Opisthorchis felineus. Genus Schistosoma and its species
Two flukes that reach people through fresh water. Opisthorchis felineus, the cat liver fluke, is eaten in raw fish and lives in the bile ducts; the schistosomes, the blood flukes, bore through the skin and live as pairs in the veins, where their eggs cause the disease.
- 11 min read
- 5 questions
- Checked 28 Oct 2026
Point at a name or a part to light it.
Opisthorchis felineus, the cat liver fluke, is a slender fluke 7 to 12 mm long that lives in the bile ducts of cats, dogs, foxes and people who eat raw fish. Its name means "testes behind": its two lobed testes lie near the hind end, behind the ovary. Its egg, 19 to 30 µm long, holds its miracidium when it is passed; a Bithynia snail eats it, and the cercariae that leave the snail encyst in the muscle of carp-family fish as metacercariae, the infective form.
Simplified: not to scale: the adult, 7 to 12 mm long, drawn beside forms well under a millimetre; the adult drawn from its ventral side as if cleared, so the organs show through; the excretory and nervous systems are left out; the uterus drawn with fewer loops than it has; the seminal vesicle, the oviduct and Mehlis' gland left out, and one male duct drawn from both testes; the sporocyst left out, and the cercaria drawn with a plain tail, without the fin it swims with.
3 sources
- Centers for Disease Control and Prevention. DPDx: Opisthorchiasis. Last reviewed 20 February 2018.
- Kiyan VS, Bulashev AK, Katokhin AV. Opisthorchis felineus and Metorchis bilis metacercariae in cyprinid fish Leuciscus idus in Nura-Sarysu River, Kazakhstan. Korean J Parasitol. 2018;56(3):267–274.
- Mehlhorn H. Opisthorchis felineus (syn. tenuicollis). In: Mehlhorn H, editor. Encyclopedia of Parasitology. Springer, 2016.
At a glance
- Opisthorchis felineus, the cat liver fluke, is a slender fluke 7 to 12 mm long in the bile ducts, and sometimes the pancreatic duct, of cats, dogs, foxes and people, from Italy to Western Siberia.
- Its cycle runs through two hosts in fresh water: a Bithynia snail, which eats the egg, and a fish of the carp family, in whose muscle the cercariae encyst. People are infected by eating the fish raw or undercooked.
- The schistosomes, the blood flukes, are the only flukes with separate sexes: the stout male carries the slender female in his gynaecophoric canal, and the pair live for years in the veins round the gut or the bladder.
- Their cycle has one intermediate host, a freshwater snail, and no metacercaria: the cercariae bore straight through the skin of people in the water.
- The disease comes from the eggs. S. mansoni and S. japonicum cause intestinal and liver disease, with eggs in the stool; S. haematobium causes urogenital disease, with blood and eggs in the urine. Praziquantel treats both flukes.
Opisthorchis felineus, the cat liver fluke
Opisthorchis felineus lives in the bile ducts and the pancreatic duct of cats, dogs, foxes and other fish-eating mammals, and of people. It is found from Portugal and Spain to Eastern Europe and Western Siberia, mainly in Italy, Germany, Belarus, Russia, Kazakhstan and Ukraine, along rivers such as the Ob, the Irtysh and the Volga. An estimated 1.2 to 1.6 million people carry it. Its relatives O. viverrini, in Southeast Asia, and Clonorchis sinensis, in East Asia, are caught the same way.
The adult
The adult is a slender lance, 7 to 12 mm long and 1.5 to 3 mm wide, narrow in front and rounded behind, thin enough for its organs to show through.
- Suckers: the oral sucker at the tip, a little larger than the ventral sucker about a fifth of the way back.
- Gut: a pharynx, then two plain caeca, without branches, that run back to near the hind end.
- Female organs: the uterus, full of small eggs, winds forward in transverse loops from the small ovary to the genital pore in front of the ventral sucker; the seminal receptacle lies behind the ovary; the yolk glands lie in about seven bunches down each side of the middle of the body.
- Male organs: two lobed testes near the hind end, one diagonally behind the other, the front one with four lobes and the hind one with five. The name Opisthorchis means "testes behind". In Clonorchis the testes lie in the same place but are deeply branched.
| Fasciola hepatica | Dicrocoelium dendriticum | Opisthorchis felineus | |
|---|---|---|---|
| Size | up to 30 by 15 mm, a broad leaf | 6 to 10 by 1.5 to 2.5 mm, a lance | 7 to 12 by 1.5 to 3 mm, a lance |
| Caeca | with many side branches | simple | simple |
| Testes | branched, behind the ovary | lobed, in front of the ovary | lobed, near the hind end, behind the ovary |
| Egg | 130 to 150 µm, no miracidium yet | 35 to 45 µm, dark brown, holds a miracidium | 19 to 30 µm, a lid in a rim and a knob, holds a miracidium |
| Second intermediate host | none: water plants | ant | carp-family fish |
| People eat | watercress | an ant, by accident | raw or undercooked fish |
The life cycle
Point at a name to light its pathway.
Opisthorchis needs two intermediate hosts, both in fresh water: a Bithynia snail, which eats the eggs and sheds cercariae, and a fish of the carp family, in whose muscle the cercariae encyst. Cats, dogs, foxes and people are infected by eating the fish raw or undercooked; the young flukes go straight up the bile duct from the duodenum.
Simplified: a map, not anatomy: the hosts drawn as icons and the parasites drawn small, nothing to scale; one cat drawn for the cats, dogs, foxes and other fish-eating mammals, one Bithynia for the snails and one fish for the many carp-family fish that can carry it; the young flukes' climb through the ampulla of Vater drawn as one step; the adults also live in the pancreatic duct.
3 sources
- Centers for Disease Control and Prevention. DPDx: Opisthorchiasis. Last reviewed 20 February 2018.
- Chai JY, Jung BK. General overview of the current status of human foodborne trematodiasis. Parasitology. 2022;149(10):1262–1285.
- Kiyan VS, Bulashev AK, Katokhin AV. Opisthorchis felineus and Metorchis bilis metacercariae in cyprinid fish Leuciscus idus in Nura-Sarysu River, Kazakhstan. Korean J Parasitol. 2018;56(3):267–274.
The adults lay eggs that pass with the bile into the gut and out in the faeces, already holding a miracidium. The egg hatches only when a freshwater snail of the genus Bithynia eats it; in Europe these are the Bithynia leachi group. In the snail the miracidium becomes a sporocyst, which makes rediae, in which the cercariae grow. The cercariae leave the snail, swim to a fish of the carp family, such as the tench, the ide, the bream, the barbel or the carp, bore through its skin and encyst in its muscle or under its scales as metacercariae, the infective form.
A cat, a dog, a fox or a person who eats the fish raw or undercooked swallows the metacercariae. They excyst in the duodenum and the young flukes climb through the ampulla of Vater into the bile ducts, without crossing the gut wall or the liver. They attach to the lining and lay eggs after 3 to 4 weeks.
Opisthorchiasis
Opisthorchiasis is often silent. The damage comes from inflammation of the bile ducts and from their blocking now and then:
- An acute phase in some people 2 to 4 weeks after a meal of infected fish, which resembles the Katayama fever of schistosomiasis: fever, a swollen face, enlarged lymph nodes, joint pains, a rash and eosinophilia, often with signs of cholestatic hepatitis. In an outbreak by Lake Trasimeno in Italy in 2022, fever was the commonest symptom.
- Chronic infection: dyspepsia, abdominal pain, diarrhoea or constipation; with heavier and longer infections an enlarged liver, and inflammation of the bile ducts, the gall bladder and, in O. felineus, the pancreatic duct.
- Cholangiocarcinoma, a cancer of the bile ducts: its relatives O. viverrini and Clonorchis cause it after long infection; whether O. felineus does is not yet proven.
Diagnosis rests on the eggs in the stool, small, with a lid set in a rim and a knob at the far end, which look almost the same as those of Clonorchis. Early in the infection, before eggs appear, and in light infections, serology helps; so does asking about raw fish.
Treatment is praziquantel, 25 mg/kg three times a day for 1 to 3 days, depending on the worm load. In the 2022 Italian outbreak, more than half of those given albendazole still passed eggs afterwards. Prevention is cooking freshwater fish well. Marinating, salting or drying does not make it safe: the metacercariae survive cold, drying and salt and are killed only by heat, and the 2022 Trasimeno outbreak came from marinated fillets. Earlier Italian outbreaks came from raw fillets of tench from the lakes of the centre of the country.
The genus Schistosoma, the blood flukes
The schistosomes are flukes of the blood. Unlike every other fluke, they have separate sexes, and they live in pairs in the small veins round the gut or the bladder. More than 250 million people needed treatment for schistosomiasis in 2024, most of them in sub-Saharan Africa; after malaria it is the parasitic disease with the greatest economic impact. Six species infect people, three of them widely (WHO counts S. guineensis and S. intercalatum together, as five).
| S. mansoni | S. haematobium | S. japonicum | |
|---|---|---|---|
| Where | Africa, the Middle East, Brazil, Venezuela, Suriname, the Caribbean | Africa, the Middle East; in Corsica, a hybrid with S. bovis | China, the Philippines, Indonesia |
| Snail | Biomphalaria | Bulinus | Oncomelania |
| Adults live in | the veins of the large intestine (inferior mesenteric) | the veins round the bladder (vesical plexus) | the veins of the small intestine (superior mesenteric) |
| Egg | 114 to 180 µm, a spine on the side | 110 to 170 µm, a spine at the end | 70 to 100 µm, rounder, a small spine |
| Eggs leave in | the stool | the urine | the stool |
| Disease | intestinal and liver | urogenital | intestinal and liver |
S. mekongi, along the Mekong in Cambodia and Laos, and S. intercalatum and S. guineensis, in the rain forests of central and West Africa, cause intestinal disease in small areas. S. japonicum is also carried by cattle, dogs, pigs, rodents and other animals, which makes it hard to control.
The adults
Point at a name or a part to light it.
Schistosomes, unlike other flukes, have separate sexes: the stout male carries the longer, slender female in a groove along his body, the gynaecophoric canal, and they live together for years in the veins round the gut or the bladder. Their eggs have no lid but a spine whose place tells the species apart: on the side near the hind end in S. mansoni, at the hind end in S. haematobium, and small in S. japonicum.
Simplified: not to scale: the adults, 10 to 20 mm long, drawn beside forms well under a millimetre; only the three eggs are drawn at one scale; the pair drawn from the ventral side as if cleared, so the male's organs show through his rolled sides; the excretory and nervous systems are left out; the female's front end drawn leaving the canal at its side, a little behind its front opening; the male's seminal vesicle and the female's oviduct, ootype and vitelline duct left out; seven testes drawn; the pair drawn as S. mansoni; the other species differ in the number of testes, the place of the ovary and the number of eggs in the uterus.
6 sources
- Centers for Disease Control and Prevention. DPDx: Schistosomiasis. Last reviewed 7 June 2024.
- Australian Society for Parasitology. Para-site, an interactive resource dedicated to parasitology: Schistosoma.
- Kincaid-Smith J, Tracey A, de Carvalho Augusto R, et al. Morphological and genomic characterisation of the Schistosoma hybrid infecting humans in Europe reveals admixture between Schistosoma haematobium and Schistosoma bovis. PLoS Negl Trop Dis. 2021;15(12):e0010062.
- Morona D. Differentiation of various species of Schistosoma, after Manson’s Tropical Diseases, Table III.2. Teaching handout, Department of Parasitology, Catholic University of Health and Allied Sciences, Bugando, 2012.
- Pichon R, Lotkowska ME, Bulathsinghalage JLD, et al. In vitro sexual dimorphism establishment in schistosomes. eLife. 2026;15:RP111066.
- Schistosoma mansoni. Wikipedia, after Rey L. Parasitologia. Rio de Janeiro: Guanabara Koogan, 1991.
Adults are long, rounded worms, 10 to 20 mm long, that look more like roundworms than like the flat leaf of Fasciola.
- The male is shorter and stouter. Behind his large ventral sucker his flat body rolls its sides under to make a long groove, the gynaecophoric canal, the canal that carries the female. Small tubercles stud his back in S. mansoni. His gut forks at the ventral sucker into two caeca that join again in the front half of his body and run on as one long caecum, about two thirds of his length. His testes, 6 to 9 in S. mansoni, lie in a row behind the ventral sucker.
- The female is longer and thread-like, and lies in the male's canal. Her suckers are small and her gut is dark with haemozoin, the pigment left from the blood she digests. In S. mansoni her ovary lies in her front half, the uterus ahead of it holds one or two eggs at a time, and yolk glands fill her hind two thirds. Without a male she does not mature.
The pair usually live 2 to 5 years, but some live for decades, up to 30 years. A female lays hundreds of eggs a day into the small veins.
The life cycle
Point at a name to light its pathway.
Schistosomes need only one intermediate host, a freshwater snail, and no second: the cercariae that leave the snail bore straight through the skin of people who wade, wash, swim or work in the water. The adults live as pairs in the veins for years, and the disease comes from their eggs, which lodge in the gut, the bladder and the liver.
Simplified: a map, not anatomy: the patient and the snail drawn as icons and the parasites drawn small, nothing to scale; one map for the three main species, which differ in their snail, the veins the adults live in and whether the eggs leave in the faeces or the urine; one ramshorn snail, Biomphalaria, drawn for all their snails; the way from the skin through the heart and lungs to the liver drawn as one step; the animal reservoirs of S. japonicum left out.
2 sources
- Centers for Disease Control and Prevention. DPDx: Schistosomiasis. Last reviewed 7 June 2024.
- Kumar and Clark’s Clinical Medicine, 10th edition. Elsevier, 2021. Chapter 20, Infectious disease; helminths, p. 574, and schistosomiasis, pp. 578–579.
The eggs, each with its spine and no lid, are laid in the small veins of the gut or the bladder wall. Many work their way through the wall into the lumen and leave in the stool (S. mansoni, S. japonicum) or the urine (S. haematobium), each holding a mature miracidium. In fresh water they hatch, and the miracidium swims to a snail of the right kind and enters it. There it becomes a mother sporocyst, which makes daughter sporocysts, in which the cercariae grow; there are no rediae.
Thousands of cercariae with forked tails leave the snail into the water, where they live for 2 to 3 days. When one meets a person who wades, washes, swims or works in the water, it bores through the skin and sheds its tail, becoming a schistosomulum. The schistosomula travel in the veins through the heart and the lungs to the liver, where they grow in the portal veins and pair. The pairs then move to the veins of the gut or the bladder and start laying eggs.
Schistosomiasis
Schistosomiasis, or bilharzia, unfolds in stages. Most of the damage is done not by the worms but by the body's reaction to their eggs.
- Cercarial dermatitis, "swimmer's itch": small, itchy red bumps where the cercariae entered. The cercariae of the schistosomes of birds and other animals cause the same rash without going further.
- Acute schistosomiasis, Katayama fever: 3 to 4 weeks later, as the worms mature and begin to lay eggs, an allergic illness with fever, a rash, muscle aches, a cough and pneumonitis, abdominal pain, diarrhoea, an enlarged liver and spleen, and eosinophilia. It is common in travellers meeting the parasite for the first time and rare in people who have lived with it since childhood.
- Chronic schistosomiasis: eggs that lodge in the tissues are walled off by granulomas, which scar.
- Intestinal and liver disease (S. mansoni, S. japonicum): abdominal pain and bloody diarrhoea, later polyps, ulcers and strictures of the bowel; eggs carried to the liver cause periportal fibrosis, with portal hypertension, oesophageal varices, an enlarged spleen and ascites, while the liver cells themselves keep working. S. japonicum, which lays more eggs, causes the more severe disease.
- Urogenital disease (S. haematobium): painless blood at the end of passing urine, the classic first sign; then frequency and pain, fibrosis and calcification of the bladder and ureters, which block the flow and damage the kidneys; and, later, squamous cell carcinoma of the bladder. In women, female genital schistosomiasis causes genital lesions, bleeding, pain and infertility, and raises the risk of HIV; in men, it affects the prostate and seminal vesicles.
- Eggs elsewhere can reach the nervous system: the brain, mostly in S. japonicum, which involves it in about 5% of cases, or the spinal cord.
- In children, anaemia, stunted growth and poorer learning, mostly reversible with treatment.
Diagnosis, treatment and prevention
- Eggs: for S. haematobium, a filtered sample of urine collected around midday; children with it nearly always have microscopic blood in the urine. For S. mansoni and S. japonicum, the stool, by the Kato-Katz technique, or a rectal snip. The spine tells the species.
- Serology shows exposure in travellers and in people from places with little transmission, but may take up to 12 weeks after infection to turn positive.
- Ultrasound and endoscopy show the damage to the bowel, the liver and the urinary tract; a liver biopsy shows the periportal fibrosis.
Praziquantel treats every species: 40 mg/kg as a single dose for S. mansoni and S. haematobium, and 60 mg/kg for S. japonicum, either dose split to spare nausea. Where the disease is common, children, adults at risk and, in the worst areas, whole communities are treated, every year where transmission is high: re-infection is common, but repeated treatment in childhood reduces, and can reverse, the severe late disease. Safe water, sanitation, health education and control of the snails reduce transmission.
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1What does the name Opisthorchis describe?
2How do people catch Opisthorchis felineus?
3What sets the schistosomes apart from every other fluke in this chapter?
4Large eggs with a spine at one end are found in a filtered midday urine sample. Which species is it?
5What causes the damage in chronic schistosomiasis?