Asymptomatically Infected Broodstock Are a Potential Infection Source for Aquareovirus Outbreaks in Hatchery-reared Japanese Flounder Paralichthys Olivaceus

Aquareovirus infection by hirame aquareovirus (HAqRV) is fatal in hatchery-reared Japanese flounder Paralichthys olivaceus. In the present study, we investigated the transmission route of HAqRV in order to develop outbreak prevention measures. HAqRV was isolated from six cases of Japanese founder juveniles from different locations in 2015 and 2016 and compared by molecular epidemiology. For three of these cases, apparently healthy broodstock that were used for Japanese flounder production were subjected to virus carrier tests using real-time PCR and ELISA targeting the viral genome and antibody against HAqRV, respectively. The viral genome was detected in the intestine and/or liver, with a detection rate of approximately 60% (n = 143). For the ELISA test, antibodies against HAqRV were detected in 100% of the examined broodstock (n = 109). Sequencing analysis of the RNA-dependent RNA polymerase gene indicated that the nucleotide sequences of the isolated virus, diseased juveniles, and apparently healthy adults were identical within the same case, but distinct among different cases. These results suggest that asymptomatically infected broodstock are a potential infection source of HAqRV outbreaks in Japanese flounder juveniles.