Andy Miller, MD, is board-certified in infectious disease and internal medicine. He is an associate professor at Weill Cornell Medicine, associate attending physician at the Hospital for Special Surgery and New York Presbyterian Hospital and a fellow of the Infectious Disease Society of America.

A bite of fresh crab or crayfish can come with an unintended surprise: a parasite. Cooked crab should be free of any problems from this parasite. But crab that isn't cooked properly could possibly have a nasty parasite called Paragonimus, which, once ingested, could cause an infection of the lung or other organs and body parts.

Paragonimus is the name of a family of flatworms (flukes) that are human parasites, mostly causing problems in the lung, brain, and spinal cord. They are roughly about 10mm long, 5mm wide, and 4mm thick. Early in the infection, they cause low fever, with lung, abdominal, and brain symptoms developing later. Lung symptoms can be as mild as mild bronchitis or can be severe, with bleeding from the lungs. When it invades the central nervous system, it typically causes a type of meningitis. In rare cases, the infection can be fatal.

Most cases are found in Asia, particularly in southeast Asia, but they can also be found in Africa and the Americas.

Southeast Asia, especially Laos, Thailand, southern China, and Vietnam see many of the cases worldwide, but there are occasional cases in the US. Crabs can also be imported from high-risk areas.

Different types of Paragonimus are found in different locations. Paragonimus westermani is the most common and is found in Asia (Japan, Taiwan, Thailand, Philippines, China, Laos, Vietnam, etc) as are Paragonimus heterotremus and Paragonimus philippinensis.

There is Paragonimus kellicotti, Paragonimus caliensis, and Paragonimus mexicanus found in the Americas. Paragonimus africanus and Paragonimus uterobilateralis have been seen in western and central Africa.

Cases sometimes occur in the United States, particularly in the midwest and southern states. These are due to Paragonimus kellicotti. This can be from eating undercooked crayfish and can be found in the Mississippi River area. Cases have been found particularly in Missouri.

The adult parasites usually live off humans and other mammals, which serve as a host for the Paragonimus. An infected person or animal may carry eggs from the parasite. If they cough these up or pass them in stool, the eggs enter the outside environment where they hatch and, while still in the larval stage invade a crustacean such as a crab or crayfish.

When another person or mammal eats the crab or crayfish, they ingest the parasite and become infected.

In a person, the parasite is ingested in its late larval stage and then passes through several stages:

In the United States, where Paragonimus illness is rare, there can be a delay in diagnosis because it may not occur to a physician to consider it a possibility.

The infection is often mistaken for TB. A standard test for TB ("Acid Fast Staining" of sputum on slides) was thought to destroy the Paragonimus eggs making it hard to detect the parasite as patients are often tested for TB. It turns out the eggs can be found more often than thought through TB tests.

It can be diagnosed by finding eggs in sputum samples (or in stool samples if the eggs are coughed up and swallowed). Tissue samples can also be sent to a pathology lab.

There are also antibody tests through the CDC (or some other labs) which can identify some Paragonimus infections and exposures.

It is possible to cure infections, including those by the most common type of Paragonimus, Paragonimus westermani. Treatment involves taking the prescription medications triclabendazole or praziquantel.

Raw or undercooked freshwater crab or crayfish can contain Paragonimus. For instance, dishes that marinate crab and crayfish with vinegar, wine, or brine to "chemically cook" them do not always kill the parasite; cooking adequately with heat does kill the parasite.

Examples of dishes that are made with uncooked marinated crab and crayfish include ceviche and "drunken crab." Fresh crab juice, an ineffective folk remedy for measles, also can contain live parasites. On the other hand, sushi and sashimi is generally prepared either with imitation crab or with cooked crab pieces and rarely include raw crab.

The FDA advises cooking crab or crayfish to 145 F (or 63 C). That means that the internal temperature of the crab/crayfish should reach 145 F (63 C). The meat should be opaque and pearly.

Sometimes food is frozen to avoid parasites. It should be noted that flukes, like Paragonimus, are more resistant to freezing than other parasites. This means eating crab that was frozen but not cooked would not be expected to be as safe as it may be with other food that can have parasites.

The parasite can actually be found in other animals, many of which are not eaten by humans. This includes tigers, leopards, mongooses, opossums, monkeys, cats, and dogs. They can also be found in wild boars. In addition, there have been reports of kitchen instruments like chopping boards being contaminated with this parasite, so it is a good idea to clean your kitchen instruments after processing raw crabs and crayfish before using them for other foods.

No, the parasite doesn't spread from one person to another. You get sick only by eating food contaminated with the same parasite.

Yes. The parasitic Paragonimus species are hermaphrodites, so they contain both ovaries and testes. They are capable of asexual and sexual reproduction.

When infected with the Paragonimus westermani parasite, people often have no symptoms. However, others develop abdominal pain, fever, and symptoms that are similar to tuberculosis. 

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Centers for Disease Control and Prevention. Paragonimus FAQs.

Johannesen E, Nguyen V. Paragonimus kellicotti: a lung infection in our own backyard. Case Rep Pathol. 2016;2107372. doi:10.1155/2016/2107372

Diaz JH. Paragonimiasis acquired in the United States: native and nonnative species. Clin Microbiol Rev. 2013;26(3):493–504. doi:10.1128/CMR.00103-12

Centers for Disease Control and Prevention. Biology.

Slesak G, Inthalad S, Basy P, et al. Ziehl-Neelsen staining technique can diagnose paragonimiasis. PLoS Negl Trop Dis. 2011;5(5):e1048.

Centers for Disease Control and Prevention. Serum/Plasma Specimens - Antibody Detection Tests Offered at CDC.

U.S. Food and Drug Administration. Selecting and Serving Fresh and Frozen Seafood Safely.

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