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Microbiology · Trematodes (flukes)

Paragonimus spp. !

  • Parasite
  • For completeness

A flatworm that comes with raw freshwater crabs or crayfish and settles in the lung. It causes bloody cough and is mistaken for tuberculosis.

Updated 2026-11-01

Also called: lung fluke, paragonimiasis

At a glance

Shape
Oval and reddish, like a coffee bean
Family
Paragonimidae
Type
Pulmonary trematode (lung fluke)
Size
Adult about 1 cm

Morphology

  • Yellow-brown egg, 80–120 µm, operculated, with a thickened opposite end.
  • Passed unembryonated in sputum or stool.
  • Adults encapsulated in lung cysts.

Transmission

  • Eating raw, marinated or undercooked freshwater crabs or crayfish with metacercariae.
  • Rarely, raw meat of animals that eat crustaceans (for example, wild boar).

Reservoir: Wild and domestic carnivorous mammals; Freshwater snails and crustaceans as intermediate hosts

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. Eggs leave in sputum or, if swallowed, in stool; they embryonate in water.
  2. The miracidium infects a snail, where sporocysts, rediae and cercariae form.
  3. Cercariae invade freshwater crabs or crayfish and encyst as metacercariae.
  4. When the crustacean is eaten raw or undercooked, the metacercaria excysts in the duodenum.
  5. It crosses the gut wall and diaphragm to the lung, where it encapsulates and matures.
  6. It lays eggs after 2–3 months; infection can last up to 20 years and sometimes reaches the brain.

Virulence factors

  • Lung cysts with haemorrhagic inflammation produce cough and haemoptysis.
  • It can cause pleural effusion and pneumothorax.
  • The cerebral form causes headache, seizures and haemorrhage.

Diseases it causes

  • Pulmonary and extrapulmonary (cerebral) paragonimiasis

Clinical picture

  • Chronic cough with rusty or bloody sputum.
  • Chest pain, breathlessness, fever and eosinophilia.
  • Negative sputum smears in a picture that looks like tuberculosis.

Diagnosis

  • Operculated eggs in sputum or stool, from 2–3 months after infection.
  • Eggs in pleural fluid or bronchoalveolar lavage.
  • Serology (immunoblot) when no eggs are found.
  • Chest CT: nodules, cavities or cysts.

Treatment

Antitrematode drugs: praziquantel or triclabendazole.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Cook freshwater crabs and crayfish thoroughly.
  • Do not use the same utensils for raw crustaceans and other food.

Test yourself

Think of the answer, then tap the question to see it.

Which food transmits Paragonimus?

Raw or undercooked freshwater crabs or crayfish carrying metacercariae.

Which disease is pulmonary paragonimiasis mistaken for?

Tuberculosis, because of chronic cough and haemoptysis.

Where are Paragonimus eggs looked for?

In sputum and in stool (swallowed eggs), plus pleural fluid or bronchoalveolar lavage.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Paragonimiasis (CDC, 2024); Foodborne trematode infections (WHO, 2021).

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Representative model, for educational purposes only.