Paragonimus: The Parasite in Raw Crab

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 isnt cooked properly could possibly have a nasty parasite called Paragonimus.

Crab on the beach
Francesca Yorke / Getty Images 


Paragonimus is the name of a family of flatworms (flukes) that are human parasites, mostly causing problems in the lung and the 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.

Where Paragonimus Is Found

Most cases are found in Asia, particularly in southeast Asia, but it 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, and 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.

How Paragonimus Spreads

The adult parasites usually live of humans and other mammals. They lay eggs which are coughed up and out or swallowed down and spread in stools. These eggs then reach freshwater where they can then end up inside of various different snails, develop further, and then spread into crabs (or similar crustaceans). Eating these crabs then spreads the infection back to people (or other hosts).

Diagnosis and Treatment

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.

Medications used to treat the infection include triclabendazole and praziquantel.

Potentially Infected Food

Raw or undercooked freshwater crab or crayfish can contain Paragonimus. For instance, dishes that marinate crab and crayfish with with vinegar, wine, or brine to "chemically cook" them do not always kill the 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 includes raw crab.

How to Safely Cook 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.

Is Paragonimus Only Found In Crabs And Crayfish?

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.

Is the Infection Contagious?

No, the parasite doesn't spread from one person to another. If someone gets sick with it, you would get sick only if you had been eating food contaminated with the same parasite.

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