Which trematode may cause serious disease, with loads of over 25,000 parasites per patient? If you answered the Clonorchis sinensis, you are correct! Clonorchis sinensis, also known as the Chinese liver fluke, lives in the liver of humans and is commonly found in the bile ducts and gall bladder . The geographic distribution of this parasite is in endemic areas in Asia including China, Japan, Korea, Taiwan, and Vietnam . It is found mainly in Asian immigrants, or following ingestion of imported, undercooked, or pickled freshwater fish. C. sinensis has also been reported in non-endemic areas including the United States.
Figure 1. Clonorchis sinensis: Chinese liver fluke
Do Hyun Park, M. D., Ph. D. and Hyun-Young Son, M. D. from the New England Journal of Medicine presented a clinical case dealing with the disease associated with C. sinensis. Briefly, an elderly man reported fatigue, fever, and abdominal pain for a week along with the consumption of raw pond smelt . A duodenoscopy was performed and after the cannulation of the common bile duct, numerous leaf-shaped worms popped out (see video below)—subsequently identified as C. sinensis . The worm burden and the duration of infection tend to reflect the clinical manifestations of the disease clonorchiasis .
Video 1. Cannulation of the common bile duct displaying numerous trematode parasite C. sinensis being popped out of a patient with the disease clonorchiasis.
Clonorchis sinensis is a trematode in the phylum Platyhelminthes . Because this parasite exhibits a three-host sequence, part of its life cycle takes place in aquatic habitats while the other part takes place in terrestrial habitats. The physical appearance of C. sinensis includes many forms: egg, miracidium, sporocyst, redia, cercaria, metacercaria, and adult . C. sinensis is classified as hermaphroditic; as a result, every single sexually mature form will produce eggs . Interestingly, each adult yields a daily production of 4000 eggs for at least six months ! C. sinensis is currently infecting an estimated 30,000,000 humans and is believed to be the third most prevalent trematode parasite in the world .
C. sinensis includes three species of hosts: the mollusk, the fish, and the mammal. Eggs are evacuated with feces and ingested by a freshwater gastropod mollusk. A dozen species have been reported as vectors, but the most frequent one is Parafossarulus manchouricus . Numerous swimming larvae, also known as cercariae, are produced by asexual multiplication and then leave the mollusk . The cercariae penetrate a variety of fish species—almost a hundred—where they then encyst in the muscle as metacercariae . Once the metacercariae are eaten by the fish, they excyst in the mammal’s stomach and mature in the bile ducts of the liver .
DPDx—a website developed and maintained by CDC’s Division of Parasitic Disease and Malaria—provided an outstanding photo depicting the life cycle of C. sinensis. First, embryonated eggs are discharged in the biliary ducts of humans . Second, eggs are ingested by a suitable snail intermediate host . Each egg releases a miracidia, which go through four developmental stages: miracidia, sporocysts, rediae, cercariae . The cercariae are then released from the snail and after a short period of free-swimming time in water, they come in contact and penetrate the flesh of freshwater fish, where they encyst as metaceriae . After ingestion, the metacercariae excyst in the duodenum and ascend the biliary tract via the ampulla of Vater . The cycle repeats over and over again from this point on.
Figure 2. The three-host sequence life cycle of C. sinensis.
C. sinensis has had large detrimental effects on humans, specifically those in areas such as Asia, where eating raw or undercooked fish is a part of their cultural upbringing. A human host with an average of infection will have or three dozen worms, and heavily infected individuals have been found with as many as 20,000 worms . These parasites reside in the biliary system of the liver and occasionally in the pancreas. Although the liver or pancreas is not attacked, they are greatly damaged due to the parasites’ migration through the biliary system . Damages include erosion of the epithelial lining of bile ducts, which leads to the development of blockage in the ducts due to the thickening of scar tissue . This erosion, along with the effects of C. sinensis’ perforation into the parenchyma of the liver leads to possible disturbance of normal hepatic functions .
An Example of the Three-Host Sequence:
Several unrelated species of hosts may be successively exploited as the parasite develops over its life cycle. The sequence of hosts consists of the series of hosts utilized during the life cycle and is often referred to as the longitudinal component of the cycle . This sequence involves two dimensions: ecological (the succession of environments) and ontogenetic (the various developmental states of the parasites) . The parasite undergoes a metamorphosis at each change of host because each time that the parasite changes host, some genes are turned off while others are activated, resulting in a morphologically, anatomically, physiologically, and ethologically distinct organism . Adults of C. sinensis demonstrate an example of the three-host sequence: the mollusk, the fish, and the mammal.
 Laboratory Identification of Parasites of Public Health Concern. “Clonorchiasis.” Centers for Disease Control and Prevention. Online. Accessed 24 Feb. 2012. <http://dpd.cdc.gov/dpdx/html/Clonorchiasis.htm>.
 Park, D. H. and H. Y. Son. “Images in Clinical Medicine: Clonorchis sinensis.” The New England Journal of Medicine. (2008); 358: e18.
 Eckroad, E. and H. Lee. 2001. "Clonorchis sinensis." Animal Diversity Web. Online. Accessed 24 Feb. 2012. <http://animaldiversity.ummz.umich.edu/site/accounts/information/Clonorchis_sinensis.html>.
 Combes C. 2001. Parasitism: The Ecology and Evolution of Intimate Interactions. Chicago: University of Chicago Press. 36-38 p.