/Paragonimiasis (Lung Fluke)

Paragonimiasis (Lung Fluke)

What is paragonimiasis?

Paragonimiasis is a disease caused by a parasite, also known as a fluke, which commonly infects the lungs and other body sites of humans and other mammals.

The parasites most commonly associated with paragonimiasis are Paragonimus westermani and Paragonimus kellicotti although there are other Paragonimus species that can also cause disease in humans.

The disease can present as pulmonary (lung) and extrapulmonary (outside of the lung, such as in the abdomen or brain).

How does infection with Paragonimus species occur?

Humans most commonly become infected by eating raw, undercooked, salted or pickled freshwater crabs or crayfish that contain the parasite inside a cyst. One study found that approximately 17 percent of harvested crabs contained the infectious cysts.

Once in the intestine, the parasites come out of the cyst, penetrate the intestine, go into the abdomen and then into the lungs. Upon reaching their destination, the parasites mature and then start making eggs. The eggs in the lungs are either coughed up or swallowed and excreted in the feces. Although this parasite has other life cycle forms, it is only infectious to humans in the form that is found in crabs and crayfish.

What are the signs and symptoms of paragonimiasis?

This disease comes on slowly and symptoms may continue for years, sometimes as long as 20 years. In the first three weeks after ingestion, symptoms may include diarrhea, abdominal pain, cough, fever and hives while the worms are migrating from the intestines to the lungs.

The chronic symptoms of paragonimiasis are caused by the body’s immune system reaction to the worms and eggs. The incubation period, or time from infection until symptoms, varies, but it usually takes about 6-10 weeks for the parasites to start making the eggs in the lungs.

The pulmonary or lung form often has no or mild symptoms. There can be chronic cough and difficulty breathing. Infection with a large number of the parasites can lead to coughing fits with blood-tinged sputum; the sputum may be brown because of the Paragonimus eggs being coughed up. Other complications include fluid in the lining of the lung (pleural effusion), air leak (pneumothorax) in the space surrounding the lung and chronic lung inflammation and scarring.

The extrapulmonary form occurs when the infection affects the liver, spleen, abdomen, skin and brain. When the skin is affected, there are raised areas (nodules) that can move around; these are caused by young parasites in the worm or larval stage.

How is paragonimiasis diagnosed?

The eggs of this parasite are characteristic and may be seen under a microscope in stool, sputum, lung fluid, cerebrospinal fluid (fluid covering the brain and spinal cord) and tissue specimens from infected patients. However, the eggs may not be present until two to three months after infection.

A blood test is also available that identifies the antibodies the body produces when infected with Paragonimus. Early in the illness, blood tests may show an increase in a certain type of cell, called an eosinophil, which is often associated with parasitic infections.

How is paragonimiasis treated?

There is a prescription drug available called praziquantel that is effective for treatment of this infection in the lungs. The usual treatment course is two days. Another prescription drug, bithionol, is also effective.

How can I prevent infection with Paragonimus species?

These types of infections are rare in the United States and are much more common in Asia, South and Central America and Africa. Thoroughly cooking freshwater crabs and crayfish at a temperature of 55 C (131 F) for five minutes and until the flesh is pearly and opaque will kill the infectious form of the parasite.

What does the Southern Nevada Health District do upon receiving a report of paragonimiasis?

The health district conducts an investigation to find out where and how a person became sick. We also work to prevent further spread of the illness in the community. Visit the Disease Report Investigation Process webpage to learn more.


Suguyama, H et.al. “Detection of Paragonimus Metacercariae in the Japanese Freshwater Crab, Geothelphusa dehaani, Bought at Retail Fish Markets in Japan.” Jpn.J.Infect.Dis., 62, 2009, 324-325.

Contact Information

(702) 759-1000

Updated on: August 20, 2018

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