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DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/paragonimus.

Paragonimiasis

[Paragonimus westermani] [Paragonimus spp.]

 Adults of P. westermani.

Adult of P. westermani.


Eggs of P. westermani in an unstained wet mount.

Egg of P. westermani in an unstained wet mount.

Causal Agents

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

Life cycle of Paragonimiasis

The eggs are excreted unembryonated in the sputum, or alternately they are swallowed and passed with stoolThe number 1. In the external environment, the eggs become embryonatedThe number 2, and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissuesThe number 3. Miracidia go through several developmental stages inside the snailThe number 4: sporocystsThe number 4a, rediaeThe number 4b, with the latter giving rise to many cercariaeThe 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 hostThe number 5. Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasiteThe number 6. The metacercariae excyst in the duodenumThe 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 adultsThe 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.

Geographic Distribution

Paragonimus spp. are distributed throughout the Americas, Africa and southeast Asia. Paragonimus westermani is distributed in southeast Asia and Japan. Paragonimus kellicotti is endemic to North America.

Clinical Presentation

The acute phase (invasion and migration) may be marked by diarrhea, abdominal pain, fever, cough, urticaria, hepatosplenomegaly, pulmonary abnormalities, and eosinophilia. During the chronic phase, pulmonary manifestations include cough, expectoration of discolored sputum, hemoptysis, and chest radiographic abnormalities. Extrapulmonary locations of the adult worms result in more severe manifestations, especially when the brain is involved.

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  • Page last reviewed November 29, 2013
  • Page last updated November 29, 2013
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
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