We evaluated the prevalence of the akoya oyster disease (AOD) in Japan and the pathogenicity of the unidentified causative agent. We obtained naïve Japanese pearl oysters Pinctada fucata martensii from a site with no history of AOD. In separate experiments, the oysters were either challenged by intramuscular injection with the hemolymph of infected oysters or reared in several major pearl oyster farms throughout Japan. Both groups of oysters experienced high mortality and exhibited discoloration of the adductor muscle and histopathological changes characterizing AOD. Our data suggest that AOD is prevalent in pearl oyster culture fields throughout Japan and that the causative pathogen remains highly pathogenic to P. fucata martensii. When groups of oysters produced by repeated selective breeding against AOD were reared in different pearl oyster fields alongside uninfected naïve oysters, the selectively bred oysters had significantly lower mortality than naïve oysters. The recent decline in outbreaks of AOD can be attributed, in part, to the introduction of selectively bred oysters that are resistant to this disease.
