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  • A parasitic infection caused by trematodes of the paragonimus genus.
  • Humans are infected from ingestion of raw or undercooked food.
  • It results in chronic lung infection and eosinophilia.


Paragonimus is a lung fluke (flatworm) that infects the lungs of humans after eating an infected raw or undercooked crab or crayfish. Less frequent, but more serious cases of paragonimiasis occur when the parasite travels to the central nervous system.

Although rare, paragonimiasis has been acquired in the United States, with multiple cases reported from the Midwest. Once the diagnosis is made, effective treatment for paragonimiasis is available from a physician.


Several species of Paragonimus cause most infections; the most important is P. westermani, which occurs primarily in Asia including China, the Philippines, Japan, Vietnam, South Korea, Taiwan, and Thailand. P. africanus causes infection in Africa, and P. mexicanus in Central and South America. Specialty dishes in which shellfish are consumed raw or prepared only in vinegar, brine, or wine without cooking play a key role in the transmission of paragonimiasis. Raw crabs or crayfish are also used in traditional medicine practices in Korea, Japan, and some parts of Africa.

Although rare, human paragonimiasis from P. kellicotti has been acquired in the United States, with multiple cases from the Midwest. Several cases have been associated with ingestion of uncooked crawfish during river raft float trips in Missouri.


Causal Agent

More than 30 species of trematodes (flukes) of the genus Paragonimus have been reported which infect animals and humans. Among the more than 10 species reported to infect humans, the most common is P. westermani, the oriental lung fluke.

Life Cycle

The eggs are excreted un-embryonated in the sputum, or alternately they are swallowed and passed with stool The number 1. In the external environment, the eggs become embryonated The number 2, and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues The number 3. Miracidia go through several developmental stages inside the snail The number 4: sporocysts The number 4a, rediae The number 4b, with the latter giving rise to many cercariae The number 4c, which emerge from the snail. The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, where they encyst and become metacercariae. This is the infective stage for the mammalian host The number 5. Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite The number 6. The metacercariae excyst in the duodenum The number 7, penetrate through the intestinal wall into the peritoneal cavity, then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults The number 8 (7.5 to 12 mm by 4 to 6 mm). The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. However, when this takes place completion of the life cycles is not achieved, because the eggs laid cannot exit these sites. Time from infection to oviposition is 65 to 90 days.

Infections may persist for 20 years in humans. Animals such as pigs, dogs, and a variety of feline species can also harbor P. westermani.


Eggs of P. kellicotti in a Pap-stained bronchial alveolar lavage Eggs of P. kellicotti in a Pap-stained bronchial alveolar lavage (BAL) specimen at 400x magnification. Images courtesy of Dr. Gary Procop.

Infection with Paragonimus spp. can result in an acute syndrome with cough, abdominal pain, discomfort, and low-grade fever that may occur 2 to 15 days after infection. Persons with light infections may have no symptoms. Symptoms of long-term infection may mimic bronchitis or tuberculosis, with coughing up of blood-tinged sputum.


The infection is usually diagnosed by identification of Paragonimus eggs in sputum. The eggs are sometimes found in stool samples (coughed-up eggs are swallowed). A tissue biopsy is sometimes performed to look for eggs in a tissue specimen.

Specific and sensitive antibody tests based on P. westermani antigens are available through CDC, and serologic tests using a variety of techniques are available through commercial laboratories.


Paragonimus infections are treatable by your health care provider. Prescription medications are available.

Prevention & Control

Never eat raw freshwater crabs or crayfish. Cook crabs and crayfish for to at least 145°F (~63°C). Travelers should be advised to avoid traditional meals containing undercooked freshwater crustaceans. Paragonimus westermani 01.jpg


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