Chlonorchis, Fasciola, Opisthorchis and Paragonimus are the main trematodes that cause infections in humans. All of them are foodborne.
Human infection with Clonorchis sinensis is endemic in China, Taiwan, Hong Kong, Korea and the countries of southeast Asia. Eggs are passed with the bile into the faeces. After reaching freshwater, the eggs are ingested by aquatic snails. Larvae emerge from the snail and encyst in the muscles and underscales of many freshwater fish species and a few shrimp species. Human consumption of raw fish allows the parasite to reach the biliary tree of the liver. Cats, dogs, pigs and several other fish eating mammals are also susceptible for the infection and are able to maintain the infection. In South-Korea over 25% of the human population is infected in certain areas; in Vietnam 13.7%.
Light infections do not cause many manifestations of the disease in humans. When the infection is more intense and of longer duration, the patient may exhibit loss of appetite, diarrhoea and fever. The infection may result in obstruction of the bile duct and liver cirrhosis. The most important clinical consequence of liver fluke infection is chronic jaundice and eventually the higher occurrence of cholangiocarcinoma.
This disease is caused by two agents: Fasciola hepatica and Fasciola gigantica. F. hepatica is found in almost all temperate regions where sheep and other ruminants are raised. F. gigantica occurs mainly in tropical areas such as Africa, south and southeast Asia, southern Europe, Hawaii and also in the former USSR.
The eggs are excreted with the faeces of humans and other final hosts. They penetrate a snail in which they develop to cercariae. They abandon the snail and develop to metacercariae. The definitive hosts become infected by ingesting metacercariae along with plants or water. The definite hosts of F. gigantica are cattle, goats, zebras and sheep. The definitive hosts of F. hepatica are sheep and cattle. Humans can also serve as (accidental) definitive hosts of the two parasites. A few human outbreaks have been recorded, mostly related to watercress consumption. Recently a few foci with human-to-human transmission in South America have been discovered. The prevalence of Fasciola in certain areas of Bolivia was found to be over 15%. Symptoms in man are abdominal pain, diarrhoea, fever and chronic inflammation and obstruction of the bile duct.
The life cycle of Opisthorchus is similar to that of Clonorchis. Man acquires the infection by ingestion of raw fish. Other definitive hosts are civet, dogs, cats and other fish eating animals. The disease is endemic in southeast Asia, the Siberian lowlands and eastern Europe. In northeastern Thailand for example 18.5% of the population was infected in 1997. Light infections usually do not cause major health problems. In more heavy infections symptoms in humans are comparable to those in Clonorchiasis. The causal relationship with cholangiocarcinoma is more pronounced than in Clonorchiasis.
Eggs are shed by the definitive host via expectoration or in faeces if bronchial secretions are swallowed. They develop to larvae and invade snails. A transformation to cercariae, which finally abandon the snail, takes place. They seek a crustacean such as crab or crayfish in which they develop to an infectious stage for the definitive host. In the definitive host they invade the lungs. Humans become infected upon eating infected raw freshwater crabs or crayfish. Human cases occur very focal related to specific feeding habits. Other definitive hosts are animals like pigs, dogs, and a variety of feline species that eat crabs or crayfish. Human infections occur in Africa, the Americas and Asia. In several provinces of Thailand the infection rate was found to be 6.5%. The symptoms of Paragonimiasis in humans are tuberculosis-like and include fever, chronic cough and thoracic pain.
Prevention of fluke infection depends on avoidance of raw or undercooked food (meat, fish, crabs etc.). Treatment of fluke infections is relatively easy with Praziquantel and/or Triclabendazole.