrea gigas have been recorded in association with the detection of the Ostreid Herpesvirus 1 (OsHV-1). A new variant called μVar has been recently described, characterized mainly by 12 consecutive deletions followed by one deletion of an adenine in the C region. The purpose of this study is to characterize the genotype (variants or OsHV-1 reference) of 300 positive samples of C. gigas analyzed between July 2008 and July 2010 collected along the French, Jersey, and Irish coasts. Samples were quantified by TaqMan PCR, amplified with conventional PCR, targeting the area of the deletion, and then sequenced. Eighty-seven percent of the samples were characterized and the OsHV-1 μVar was detected in 257 oyster samples. The genotype OsHV-1 reference was never detected during the 25 months of the present survey. Thirty-eight samples could not be determined and the majority of them had a low viral load. A novel genotype containing only 9 consecutive deletions named OsHV-1 μVar Δ9 was found in 5 samples. These observations indicate the emergence of different OsHV-1 variants.
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