Investigation of the latency of Cyprinid herpesvirus 2 in apparently healthy farmed gibel carp, Carassius auratus gibelio

Cyprinid herpesvirus 2 (CyHV-2) is a highly contagious pathogen that causes herpesviral haematopoietic necrosis (HVHN) disease to goldfish, Carassius auratus and gibel carp, C. auratus gibelio. In this study, the virus persistence or latency in apparently healthy farmed gibel carp and reactivation in vitro were investigated. All of the freshly dissected kidney tissues of 42 individuals of asymptomatic farmed gibel carp from six culture regions tested positive in PCR, indicating all fish were CyHV-2 carriers. The viral mRNA was detected in two of 42 fish: one fish expressed virus immediate-early (IE) gene ORF147 only, and another fish expressed IE gene ORF147, early (E) gene ORF35, and late (L) gene ORF72. Following the explant cultivation, the viral DNA copies in the kidney significantly increased in three fish including two individuals that expressed IE gene before explant cultivation. After explant cultivation, the expression of both the E and L genes was detected in the two fish in which active viral gene transcription was detected initially, and the infectious virus in those two fish was confirmed by an infection experiment. The study suggests CyHV-2 can establish persistent or latent infection in asymptomatic farmed gibel carp, and the different viral mRNA expression cascades were detected in different individuals. The persistent and latent virus can reactivate and be infectious, indicating that the apparently healthy gibel carp can be a source of infection.