Seasonal reactivation enables Cyprinid herpesvirus 3 to persist in a wild host population.

Emerging infectious diseases are of growing concern in wildlife conservation and animal health. To better understand the consequences of these diseases, a key question lies in how they persist in host populations after they emerge. Using a gene expression approach, we investigated the mechanisms underlying the persistence of an emerging virus, Cyprinid herpesvirus 3 (CyHV-3), which has been spreading to wild populations of common carp (Cyprinus carpio) in Japan since 2003. Seasonal expression patterns of CyHV-3 genes in wild seropositive carp indicated that replication-related genes were transcribed only during the spring when water temperatures were permissive to CyHV-3 replication. In contrast, possible latency-related genes, which are expressed when CyHV-3 do not multiply, were also transcribed under nonpermissive conditions. These observations suggest that CyHV-3 may persist in carriers by establishing latent infection and then reactivating periodically coincident with the spring temperature increase when carp aggregate for mating, allowing successive virus transmissions between hosts during mating every year. Our results revealed that the life cycle of CyHV-3 may fit perfectly into the ecology of its host, resulting in the long-term persistence of this emerging virus in wild common carp populations.