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Human paragonimiasis is acquired through ingestion of raw or undercooked crabs or crayfish, and is usually a lung infection. After ingestion, metacercariae excyst in the small intestine and release larvae that penetrate the duodenal wall and enter the peritoneal cavity. The larvae migrate for approximately 1 week, then penetrate the diaphragm, enter the pleural cavity, and migrate directly through lung tissue to reach the bronchi. There they form cystic cavities and develop into adult worms in 5-6 weeks. The adult parasites are reddish brown and ovoid, measuring 7.5-12 mm by 4-6 mm. Adult worms induce an inflammatory response in the lungs, generating a fibrous cyst that contains a purulent, bloody effusion and eggs released by the flukes which are passed into the environment via expectoration, or may be swallowed and passed with feces. When deposited in fresh water, eggs hatch to release miracidiae, which then invade specific snail hosts. Thousands of cercariae are later released from the infected snail, which encyst (as metacercariae) in the gills, muscles, legs, and viscera of freshwater crustaceans (crabs or crayfish).


The clinical picture of chronic paragonimiasis resembles chronic bronchitis or tuberculosis. Persons may cough up coffee-colored or blood-tinged sputum, often accompanied by chest pain and/or shortness of breath. The sputum may be peppered consisting of clumps of eggs produced by the adult fluke living in the lung.

Peripheral eosinophilia is common and can be intense, especially during the early larval migration stages. Many patients have a spectrum of abnormalities on chest radiographs: lobar infiltrates, coin lesions, cavities, calcified nodules, hilar enlargement, pleural thickening and effusions. Ring-shaped opacities of contiguous cavities giving the characteristic appearance of a bunch of grapes are highly suggestive of pulmonary paragonimiasis. Central nervous system disease may provide similar “grapebunch” findings, characteristically seen in the temporal and occipital lobes on computed tomography of the brain. CNS involvement occurs in up to 25% of hospitalized patients and may be associated with Paragonimus-induced meningitis. CNS symptoms may include headaches, seizures, and visual disturbances. Paragonimus flukes may also invade the liver, spleen, intestinal wall, peritoneum, and abdominal lymph nodes.

Sputum examined microscopically may reveal Paragonimus eggs released by the flukes in the lungs. Keep in mind that the acid-fast stain that is used for TB testing of sputum destroys eggs. The eggs may also be found by multiple stool exams on different days as a result of coughed-up eggs that are swallowed. The microscopic eggs are yellowish brown, 80-120 µm long by 45-70 µm wide, thick-shelled, and with an obvious operculum. Serologic tests can be especially useful for early infections (prior to maturation of flukes) or for ectopic infections where eggs are not passed in stool.

Ectopic lesions from aberrant migration of flukes can involve any organ, including abdominal viscera, the heart, and the mediastinum. The infection can also affect the liver, spleen, abdomen, and skin. The most clinically recognizable ectopic lesions arise from cerebral paragonimiasis, which, in highly endemic countries, more commonly affects children. These children present with eosinophilic meningoencephalitis, seizures, or signs of space-occupying lesions. Many patients with central nervous system disease also have pulmonary infections. P. skrjabini often produces skin nodules, subcutaneous abscesses, or a type of creeping eruption known as “trematode larva migrans.”


Praziquantel is the drug of choice: adult or pediatric dosage, 25 mg/kg given orally three times per day for 2 consecutive days. For cerebral disease, a short course of corticosteroids may be given with the praziquantel to help reduce the inflammatory response around dying flukes.

Alternative: Triclabendazole, 2 doses of 10 mg/kg given 12 hours apart in patients 6 years of age and older.

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Page last reviewed: December 8, 2021