Organism The trematode fluke Opisthorchis sinensis (previously called Clonorchis).
Clinical features The adult fluke lives in the branches of the bile duct resulting in trauma and inflammation. Dilation of the biliary system causes a distortion of the liver architecture, which can lead to biliary stasis, hepatic engorgement, fatty infiltration and finally cirrhosis. O. sinensis is a risk factor for cholangiocarcinoma. Migration of the flukes up the pancreatic duct can damage the pancreas leading to recurrent pancreatitis.
Diagnosis The small operculated egg is found on faecal examination (Fig. 9.1).
Transmission Humans are infected by eating raw fish, which includes pickled, smoked or undercooked fish. Eggs passed in the faeces develop into miracidia, which are swallowed by snails of the genus Bulimus, Bi-thynia or Parafossarulus. These pass through the sporocyst and redia stages in the snail and produce free-swimming cercaria. Seeking out a suitable fish, cercaria penetrate between the scales and encyst in the flesh. The parasite also attacks dogs, cats, rats and pigs, which form reservoirs of infection (Fig. 9.3).
Period of communicability Eggs may be passed for as long as 30 years, but reservoir animals are also an important source of human infection.
Occurrence and distribution Distribution is very similar to Fasciolopsis, being found in Incubation period Approximately 4 weeks. China, Japan, Korea, Taiwan, Thailand,
Laos, Cambodia and Vietnam (lower Mekong valley). Some 30 million people suffer from the disease.
Control and prevention Control is by the proper cooking of fish. Members of the carp family (Cyprinidae), the so-called 'milk fish', are eaten raw as a delicacy. They are grown in fish farms as part of a system of aquaculture, fertilized by human faeces. Regulation of this practice is required to reduce this unpleasant infection. Other foods, such as fish paste often added to food after it has been cooked to improve the taste, are made from raw fish and are a potent source of infection.
Treatment Treatment is with praziquantel 25 mg/kg three times a day for 1-2 days.
Surveillance When a case is identified, search should be made for the culprit food source.
There are a number of less common trematodes that have the same life cycle as O. sinensis (Fig. 9.3). O. viverrini is found in Thailand and Laos where raw fish paste is a favourite food additive. O. felineus occurs in Central and Eastern Europe, similarly causing disease of the liver. As suggested by its name, it is mainly a disease of cats, but humans can become infected. Heterophyes heterophyes and Metagonimus yokogawai, found in Asia and the Far East, do not attack the liver, but remain in the intestines. The eggs of all of these flukes are very similar (Fig. 9.1).
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