Simplified Techniques for Killing the Carcinogenic, Opisthorchis Viverrini Metacercariae in Cyprinid Fish

Consumption of fluke-free fish is the most important factor in controlling Opisthorchis viverrini (OV) infection in endemic areas such as northeast Thailand and thereby reducing the risk of cholangiocarcinoma. Cooking fish is the best way to avoid infection; however, the cultural practice of eating raw or fermented fish is difficult to change. We investigated the food preparation process, using freezing, heating and fermentation to kill OV metacercariae in fish. Uncooked cyprinid fish infected with OV were divided into three groups: refrigerated at 4 °C for 24, 48 or 72 h (control group); frozen at -20 °C for 24, 48 or 72 h; or heated by microwaving (at 400 or 800 W) or boiling at 90 °C for 1, 5 or 10 min. Moreover, pickled (fermented) fish were divided into two groups: refrigerated at 4 °C (control) or frozen at -20 °C for 24 or 48 h. The infectivity of recovered metacercariae was confirmed by infecting hamsters with OV and then evaluating the recovery of adult worms after 1 month. We found that a heating process, by boiling or microwaving at 400 or 800 W for at least 5 min, could kill OV metacercariae, and freezing pickled fish at -20 °C for 48 h could kill OV metacercariae in all sizes of fish. The present study found that heating and freezing processes, as well as the fermentation process under optimal conditions, could kill OV metacercariae in a timely manner. This knowledge is valuable for implementation in endemic areas to control OV infection and cholangiocarcinoma.