Life Cycle of Clonorchis Sinensis
Clonorchis sinensis, or Chinese liver fluke, is a parasite or trematode native to Eastern and South Pacific Asia. This creature lives in the human liver, most commonly in the gall bladder and common bile duct. Chinese liver flukes have a complicated life cycle that involves several different types of animal hosts. Understanding this life cycle is an important part of controlling the spread of the parasite.
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Egg
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The liver fluke discharges its eggs into the bile ducts and into the host animal's waste. From there, the eggs make their way into bodies of fresh water, where they are ingested by snails. The snail is the initial host for the fluke. Once inside the snail, the egg hatches into a form called the miracidium, a simple organism with two eye spots and primitive sensory organs. Miracidia may also hatch into the water and swim free until they find an appropriate snail.
Snail
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Inside the snail, the miracidium develops into several other forms. First, it becomes a sporocyst, a hollow sack with a developing larva inside. Eventually a form of the fluke called a redia emerges from the sporocyst. This form looks like a simple worm and lacks an intestine or esophagus. This form continues to develop until it becomes the cercaria, a stage that looks like a small adult fluke with a tail. Once the fluke reaches this stage, it bores its way out of the snail host and swims freely again.
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Fish
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Clonorchis sinensis cercariae swim through the water until they find an appropriate fish to serve as their second intermediate host. These creatures then bore into the fish's body, forming a parasitic attachment to the fish. The cercariae create a cyst in the muscle of the fish, becoming metacercariae. It loses its eye spots and tail and becomes rounded.
Adult
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The liver fluke reaches its adult form only after ingestion by its final host–a human being. Once a human ingests the flesh of a fish containing metacercariae, the acid-resistant cyst passes through the stomach and into the small intestines. From there, the liver fluke moves through the bile tract to the liver, which is its final habitat. After about a month, the fluke matures and begins to produce eggs.
Considerations
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Humans are most likely to contract liver flukes by eating raw, undercooked or pickled freshwater fish from Asia. Clonorchis sinensis also occurs outside its natural range, and is spread primarily by imported fish. Proper cooking of fish before consumption destroys metacercariae and renders the fish safe to eat.
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References
- Photo Credit Dry fish on a dish in the form of a fish image by terex from Fotolia.com
