THE LIFE CYCLE (~3 MONTHS)
Embryonated eggs are discharged in the biliary ducts and in the stool. (Image)
Eggs are ingested by a suitable snail intermediate host. (List of selected species)
ASEXUAL REPRODUCTION TAKES PLACE IN THE SNAIL HOST
Each egg releases a miracidia*, which goes through several developmental stages:
DEVELOPMENT INTO METACERCARIAE TAKES PLACE IN FRESHWATER FISH
The cercariae are released from the snail and, after a shot period of time swimming free in the water, they come into contact with and penetrate the flesh of freshwater fish. (List of selected species) They discard their tails and encyst as metacercariae***** in the subcutaneous tissue or in the muscles.
DEVELOPMENT INTO ADULT FLUKES TAKES PLACE IN HUMANS AND RESERVOIRS
Infection of humans and reservoir hosts (List of hosts) occurs by ingestion of undercooked, salted, pickled, or smoke freshwater fish.
After being acted on by peptic acid within the stomach, the metacercariae encyst in the duodenum and jejunum
They then ascend the biliary tract through the ampulla of Vater, using their suckers to move against the flow of bile and migrate to the smaller biliary radicles, especially in the left lobe of the liver. At this point maturation occurs, which takes approximately 1 month. The adults flukes (Image) reside in small and medium sized biliary ducts, and can live up to 30 years. These adult flukes then shed eggs in the bile.
* Miricidia: 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.
(Metacercariae of Clonorchis sinensis collected from digested fish. Courtesy of Sung-Tae Hong http://www.atlas.or.kr/atlas/alphabet_view.php?my_codeName=Clonorchis%20sinensis)
Information for this page was obtained from The University of Michigan Animal Diversity Web and from DPDx: Parasites and Health)
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