The eggs of the Pacific oyster, Crassostraea gigas, become infertile when infected by the parasite Marteilioides chungmuensis. Histologically, M. chungmuensis infects the oyster oocyte cytoplasm, and the ovaries take on a "lumpy" appearance once infected, which lowers commercial value of the oyster. This has a negative economic impact on oyster farms in South Korea and Japan. In this study, we compared traditional diagnostic methods (histology) with two molecular-based methods (polymerase chain reaction [PCR] amplification and in situ hybridization [ISH]) to identify M. chungmuensis-infected oysters. The efficacy of PCR and ISH to identify M. chungmuensis-infected oysters was compared to that of routine histology in 100 oysters. Thirty infections were identified using PCR and 16 using histology, whereas 31 infections were identified using ISH. The ISH and PCR assays were more sensitive compared to using histology with standard epidemiological methods. We strongly recommend that early parasitic invasion should be monitored with PCR/ISH methodologies as a basis for developing effective diagnostic techniques to identify M. chungmuensis-infected oysters.
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