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 oC for 24, 48 or 72 h (control group); frozen at -20 oC for 24, 48 or 72 h; or heated by microwaving (at 400 or 800 W) or boiling at 90 oC for 1, 5 or 10 min. Moreover, pickled (fermented) fish were divided into two groups: refrigerated at 4 oC (control) or frozen at -20 oC 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 oC 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.

In conclusion, heating at 70 and 80°C groups after 10, 15, and 30 min can kill O. viverrini eggs. This study suggests that destroying the Opisthorchis viverrini eggs in feces using heat is a possible approach for controlling egg contamination in the environment.

Microwaving at 400 or 800 W for at least 5 min, could kill OV metacercariae.

PATHOGENICITY/TOXICITY: There is consensus that the biology and pathologic characteristics of Opisthorchis and Clonorchis are the same, essentially low grade biliary tract pathogens.  The majority of Opisthorchis infections (around 80%) are light infections ^{Footnote1Footnote2}. Classified by less than 1000 eggs/g of feces, a light infection is usually asymptomatic except for eosinophilia ^Footnote2. With this stage, there is no noticeable damage to liver function ^Footnote2. A heavy infection is classified by 10,000-30,000 eggs/g of feces ^Footnote2. The clinical presentation of a heavy infection includes diarrhea, constipation, abdominal pain, anorexia, indigestion, gastrointestinal bleeding, lassitude, mild fever, jaundice, enlarged or non-functional gall bladder, cholecystitis, cholangitis, liver abscess, and gallstones ^{Footnote2Footnote3Footnote7}. Clinical presentation of untreated chronic infection includes oedema of the legs, ascites, mild cirrhosis, hepatomegaly, and biliary epithelium hyperplasia and inflammation ^Footnote2. Cholangiocarcinoma is strongly associated with O. viverrini infection and has a high mortality rate ^Footnote2. It usually develops 30-40 years after initial infection, and its victims usually die within 3-6 months ^{Footnote2Footnote8}.

EPIDEMIOLOGY: O. viverrini is endemic in Thailand, Laos, and Cambodia. O. felineus is reported in North Europe and Asia ^Footnote2. Infection is more common from September to February. In endemic areas, infection rates can be up to 90% in humans, and 97% in fish ^Footnote5. Infection is uncommon in children under 5 years of age. Prevalence of Opisthorchiasis is higher in areas that are poor or unsanitary ^{Footnote9Footnote10}.

HOST RANGE: Humans, snails, fish, cats, dogs, reptiles, amphibians, and fish-eating animals ^{Footnote4Footnote11-13}

INFECTIOUS DOSE: Possibly as low as 1-2 metacercariae ^Footnote8.

MODE OF TRANSMISSION: Opisthorchis spp. are transmitted when raw or undercooked fish containing metacercariae is ingested ^Footnote3.