Most people who pick up a flatworm parasite get it from a meal: raw or undercooked meat, fish, shellfish or water plants that carry the worm’s young stages.1PubMed Central. General overview of the current status of human foodborne trematodiasis The big exception is the blood flukes, which need no meal at all and slip in through bare skin during a swim or a wade in freshwater.2World Health Organization. Schistosomiasis Many of these worms pass through snails first, which is why freshwater, and whatever grows or swims in it, matters so much. Once inside, the harm depends less on the worm being there than on where it settles and how long it stays, ranging from a quiet gut passenger to a cause of liver scarring or cancer.
The Two Kinds of Flatworms That Infect People
Flukes are flat, leaf-shaped parasitic worms that usually move between a snail, a second animal or plant, and a final host such as a person.3PubMed Central. The physical soldier caste of an invasive, human-infecting flatworm is morphologically extreme and obligately sterile Inside the snail, a single larva can multiply into a colony of dozens to thousands of copies.4PubMed Central. The physical soldier caste of an invasive, human-infecting flatworm is morphologically extreme and obligately sterile
Tapeworms look nothing like that. They are long, segmented ribbon worms that live in the gut as adults. Cattle, pigs and some fish carry their larvae in muscle or organs, and people catch them by eating that flesh.5Biron Health Group. How do you get tapeworms? The beef tapeworm, the pork tapeworms and the fish tapeworm are the main ones in people.6Biron Health Group. How do you get tapeworms? Once you know which animal a worm uses, you know what to avoid: snail-infested water for one, raw carp for another, rare pork for a third.
The two groups also behave differently once inside. Flukes attach in organs such as the bile ducts or blood vessels, while adult tapeworms usually hang in the intestine. Some tapeworm larvae, though, form cysts in tissues instead, and that is far more serious.
Catching Blood Flukes Through the Skin
Blood flukes, the cause of schistosomiasis, cycle between freshwater snails and people.7PubMed Central. Transmission potential of human schistosomes can be driven by resource competition among snail intermediate hosts Eggs leave infected people in urine or feces, hatch in water and infect snails.8PubMed Central. Transmission potential of human schistosomes can be driven by resource competition among snail intermediate hosts The snails then release swimming larvae, and those larvae infect people who touch the water.9World Health Organization. Schistosomiasis
The larvae get through skin quickly by breaking down its proteins, including the stretchy protein elastin.10PubMed. Schistosome invasion of human skin and degradation of dermal elastin are mediated by a single serine protease Their enzymes also cut collagen and other proteins that hold the skin layers together.11PubMed Central. Proteomic Analysis of Human Skin Treated with Larval Schistosome Peptidases Reveals Distinct Invasion Strategies among Species of Blood Flukes Once inside, the worms mature in the blood vessels, where they mate and lay eggs.12PubMed Central. Transmission potential of human schistosomes can be driven by resource competition among snail intermediate hosts
Risky contact means swimming, wading, bathing or washing in lakes, rivers or canals where the disease occurs. The worms live in freshwater, not the sea or chlorinated pools.
Catching Flukes and Tapeworms From Food
- Fish and shellfish: freshwater fish, crabs and crayfish, and freshwater snails or clams can carry fluke larvae when eaten raw or undercooked.13PubMed Central. General overview of the current status of human foodborne trematodiasis
- Liver flukes: Clonorchis and Opisthorchis come from raw freshwater fish.
- Water plants: the sheep liver fluke Fasciola comes from raw watercress or other freshwater plants with larvae attached.14Centers for Disease Control and Prevention. About Fasciola
Tapeworms come from meat and fish. Eating raw or undercooked beef or pork is the main risk for a gut tapeworm,15Centers for Disease Control and Prevention. About Human Tapeworm and some raw freshwater fish carry the fish tapeworm.16Biron Health Group. How do you get tapeworms? Inspection often misses these larvae, so a carcass that looked fine can still carry them.17PubMed Central. Foodborne Parasites and Their Complex Life Cycles Challenging Food Safety in Different Food Chains Thorough cooking or proper freezing is more reliable than inspection. Marinating, pickling or salting does not reliably kill larvae.
Sushi is often blamed, but the riskiest fish for flukes are freshwater species, not most sea fish. The fish tapeworm can come from freshwater and from some migrating fish. The freezing used when raw fish is prepared for service lowers parasite risk.
The main liver fluke risk comes from raw or undercooked freshwater fish, such as carp-type species eaten in parts of East and Southeast Asia. If you eat raw fish, ask whether it came from fresh water or the sea.
Fluke larvae in fish usually cause no immediate illness, so a person can eat an infected fish and feel perfectly well for weeks or years. That delay is why people often cannot connect an infection to a particular meal. If you ate raw freshwater fish in an affected region, mention it to a doctor later, even if you feel fine.
What Flukes Do to the Liver and Bile Ducts
In blood fluke infection, the main damage comes from eggs trapped in the liver, which the body walls off in small inflamed lumps.18PubMed Central. Schistosome egg antigen stimulates the secretion of miR-33-carrying extracellular vesicles from macrophages to promote hepatic stellate cell activation and liver fibrosis in schistosomiasis Those lumps slowly turn into scar tissue.19PubMed Central. The Schistosoma Granuloma: Friend or Foe? Meanwhile the worms dodge the immune system by disguising themselves and coating their surface with a resistant outer layer.20PubMed Central. Immune Evasion Strategies of Schistosomes
Because the eggs, not the adult worms, cause the damage, trouble depends on how many eggs get stuck. A few can pass unnoticed, while a long, heavy infection leaves a lot of scarring. As scar tissue builds up around the small blood vessels in the liver, blood flowing through the organ meets resistance, and over years this can raise pressure in the veins feeding the liver. The liver and spleen can become enlarged. The condition is slow, so a person may only learn of it years after the original exposure.
Liver flukes in the bile ducts cause a different kind of harm. Heavy, long-lasting infection can cause upper belly pain, indigestion, fever and loose stools.21PubMed Central. Clonorchis sinensis and Cholangiocarcinoma Over time it can lead to gallstones, liver abscess, infected bile ducts and pancreatitis.22PubMed Central. Clonorchis sinensis and Cholangiocarcinoma The worms’ grazing and secretions keep wounding the duct lining, causing scarring and abnormal growth.23PubMed Central. Mechanistic insights into liver-fluke-induced bile-duct cancer
That is why two Asian liver flukes, Clonorchis and Opisthorchis, are officially classed as known causes of cancer.24Parasite. Clonorchis sinensis and cholangiocarcinoma: molecular mechanisms and biomarker advances Infected people have roughly 3 to 14 times the risk of bile-duct cancer, depending on the study.25Parasite. Clonorchis sinensis and cholangiocarcinoma: molecular mechanisms and biomarker advances In Korea about 1 in 10 bile-duct cancers is linked to this fluke.26PubMed Central. Clonorchis sinensis and Cholangiocarcinoma
The immune system does respond. Worm infections often raise a type of white blood cell called eosinophils, which can attack larvae; your blood test report may list them. A high eosinophil count on a blood test can be a clue a doctor follows up.
Tapeworm Effects From Anemia to Brain Cysts
The fish tapeworm absorbs about 80% of the vitamin B12 in food.27PubMed Central. Diphyllobothrium latum Mimicking Subacute Appendicitis As a result, up to 40% of infected people develop low B12.28Cleveland Clinic Journal of Medicine. An unusual cause of vitamin B12 and iron deficiency Yet only about 2% develop anemia with symptoms such as tiredness.29Cleveland Clinic Journal of Medicine. An unusual cause of vitamin B12 and iron deficiency
The loss can go unnoticed because the body stores a fair amount of B12 in the liver, so a drop may take a long time to cause trouble. If it goes unaddressed for a long time, the nerves can be affected, producing tingling or numbness. A blood test for B12 can find the problem, and treating the worm usually lets levels recover.
Pork tapeworm larvae can lodge in the brain, a condition called neurocysticercosis.30PubMed Central. Seizures and Epilepsy in Association With Neurocysticercosis The most common results are seizures, especially starting in adulthood, chronic headaches and raised pressure in the skull.31PubMed. Neurocysticercosis The cysts pass through stages until they become calcified spots,32PubMed Central. Seizures and Epilepsy in Association With Neurocysticercosis but even calcified spots can keep triggering seizures.33PubMed Central. Seizures and Epilepsy in Association With Neurocysticercosis
Brain cysts differ from gut worms in how they arrive. Eating undercooked pork gives a gut worm. Brain cysts come from swallowing the eggs, often through food or hands contaminated by a person carrying the gut worm, so the risk includes poor hand hygiene, not only meat.
A hydatid cyst is a fluid-filled tapeworm cyst, often in the liver. If one ruptures, about 17% to 25% of people get minor allergic reactions like hives.34PubMed Central. Intraperitoneal rupture of the hydatid cyst: Four case reports and literature review Life-threatening allergic shock happens in about 1.4% of ruptures.35PubMed Central. Intraperitoneal rupture of the hydatid cyst: Four case reports and literature review
An ordinary gut tapeworm is usually much milder. Many people notice nothing. Some have mild belly discomfort or appetite changes, and segments may show up in stool.
How Doctors Find Flatworm Infections
Doctors often look for eggs in a stool sample under a microscope, a cheap and simple test.36Journal of Helminthology. Advantages and limitations of diagnostic procedures for zoonotic helminth infections in feces More sensitive DNA tests can identify which worm is present,37Journal of Helminthology. Advantages and limitations of diagnostic procedures for zoonotic helminth infections in feces and one such test correctly identified every worm group it was tried on.38PubMed. Improving the specific diagnosis of trematode, cestode and nematode infections by a multiplex single-tube real-time PCR assay Brain or liver cysts are usually found with MRI, CT or ultrasound instead.39PubMed Central. Seizures and Epilepsy in Association With Neurocysticercosis
One stool test can miss an infection, because eggs are shed unevenly. Doctors may ask for several samples, and a negative result in someone with clear symptoms can be repeated.
It helps to tell your doctor about recent travel and freshwater swimming, and any raw fish, meat or water plants you ate. Say how long ago, since symptoms can appear months later. If you have seen worm segments, mention that too.
Treatment With Praziquantel and Albendazole
Praziquantel is the standard drug for all forms of blood fluke infection, and it is also used for many other fluke and gut tapeworm infections.40PubMed. Praziquantel: An update on the mechanism of its action against schistosomiasis and new therapeutic perspectives It floods the worm’s muscles with calcium, causing cramping and paralysis.41PubMed. The Mechanism of Action of Praziquantel: Can New Drugs Exploit Similar Mechanisms? But it works mainly on adult worms, not young ones,42PubMed. Praziquantel: An update on the mechanism of its action against schistosomiasis and new therapeutic perspectives so a repeat dose may be needed to catch worms that were young at first.
Doctors often repeat the dose or recheck the person some weeks later. Follow-up stool or urine tests for eggs, a few weeks to months after treatment, show whether the worms are gone.
Albendazole is used for tapeworm cysts, given before and after surgery or drainage of hydatid cysts.43PubMed. Treatment options for hepatic cystic echinococcosis It lowers the risk of the infection spreading if a cyst spills.44PubMed. Treatment options for hepatic cystic echinococcosis For brain cysts, combining albendazole with praziquantel raises drug levels and may kill cysts better.45PubMed Central. Pharmacokinetics of combined treatment with praziquantel and albendazole in neurocysticercosis
Gut infections often need only one day or a few days of tablets. Brain cyst treatment is supervised because dying cysts can inflame the tissue around them.
Protecting Yourself From Flatworms
In the kitchen, cook beef, pork and freshwater fish until no pink remains. Avoid raw watercress and other water plants from areas with livestock. Wash your hands well, since swallowing pork tapeworm eggs leads to brain cysts. Deep freezing kills many fish parasites, but home freezers may not get cold enough.
For blood flukes, avoid swimming or wading in freshwater where schistosomiasis occurs, since even brief contact lets larvae through the skin. Travelers can ask locally whether a lake carries the disease, and choose a pool or the sea instead. Towel drying does not reliably prevent infection. If you had contact, get tested after you return.
Where Blood Flukes May Spread Next
Climate change is shifting where the host snails can live, and growing towns and their outskirts are creating new local snail habitat.46PubMed Central. Climate and urbanization drive changes in the habitat suitability of Schistosoma mansoni competent snails in Brazil Parts of Southeast Asia, the Pacific Islands, the Mediterranean and the Amazon could become new hotspots.47PubMed Central. Impact of Climate Change on Schistosomiasis Transmission and Distribution—Scoping Review Risk may rise across much of Uganda, Tanzania and south-west Kenya over the next 20 years.48PubMed Central. Predicting the effects of climate change on Schistosoma mansoni transmission in eastern Africa
Mass treatment helps but does not end the problem. In modeled heavily affected communities, treating everyone with praziquantel cut infection from about half of people to about a fifth.49PubMed Central. Population-level impact of mass drug administration against schistosomiasis with anthelmintic drugs targeting juvenile schistosomes: a modelling study Even so, reinfection kept about 1 in 7 infected after five years of treatment.50PubMed Central. Population-level impact of mass drug administration against schistosomiasis with anthelmintic drugs targeting juvenile schistosomes: a modelling study