Aetiology of the disease is a Helminth, trematode (flatworm): Clonorchis sinensis i.e. the Chinese liver fluke.
It is a flattened worm, 10-25 mm long, 3-5mm wide, usually spatula-shaped, yellow-brown in colour (owing to bile staining); has an oral and a ventral sucker and is a hermaphrodite.
Eggs measure 20-30*15-17 micrometres, are operculate and are among the smallest trematode eggs to occur in man.
Symptoms
The incubation period varies with the number of warms present. Symptoms begin with the entry of immature flukes into the biliary system one month after encysted larvae (metacercaria) are ingested.
Most patients are asymptomatic but may have eosinophilia. Gradual onset of discomfort in the right upper quadrant, anorexia, indigestion, abdominal pain or distension and irregular bowel movement.
Patients with heavy infection experience weakness, weight loss, epigastric discomfort, abdominal fullness, diarrhoea, anaemia, and oedema. In later stages, jaundice, portal hypertension, ascites and upper gastrointestinal bleeding occur (Bean & Griffins, 1990).
An acute illness occasionally develops 2-3 weeks after initial exposure. Adult worms can live many years.
Source
Snails are the first intermediate host. Some 40 species of river fish serve as the second intermediate host. Humans, dogs, cast, and many other species of fish-eating mammals are definitive hosts.
Mode of transmission
People are infected by eating raw or under-processed freshwater fish contain encysted larvae (metacercaria). During digestion, the larvae are freed from the cysts and migrate via the common bile duct to biliary radicles.
Eggs in faeces contain fully developed miracidia; when ingested by a susceptible operculate snail, they hatch in its intestine, penetrate the tissues and asexually generate larvae (cercariae) that migrate into the water.
On contact with a second intermediate host, the cercariae penetrate the host and encyst, usually in muscle, occasionally on the undesirable of scales. The complete life cycle from person to snail to fish to person requires at least 3 months.
Control and Prevention
Safe disposal of sewage or wastewater to prevent contamination of rivers; treatment of wastewater used for aquaculture; irradiation of freshwater fish; freezing; heat treatment (canning); good hygiene practices during production and processing.
Thorough cooking of freshwater fish. Consumers should avoid consumption of raw or undercooked freshwater fish. Other measures include control of snails with molluscicides where feasible; drug treatment of the population to reduce the reservoir of infection; elimination of stray dogs and cats.
Occurrence is common in endemic part of western Pacific (China, Japan, Korean peninsula, Malaysia, Vietnam). In Europe (eastern part of Russian Federation).
References
- Bean, N., & Griffins, P. (1990). Foodborne Diseases Outbreaks in the United States, 1973-1987: Pathogens, Vehicles and Trends. Journal of Food Microbiology, 53, 804-817.
