Clonorchis sinensis
human liver fluke
(Also: Chinese liver fluke)

Clonorchis sinensis is found mainly in eastern Asia and south Pacific Asia. Its common name, Chinese liver fluke, comes from its abundance in these areas. Clonorchis sinensis is distributed over multiple countries, including China, Korea, Vietnam, Taiwan, Japan, and others. (Chappell, 1979)

Part of the Clonorchis sinensis life cycle takes place in an aquatic habitat. The first intermediate host is always a snail of some sort, mainly Parafossarulus manchouricus and species from the genus Bulinus. "Selection" of the second intermediate host is less host-specific since cercaria are stimulated to swim by shadows and motion. However, since C. sinensis is mainly in specific regions of the world, certain kinds of freshwater fish are normally found to be second intermediate hosts. There are 12 species of fish that are mainly responsible for passing the infection to humans. Some of these fishes include Pseudorasbora parva (in the Japan region) and Ctenopharyngodon idellus (in the Canton region).

Because the definitive host can be any kind of fish-eating vertebrate, its life cycle is no longer confined to an aquatic environment and its geographic range has the potential to substantially increase. Possible hosts include humans, reptiles, birds, pigs, dogs, and cats. (Brusca and Brusca, 1990; Buchsbaum et al., 1987; Chappell, 1979; Clay and Rothschild, 1952; Swellengrebel and Sterman, 1961)

Egg - The egg is oval-shaped, 26-30 micrometers in length, and has a thick yellow-brown shell surrounding it. The C. sinensis egg has an operculum covering the anterior end with a conspicuous rim that protrudes from the sides of the egg. On the posterior side, each egg has either a small knob or a little curved spine.

Miracidium - This larval stage is ciliated and slightly oval in shape. It has 2 simple eyespots and lateral papillae which protrude outwards and serve as sensory organs.

Sporocyst - The sporocyst resembles a hollow and simple sac. Oftentimes, the developing rediae are visible inside the sac.

Redia - At this larval stage, it retains a very simple worm structure. In some ways, it still resembles a sac. It has a pharynx but no esophagus or intestine. Developing cercariae are visible in the rest of its body.

Cercaria - In this stage, C. sinensis resembles a small adult with a tail, which it loses upon penetration of the second intermediate host. The tail has dorsal and ventral fins on it to aid in locomotion. It is brownish in color. Unlike an adult, it has two eyespots, penetration glands and a stylet at its anterior end, and a cuticle with small spines.

Metacercaria - In this form, C. sinensis is encysted and does not look like a fluke. It has lost larval organs such as the eyespots, the stylet, and the tail. The round cyst has very thick walls and within it, the maturing fluke is visible as are its suckers.

Adult - As an adult, C. sinensis is a very narrow fluke, 10-25 mm. in length, flattened dorsal-ventrally, with Trematode characteristics such as an oral/anterior sucker, a ventral sucker (also known as the acetabulum), no blood circulatory system, and no body cavity. The common genital pore is just anterior to the acetabulum. The fluke is tapered at the anterior end and rounded at the posterior end. The intestine is bifurcated and ends blindly. A thick and elastic cuticle lacks any kind of spines or scales and can either be a translucent gray color or yellow color (due to absorption of bile). When stained on a slide, its branched testes, lobed ovary, and follicular vitellaria are apparent as is the long and convoluted uterus. The pharynx and esophagus are also visible. (Chappell, 1979; Clay and Rothschild, 1952; Swellengrebel and Sterman, 1961)

The life cycle is a three host-system: a snail is always the first intermediate host, a freshwater fish is normally the second intermediate host, and there is a wide range of definitive hosts. C. sinensis is hermaphroditic therefore every single sexually mature fluke will produce eggs. Each adult yields a daily production of 4000 eggs for at least six months which are produced by cross-fertilization. The eggs are passed out with the feces of the definitive host. An egg will not hatch until it is ingested by a proper first intermediate host.

Once ingested and in the alimentary canal, a miricidium emerges from the egg. It remains in this stage for only about 4 hours, after which it becomes a sporocyst.

The sporocyst is normally located in the wall of the first intermediate host's intestine. It can be found in other organs as well. In the next 20 days, germinal cells in the sporocyst undergo asexual reproduction to produce a new "generation". Each new organism is a redia and they emerge when they are mature.

Each redia contains germinal cells which will undergo further asexual reproduction. The next "generation" are cercariae (sing. cercaria). It travels to the hepatopancreas of the snail while the cercariae are developing within itself. At some point, though it is not yet entirely known, up to 50 cercariae emerge from the redia's birth pore, still immature.

The cercaria leaves its snail host one month after the initial infection of the snail by the eggs. It emerges due to some sort of stimuli (i.e. light, temperature, pH, humidity) although the specific stimulus is unknown. Upon contact with the second intermediate host, it attaches with its oral sucker and penetrates the host. It loses its tail during this process of penetration. After 35 days of infection, the cercaria encysts under a scale or in a muscle.

The metacercaria remains encysted in various tissues until the second intermediate host is ingested by a vertebrate. Excystation takes place in the duodenum and a juvenile adult fluke emerges.

A juvenile adult reaches the bile ducts within 4-7 hours after ingestion of the intermediate host. From the duodenum, it penetrates the gut wall and is carried to the biliary system via the ampulla of Vater. After one month it matures enough to produce eggs. (Brusca and Brusca, 1990; Chappell, 1979; Clay and Rothschild, 1952; Swellengrebel and Sterman, 1961)

The cercaria emerges from the snail, positions itself in an upside-down position, and sinks towards the bottom of the water. Though the exact way a C. sinensis cercaria sense stimuli is unknown, it is stimulated to swim by passing shadows and movement in the water. Any kind of stimulation caused it to swim rapidly back up and then resumes its sinking. (Chappell, 1979; Clay and Rothschild, 1952)

Bristles and small spines probably act as tactile receptors, and these animals also may have reduced chemoreceptors. (Brusca and Brusca, 1990)

The redia is the first stage that actually feeds. It feeds actively on the tissues of the first intermediate host, normally the digestive and reproductive systems.

Clonorchis sinensis is considered a parasite as an adult fluke. It is found in the biliary systems of its hosts, which range from reptiles to humans, absorbing bile as its source of nutrients. (Buchsbaum et al., 1987)

These animals are probably not preyed on directly but are ingested. Egg and larval mortality are high since the parasites often do not reach appropriate hosts.

The first intermediate host is always a snail of some sort, mainly Parafossarulus manchouricus and species from the genus Bulinus. "Selection" of the second intermediate host is less host-specific since cercaria are stimulated to swim by shadows and motion; however, since C. sinensis is mainly found in specific regions of the world, certain kinds of freshwater fish are normally found to be second intermediate hosts, simply due to their naturally occurring populations.

Parasitism by C. sinensis has had large, detrimental effects on humans, especially those in areas such as Asia, where eating raw or undercooked fish is a cultural practice. A human host with an average infection will have two or three dozen worms; heavily infected individuals have been found with as many as 20,000 worms. It is found mainly in the biliary system of the liver but it has also been occasionally found in the pancreas. The fluke does not attack the liver or pancreas themselves however it is greatly damaged due to its migration through the biliary system. Erosion of the epithelial lining of bile ducts is common which leads to the development of blockage in the ducts due to the thickening of scar tissue. This erosion plus the effects of the fluke's perforation into the parenchyma of the liver leads to possible disturbance of normal hepatic functions. Experiments have shown that clonorchiasis in rabbits leads to higher levels of potassium and cholesterol in the blood while calcium levels are lowered. Lipid metabolism and hydrolyzing activity are impaired, as well as glycogen synthesis. Blood sugar levels are erratic as rabbits showed signs of hypo- and hyperglycemia.

Eggs and worms that somehow migrate out of the biliary system may become nuclei of eventual gallstones. Metacercariae travelling through the body to the appropriate locations for excystation can cause high fevers in the host. (Swellengrebel and Sterman, 1961; von Brand, 1952)

It has no known positive effect.

The most effective drug for treating clonorchiasis is Praziquantal.

In the areas where we find C. sinensis infections most prevalent, we find that it is customary for these people to eat raw or undercooked fish. It is also common for people to use human feces, which may be highly saturated with C. sinensis eggs, to fertilize fish ponds which primarily farm fish that feed on algae and plants. Animals defecating in the water also add to the problem. This simple action perpetuates the life cycle of C. sinensis because once eggs are introduced into the water, they are free to be ingested by the first intermediate host which will start the cycle again.

Also contributing to the difficulty in avoidance of contact with any stages of C. sinensis is the fact that metacercariae can remain viable even after the fish has been pickled, salted, dried, or smoked. (Brusca and Brusca, 1990; Swellengrebel and Sterman, 1961)