Tohru Mekata

One hundred sixty-four Edwardsiella specimens isolated from cultured fishes, red seabream Pagrus major, Japanese flounder Paralichthys olivaceus, thread-sail filefish Stephanolepis cirrhifer, greater amberjack Seriola dumerili and Japanese eel Anguilla japonica were analyzed by PCR for identification, and 124 of these isolates were examined using biochemical tests for characterization. Hundred and four strains from red seabream, three from Japanese flounder, and six from other fish species were identified as E. anguillarum, whereas one strain from seabream and 50 strains from flounder were identified as E. piscicida. Among the E. anguillarum strains, all strains from Japanese eel were motile and ornithine decarboxylase (ODC)-positive, whereas all strains from other fish species were non-motile and ODC-negative. The findings of this study reveal the existence of two distinct types of E. anguillarum differing in motility and ODC production. Susceptibility testing using nine antimicrobials (tetracycline, erythromycin, lincomycin, ampicillin, sulfamonomethoxine, florfenicol, thiamphenicol, oxolinic acid, and fosfomycin) showed that all strains were sensitive to fosfomycin, which had a relatively narrow range of minimum inhibitory concentrations compared to other drugs.

The Akoya pearl oyster (Pinctada fucata (Gould)) is the most important species for pearl cultivation in Japan. Mass mortality of 0-year-old juvenile oysters and anomalies in adults, known as summer atrophy, have been observed in major pearl farming areas during the season when seawater temperatures exceed about 20 °C since 2019. In this study, we identified a novel birnavirus as the pathogen of summer atrophy and named it Pinctada birnavirus (PiBV). PiBV was first presumed to be the causative agent when it was detected specifically and frequently in the infected oysters in a comparative metatranscriptomics of experimentally infected and healthy pearl oysters. Subsequently, the symptoms of summer atrophy were reproduced by infection tests using purified PiBV. Infection of juvenile oysters with PiBV resulted in an increase in the PiBV genome followed by the atrophy of soft body and subsequent mortality. Immunostaining with a mouse antiserum against a recombinant PiBV protein showed that the virus antigen was localized mainly in the epithelial cells on the outer surface of the mantle. Although the phylogenetic analysis using maximum likelihood method placed PiBV at the root of the genus Entomobirnavirus, the identity of the bi-segmented, genomic RNA to that of known birnaviruses at the full-length amino acid level was low, suggesting that PiBV forms a new genus. The discovery of PiBV will be the basis for research to control this emerging disease.

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.

Carp edema virus disease (CEVD), or koi sleepy disease, is caused by CEV. Here, we report the complete genome sequence of CEV strain FTI2020, isolated from koi carp. This sequence information has great potential for improving our understanding of the genetic characteristics of this piscine poxvirus.