Liver flukes are parasitic flatworms that can inhabit the liver and bile ducts of various mammals, including humans. These organisms have complex life cycles and can lead to significant health issues. Understanding how these parasites infect humans, how they are detected, and their treatments and prevention is important for public health. Their presence, often indicated by their eggs, is identified through stool sample examination.
What Are Liver Flukes?
Liver flukes are a type of parasitic flatworm belonging to the class Trematoda, characterized by their flattened, leaf-like bodies. They range in size, with some species like Fasciola hepatica reaching up to 3 centimeters long. Many liver fluke species are hermaphroditic, possessing both male and female reproductive organs. These parasites primarily reside in the bile ducts and liver of their hosts, feeding on blood and liver cells.
The main species of liver flukes that infect humans include Clonorchis sinensis (Chinese liver fluke), Opisthorchis viverrini (Southeast Asian liver fluke), and Fasciola hepatica (common liver fluke or sheep liver fluke), which is found worldwide.
How Liver Flukes Infect Humans and Appear in Stool
The life cycle of liver flukes involves multiple hosts, beginning with eggs passed in the feces of an infected mammal. If these eggs reach fresh water, they develop into a larval stage called miracidia, which then infect specific freshwater snails. Inside the snail, the miracidia undergo further development, multiplying into cercariae. These cercariae are then released from the snail and swim freely in the water.
For Clonorchis and Opisthorchis species, the cercariae penetrate freshwater fish or shrimp, where they encyst and become metacercariae, the infective stage. Humans become infected by consuming raw, undercooked, dried, salt-cured, or pickled freshwater fish or crustaceans containing these metacercariae. In the case of Fasciola hepatica, the cercariae encyst on aquatic plants, such as watercress, and humans become infected by eating these contaminated plants or drinking contaminated water.
Once ingested, the metacercariae excyst in the human small intestine. The immature flukes then migrate through the intestinal wall into the abdominal cavity and subsequently penetrate the liver, eventually reaching the bile ducts. Within the bile ducts, they mature into adult flukes, which can live for many years. These adult flukes produce eggs, which are then released into the bile and passed out of the body with the feces.
Recognizing Liver Fluke Infection
Recognizing a liver fluke infection can be challenging as symptoms often vary depending on the fluke species, stage, and intensity of the infection. In the acute phase, as immature worms migrate through tissues, individuals may experience fever, chills, abdominal pain, nausea, vomiting, and an enlarged liver. Skin rashes and elevated eosinophil levels, a type of white blood cell, may also be present. These initial symptoms can last for several weeks to months.
In the chronic phase, when adult flukes reside in the bile ducts, symptoms can include intermittent abdominal pain, indigestion, fatigue, and jaundice. Chronic infection can also lead to inflammation of the bile ducts (cholangitis), gallstones, and, in severe cases, liver scarring. Some infected individuals may show no symptoms at all, especially with mild infections.
The primary method for diagnosing liver fluke infection involves microscopic examination of stool samples to identify the characteristic eggs. Multiple stool samples may need to be tested over time to increase detection chances. Blood tests can also be supportive, revealing elevated eosinophil counts or the presence of antibodies against the parasites, which can be detected weeks before eggs appear in stool. Imaging techniques such as ultrasound, CT scans, and MRI may be used to assess damage to the liver and bile ducts.
Treating and Preventing Liver Fluke Infections
Medical treatment for liver fluke infections primarily relies on antiparasitic medications. For infections caused by Clonorchis sinensis and Opisthorchis viverrini, praziquantel is the standard treatment. This medication works by paralyzing the worms, allowing the body to clear them.
For Fasciola hepatica infections, triclabendazole is the drug of choice. This medication is effective against both immature and adult stages of the fluke. Prompt diagnosis and treatment are important to prevent long-term complications such as bile duct obstruction, inflammation, and the development of bile duct cancer. In some cases of biliary tract obstruction, manual extraction of adult flukes may be necessary through endoscopic procedures.
Preventing liver fluke infections centers on breaking the parasite’s life cycle. Thoroughly cooking freshwater fish, crustaceans, and aquatic vegetables is a primary preventive measure, as heat kills the infective metacercariae. Practicing good personal hygiene, including handwashing, and ensuring the safe disposal of human feces can also help prevent eggs from contaminating water sources and infecting intermediate hosts.