Liver flukes are parasitic flatworms that infect humans and animals, residing primarily within the liver and bile ducts. These parasites can lead to fascioliasis. Understanding their life cycle and common sources of contamination is important for prevention.
What Are Liver Flukes?
Liver flukes are parasitic flatworms, or trematodes, that inhabit the liver, gallbladder, and bile ducts of their hosts. They are typically leaf-shaped and feed on blood. Several species infect humans, including Clonorchis sinensis (Chinese liver fluke), Opisthorchis viverrini (Southeast Asian liver fluke), Fasciola hepatica (common liver fluke or sheep liver fluke), and Fasciola gigantica. These parasites require a host to survive and reproduce.
The Liver Fluke Life Cycle
The life cycle of liver flukes is intricate, involving multiple hosts and environmental stages. Adult liver flukes in an infected host, such as humans or animals, release immature eggs into the environment through feces. If these eggs reach freshwater, they develop into a larval stage called miracidia. These miracidia infect specific freshwater snails, which serve as the first intermediate host.
Inside the snail, miracidia undergo further developmental stages, transforming into cercariae. These cercariae, equipped with tails, emerge from the snail and swim in the water. They then encyst on aquatic vegetation, such as watercress, or within freshwater fish or crustaceans, forming metacercariae. This metacercariae stage represents the infective form for the definitive host.
How Humans Become Infected
Humans become infected with liver flukes by consuming the metacercariae stage of the parasite. This most commonly occurs through eating raw or undercooked freshwater fish, crustaceans, or aquatic plants that contain these cysts. For instance, Clonorchis sinensis and Opisthorchis species are often contracted through raw, undercooked, dried, salt-cured, or pickled freshwater fish or shrimp.
Fasciola hepatica and Fasciola gigantica infections are primarily linked to consuming raw watercress or other aquatic plants contaminated with metacercariae. Accidental ingestion of contaminated water, or eating vegetables washed or irrigated with contaminated water, also poses a risk. The cysts then release immature flukes in the human small intestine, which migrate to the liver and bile ducts to mature into adult worms, beginning the cycle anew.
Where Liver Fluke Infections Occur
The geographical distribution of liver fluke infections is closely tied to the presence of their intermediate hosts (snails, fish, or aquatic plants) and local culinary practices. Clonorchis sinensis is widely found across East Asia, including countries like China, South Korea, Taiwan, and Vietnam. Opisthorchis viverrini is prevalent in Southeast Asia, particularly in Thailand, Laos, Cambodia, and Vietnam, while Opisthorchis felineus is found in parts of Europe, such as Russia, Belarus, Ukraine, Germany, and Italy. Fasciola hepatica has a global distribution, occurring on every continent except Antarctica, with high infection rates in areas with sheep or cattle, such as parts of Bolivia, Peru, Portugal, and Puerto Rico. Fasciola gigantica is more common in tropical regions, including parts of Africa and Asia.
Avoiding Liver Fluke Infection
Preventing liver fluke infection involves avoiding the consumption of the infectious metacercariae stage. Thoroughly cooking freshwater fish, crustaceans, and aquatic plants is a primary prevention method. For fish, cooking to an internal temperature of at least 63 degrees Celsius (145 degrees Fahrenheit) is recommended to kill any larvae. It is also advisable to avoid consuming raw, dried, salt-cured, or pickled freshwater fish or shrimp from regions where these infections are common.
Raw watercress and other wild edible aquatic plants should be cooked well before consumption. Practicing good hygiene, such as washing hands and utensils thoroughly after handling raw fish, and avoiding drinking or cooking with potentially contaminated water, especially in areas with poor sanitation, further reduces the risk of infection.