Report on the occurrence of brown ring disease (BRD) in Manila clam, Ruditapes philippinarum, on the west coast of Korea

This is the first report on the occurrence of Vibrio tapetis, the brown ring disease (BRD) agent in Manila clam, Ruditapes philippinarum, in Asian waters. Brown conchiolin deposits, a distinct sign of BRD, were observed on the inner shells of clams collected from Anmyeondo Island, on the west coast of Korea. The infection intensity, based on the appearance of conchiolin deposits, ranged from 1–2, indicating that BRD of clams is in the initial phase. There was a negative correlation between the level of BRD and the condition index of the clams (P < 0.05). Extrapallial fluid from infected clams was collected and incubated in marine broth at 18 °C for 20 h to obtain genomic DNA of the BRD agent. Vibrio tapetis-specific 16S rRNA was amplified by polymerase chain reaction using a V. tapetis-specific primer pair. The DNA sequences of the BRD agent isolated in this study showed 99.0–99.3% similarity to V. tapetis reported from France and Norway, suggesting that the causative agent of BRD observed in this study was V. tapetis or taxonomically close to V. tapetis reported in Europe.