Occurrence of Ovary Enlargement Disease of Pacific Oyster by Introduction of Infected Spat in Eastern Hokkaido, Japan

Pacific oysters Crassostrea gigas infected with the ovarian protozoan parasite Marteilioides chungmuensis often develop ovary enlargement disease characterized by nodular-like ovaries, resulting in a loss of marketability due to their unattractive appearance. ​This disease was previously limited to oyster farms in western Japan, but recently a new occurrence was recognized in an open water oyster farm in eastern Hokkaido, the northernmost region of Japan. ​In this study, we developed a quantitative real-time PCR to detect the causative parasite and conducted an epidemiological survey in the oyster farm from April 2021 to March 2022. ​Prior to introduction to the farm, parasite DNA was detected in oysters originating from Miyagi Prefecture (Miyagi oysters), then infection of the parasite was histologically confirmed, suggesting that Miyagi oysters were already infected with the parasite before introduction to the oyster farm in Hokkaido, leading to the development of the disease. ​Conversely, in oysters produced in a local hatchery in Hokkaido (local oysters), parasite DNA was not detected prior to introduction to the farm, but detected in August and September after introduction. ​Although infection was not confirmed histologically in local oysters during the survey period, transmission of the parasite in the survey area needs to be further examined.