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

Abstract

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.