Ostreid herpesvirus 1 latent infection and reactivation in adult Pacific oysters, Crassostrea gigas

Ostreid herpesvirus 1 (OsHV-1) is one of the most economically important pathogens of Pacific oysters. Un-derstanding the pathogenesis of this virus is critical to developing tools to control outbreaks on shellfish farms. OsHV-1 is genetically related to vertebrate herpesviruses, which have a lytic and a latent stage, with the latent stage capable of being reactivated to the lytic stage. Here, OsHV-1 latency in Pacific oysters was investigated in experimentally and naturally infected oysters. Lytic infection in one-year-old oysters injected with the Tomales Bay strain of OsHV-1 was detectable between 1 and 4 days post-injection (dpi) but was not detectable after 5 dpi. The injected oysters shed 1 x 102 to 1 x 104 DNA copies/ml into the water during the 4-day acute phase. Lytic shedding was not detectable in two-year-old oysters injected similarly with the same strain of OsHV-1; however, the OsHV-1 genome was detectable by qPCR in the adductor muscle, gill, mantle, and hemocytes within the first 3 dpi, after which it became undetectable. No OsHV-1 was detectable in the adductor muscle, gill, or mantle from experimentally infected oysters on days 15 and 21 post-injection or from oysters sampled 9 months after sur-viving an OsHV-1 mortality event; however, OsHV-1 DNA could be detected in hemocytes of both experimentally infected oysters at 21 dpi and naturally infected oysters using nested PCR. In addition, lytic viral gene tran-scription was detectable in hemocytes of experimentally infected oysters between 1 and 21 dpi and in hemocytes of naturally infected oysters. Furthermore, OsHV-1 reactivation from latency was induced in experimentally infected oysters at 21 dpi and in naturally infected oysters 12 months after an OsHV-1 outbreak.