Nocardia seriolae infection in the cultured eel Anguilla japonica in Korea.

Mass mortality occurred at an Anguilla japonica eel farm equipped with a recirculating aquaculture system in Gimcheon, Korea, from late spring to early summer 2015. The cumulative 3-month mortality was 16% (approximately 24,300-150,000 fish). The majority of affected fish displayed ulcerative lesions that progressed to petechial haemorrhages and small white granulomas in the major organs. A Gram-positive, acid-fast, nonmotile bacterium was isolated from internal organ lesions. Phylogenetic analysis of 16S rRNA identified the species as Nocardia seriolae and the strain was designated EM150506. Afterwards, naïve eels were injected with 1.8 × 107 colony-forming units per fish to confirm the strain's pathogenicity, which resulted in a 20% mortality rate within 4 weeks. However, surviving fish still exhibited white N. seriolae colonies in internal organs. To our knowledge, this is the first report of a N. seriolae infection in cultured eel.

Development of PCR method for detecting Kudoa iwatai (Myxozoa: Multivalvulida) from rock bream Oplegnathus fasciatus.

We developed a PCR assay targeting the 28S rDNA of Kudoa iwatai (Multivalvulida: Myxozoa) and investigated the prevalence of infection in rock bream Oplegnathus fasciatus, which is commercially an important aquaculture species in Korea, with this assay. Detection limit of the PCR assay was 2.5 fg/µl with plasmid DNA and 8.6 × 103 spores/ml with purified spores, respectively. This PCR assay did not amplify DNA of other Kudoa species (Kudoa septempunctata, Kudoa lateolabracis, Kudoa thyrsites) tested. Sliced muscles of whole body from 318 rock bream (wild and cultured) were examined by this PCR assay and also with the naked eyes. All of the wild fish did not produce amplicons nor did harbor visible Kudoa cysts (0/70). Three of the cultured fish were PCR-positive and also harbored visible Kudoa cysts (3/248, 1.2%). The sequences of amplicons (574 bp) were 100% identical with those of the K. iwatai already registered in Genbank. When the visceral organs of these three fish were examined, visible cysts were not found, but one stomach sample was found to be PCR-positive. There was no difference in the prevalence of infection estimated by PCR assay and the presence of visible Kudoa cysts in our samples. This is thought to be because the development of K. iwatai is already completed and only mature Kudoa cysts existed in our samples.