Insight into OsHV-1 persistence phase in Pacific oyster spat

The ostreid herpesvirus type 1 (OsHV-1) is a major pathogen of the Pacific oyster (Magallana gigas), causing recurrent episodes of spat mortality worldwide. While OsHV-1 replication and associated outbreaks occur at moderate seawater temperatures (16-22 °C), the virus becomes largely undetectable during colder months, raising the question of how it persists between epidemic episodes. In this study, we investigated the potential of Pacific oyster spat to act as reservoirs of OsHV-1. Our approach combined two-year field monitoring in a major French production area (Marennes-Oléron Bay, France) with controlled cohabitation assays, molecular detection by qPCR and ultra-deep DNA/RNA sequencing. Our results show that OsHV-1 can persist in asymptomatic spat at seawater temperatures below 13 °C, and can reactivate into a lytic phase once temperatures rise and remain above 16 °C. Sequencing analyses confirmed the presence of viral genomic fragments, but revealed extremely limited transcriptional activity. These findings support the hypothesis that OsHV-1 establishes a latent state in Pacific oyster spat, with a non-integrated viral genome during the persistent phase, and that early cohorts play a particularly important role in this process. They also emphasise the necessity of targeted molecular approaches to fully elucidate the mechanisms underlying herpesvirus persistence in marine invertebrates.