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Lung Fluke (Paragonimus)

Lung Fluke (Paragonimus)


Disease History, Characteristics, and Transmission

Scope and Distribution

Impacts and Issues



Lung fluke infection, or paragonimiasis, is a potentially serious illness that is caused by over 30 species of trematodes (parasitic flatworms) of the genus Paragonimus. Among the more than 10 species reported to infect humans, the most common is P. westermani, found in tropical and subtropical regions of the Far East.

Lung flukes are not transmitted from person to person. Humans contract paragonimiasis when they eat inadequately cooked or pickled flesh that is infected. Most reported cases result from consuming raw freshwater crabs or crayfish. However, some species of lung fluke occur only in domestic or wild animals. Humans can contract the illness from consuming the raw flesh of these creatures. Paragonimus infect wild boars, wild and domestic canids, wild and domestic felids, raccoons, mongooses, rats, and weasels.

Disease History, Characteristics, and Transmission

Until the last quarter of the twentieth century, the public health importance of lung fluke infections was grossly underestimated. As a result, knowledge of the epidemiology of some species of fluke is still limited.

Both snails and crustaceans serve as intermediate hosts of the parasitic flatworms that cause paragonimiasis. Lung fluke eggs hatch in freshwater into miracidia (early-stage larvae), which penetrate the snails. The miracidia develop into very short-tailed cercariae (larvae in the final free-swimming stage) that penetrate and form cysts in the gills or muscles of freshwater crayfish and crabs. Humans or other animals then consume the raw infected crustaceans. The eggs hatch in the duodenum and the young flukes penetrate the gut wall, and eventually, the pleural cavity. Within two to three weeks, the worms, which are hermaphroditic (having both male and female reproductive organs), penetrate beneath the lungs where they meet and cross-fertilize. Adult trematodes become partly encapsulated in the lung tissues of their definitive host, where they lay eggs. The eggs pass into the alveoli. They are then passed in sputum (matter coughed up from the respiratory tract) or swallowed and are passed later in feces. Eggs that reach water hatch and begin the cycle again. The time from infection to oviposition is 65–90 days.

Lung fluke infections can be serious illnesses. Onset of symptoms generally occurs between six and ten weeks after infection. A classical symptom is bloody sputum in which eggs can be found. Other symptoms include cough, difficulty breathing, diarrhea, abdominal pain, fever, and hives. The infection is often mistaken for pulmonary tuberculosis. The parasite can migrate from the lungs to other organs including the brain and striated muscles. Lung flukes that become localized in the brain can create major neurological symptoms. Humans infected with Paragonimus usually display symptoms of epilepsy for the first time in adult life. Infections can persist for years, with some cases reported in which people suffered for twenty to forty years.

Scope and Distribution

At the start of the twenty-first century, over 21 million people worldwide were estimated by tropical disease specialists to be infected with lung fluke. Paragonimus westermani is the most common parasite responsible for human infection in the Far East, where it has a large range of mammalian reservoirs. Human infections with P. heterotremus are well known in Thailand and with P. pulmonalis in the Far East. P. africanus and P. uterobilateralis are distributed among humans in West Africa. P. mexicanus affects people in parts of Central and South America.

The best method of avoiding infection is to only consume properly cooked food. The preferred treatment is praziquantel tablets with bithionol tablets as an alternative drug. Praziquantel stops worms from developing or multiplying in the body.

Impacts and Issues

Paragonimiasis once tended to be limited to regions where the appropriate crustaceans formed part of the human diet in one culinary delicacy or another. However, the rise of a global cuisine has contributed to the spread of lung flukes. In 2006, two people who consumed live, imported, freshwater crabs in an Orange County, California, restaurant contracted paragonimiasis.

The invasion of nonnative species into American and Canadian waters poses a further threat to human health. The mitten crab, Eriocheir sinensis, was first spotted in fisheries on the west coast in 1992. This Yellow Sea native is a Chinese delicacy and crabs were imported live to markets in Los Angeles and San Francisco before California outlawed their possession. A female can carry from 250,000 to 1 million eggs. The crabs, adept on land, climb easily over levees as they migrate upstream. Mitten crabs can carry lung fluke, though no infected crabs have been detected in North American waters.


ALVEOLI: An alveolus (alveoli is plural) is a tiny air sac located within the lungs. The exchange of oxygen and carbon dioxide takes place within these sacs.

DEFINITIVE HOST: The organism in which a parasite reaches reproductive maturity.

INTERMEDIATE HOST: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.

OVIPOSITION: Ovum is Latin for “egg” to oviposition is to position or lay eggs, especially when done by an insect.

PLEURAL CAVITY: The lungs are surrounded by two membranous coverings, the pleura. One of the pleura is attached to the lung, the other to the ribcage. The space between the two pleura, the pleural cavity, is normally filled with a clear lubricating fluid called pleural fluid.

TREMATODES: Trematodes, also called flukes, are a type of parasitic flatworm. In humans, flukes can infest the liver, lung, and other tissues.

See AlsoHelminth Disease; Parasitic Diseases.



Peters, Wallace, and Geoffrey Pasvol. Tropical Medicine and Parasitology. London: Mosby, 2002.

Web Sites

International Society for Infectious Diseases. “ProMed Mail: Mitten Crab—USA and Canada.” August 1, 1999. <> (accessed May 26, 2007).

International Society for Infectious Diseases. “ProMed Mail: Paragonimiasis from Eating Raw Imported Freshwater Crab.” August 20, 2006. <> (accessed May 26, 2007).

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