Paragonimiasis is infection with the lung fluke Paragonimus westermani and related species. Humans are infected by eating raw, pickled, or poorly cooked freshwater crustaceans. Symptoms include chronic cough, chest pain, dyspnea, and hemoptysis. Allergic skin reactions and CNS abnormalities due to ectopic flukes, including seizures, aphasia, paresis, and visual disturbances, can also occur. Diagnosis is by identifying eggs in sputum, stool, or pleural or peritoneal fluid. Serologic tests are also available. Praziquantel is the treatment of choice; bithionol is an alternative.
Although > 30 species of Paragonimus exist and 10 have been reported to infect humans, P. westermani is the most frequent cause of disease. The most important endemic areas are in the Far East, principally Korea, Japan, Taiwan, the highlands of China, and the Philippines. Endemic foci also exist in West Africa and in parts of South and Central America.
Eggs passed in sputum or feces develop for 2 to 3 wk in freshwater before miracidia (first larval stage) hatch. The miracidia invade snails; there, they develop, multiply, and eventually emerge as cercariae (free-swimming larvae). Cercariae penetrate freshwater crabs or crayfish and encyst to form metacercariae. Humans become infected by eating raw, pickled, or poorly cooked crustaceans. Metacercariae excyst in the human GI tract, penetrate the intestinal wall, move into the peritoneal cavity, then through the diaphragm into the pleural cavity; they enter lung tissue, encyst, and develop into hermaphroditic adult worms, which grow to about 7.5 to 12 mm by 4 to 6 mm. Worms may also reach the brain, liver, lymph nodes, skin, and spinal cord and develop there. However, in these organs, the life cycle cannot be completed because the eggs have no way to exit the body. Adult flukes may persist for 20 to 25 yr.
Other hosts include pigs, dogs, and a variety of feline species.
Symptoms and Signs
Most damage is to the lungs, but other organs may be involved. About 25 to 45% of all extrapulmonary infections affect the CNS. Manifestations of pulmonary infection develop slowly and include chronic cough, chest pain, hemoptysis, and dyspnea; the clinical picture resembles and is often confused with TB. Cerebral infections manifest as space-occupying lesions, often within a year after the onset of pulmonary disease. Seizures, aphasia, paresis, and visual disturbances occur. Migratory allergic skin lesions similar to those of cutaneous larva migrans are common in infections with P. skrjabini but also occur with other species.
Diagnosis is by identifying the characteristic large operculated eggs in sputum or stool. Occasionally, eggs may be found in pleural or peritoneal fluid. Eggs may be difficult to find because they are released intermittently and in small numbers. Concentration techniques increase sensitivity.
X-rays provide ancillary information but are not diagnostic; chest x-rays and CT may show a diffuse infiltrate, nodules and annular opacities, cavitations, lung abscesses, pleural effusion, and pneumothorax. Serologic tests may assist in diagnosis of light or extrapulmonary infections.
Praziquantel 25 mg/kg po tid for 2 days cures 80 to 100% of pulmonary infections and is the drug of choice. Bithionol 30 to 50 mg/kg po every other day for 10 to 15 doses is an alternative but has more adverse effects. Praziquantel is used to treat extrapulmonary infections, but multiple courses may be required. Surgery may be needed to excise skin lesions or, rarely, brain cysts.
The best prevention is to avoid eating raw or undercooked freshwater crabs and crayfish from endemic waters.
Last full review/revision December 2009 by Richard D. Pearson, MD