Liver and Intestinal Flukes
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Direct answer
Liver and intestinal flukes are trematode helminths acquired by eating raw aquatic plants, fish or crustaceans that carry the infective metacercariae. Fasciola hepatica is the liver fluke of sheep and cattle that also infects humans, Clonorchis sinensis and Opisthorchis viverrini infect the bile ducts, and Fasciolopsis buski is the giant intestinal fluke. Triclabendazole is the drug of choice for fascioliasis, while praziquantel treats most of the other flukes.
What you must remember
- Life cycle: Every fluke here uses a snail as the first intermediate host, and humans acquire infection by ingesting metacercariae from a second host or plant.
- Fasciola hepatica: Large operculated eggs cause acute fascioliasis with fever and eosinophilia, and chronic infection causes biliary obstruction.
- Clonorchis sinensis: Infects intrahepatic bile ducts after eating raw freshwater fish and is linked to cholangiocarcinoma.
- Opisthorchis viverrini: A Southeast Asian liver fluke that, like Clonorchis, is a recognised cause of bile duct cancer.
- Fasciolopsis buski: The largest intestinal trematode of humans, acquired from water chestnut and water caltrop, causing diarrhoea and abdominal pain.
- Paragonimus westermani: A lung fluke acquired from crabs and crayfish, whose eggs appear in sputum and cause haemoptysis and cough.
- Diagnosis: Operculated eggs in stool, sputum or duodenal aspirate, found by sedimentation or the formol-ether concentration method, establish the diagnosis.
- Carcinogenicity: IARC classes Clonorchis sinensis and Opisthorchis viverrini infection as Group 1 carcinogens, meaning chronic infection causes cancer in humans.
Working from the egg to the drug
A patient with eosinophilia, right upper quadrant pain and a history of eating raw fish or greens presents the classic fluke picture. The workup moves from the egg to the organ and then to the drug.
- Identify the egg: Operculated eggs in stool or sputum point to trematodes, and the size and shape of the egg separate the genera.
- Match the exposure: Watercress points to Fasciola, raw fish to Clonorchis or Opisthorchis, and crabs or crayfish to Paragonimus.
- Choose the specimen: Sputum is used for Paragonimus, stool for intestinal flukes, and bile or duodenal aspirate for biliary flukes.
- Assess the organ: Ultrasound or CT looks for bile duct dilatation, abscesses or nodules, and liver enzymes show the degree of damage.
- Choose the drug: Triclabendazole is used for fascioliasis, while praziquantel is used for the other flukes, with the dose and duration set by the species and the current guideline.
- Confirm cure: Repeat the stool or sputum examination after treatment, because the disappearance of eggs confirms that the worms have been cleared.
Where the exam tries to catch you
Fluke questions usually test the match between the species and its drug, and between the species and its cancer risk.
- Trap: Treating Fasciola hepatica with praziquantel. Correct: Triclabendazole is the drug of choice for fascioliasis, because praziquantel works poorly against this liver fluke.
- Trap: Assuming watercress is safe because it is a vegetable. Correct: Watercress and other aquatic plants harbour Fasciola metacercariae, so raw aquatic greens are a recognised source of infection.
- Trap: Calling Paragonimus an intestinal fluke. Correct: Paragonimus westermani is a lung fluke whose eggs are coughed up in sputum, although swallowed eggs can also appear in stool.
- Trap: Overlooking biliary flukes when a patient has bile duct cancer. Correct: Clonorchis and Opisthorchis are linked to cholangiocarcinoma, so a history of raw fish eating matters.
Frequently asked questions
Which fluke is the giant intestinal fluke of humans?
Fasciolopsis buski is the largest intestinal trematode of humans and is acquired by eating water chestnut or water caltrop. Infection causes diarrhoea and abdominal pain, and heavy worm burdens can cause intestinal obstruction.
What is the drug of choice for fascioliasis?
Triclabendazole is the drug of choice for fascioliasis caused by Fasciola hepatica. Praziquantel is generally less effective against this liver fluke, so the two drugs should not be used interchangeably.
How is Paragonimus infection diagnosed?
Paragonimus westermani infection is diagnosed by finding its operculated eggs in sputum or stool, often with a history of eating raw or undercooked crabs or crayfish. Serology and chest imaging support the diagnosis when eggs cannot be found.
Which liver flukes are associated with cholangiocarcinoma?
Clonorchis sinensis and Opisthorchis viverrini are the liver flukes associated with cholangiocarcinoma, a cancer of the bile ducts. IARC classifies both as Group 1 carcinogens, so chronic infection with either is a recognised cancer risk.
What is the characteristic feature of the Fasciola egg?
Fasciola hepatica eggs are large, operculated and yellow-brown, and they are passed in bile into the faeces. The size of the egg helps distinguish it from the smaller eggs of the other liver flukes.