What is Paragonimus?

Paragonimus is a parasitic lung fluke (flat worm). Cases of  infection occur after a person eats raw or undercooked infected crab or  crayfish. Paragonimus infection also can be very serious if the fluke  travels to the central nervous system, where it can cause symptoms that mimic meningitis.


Where is Paragonimus  found?

Paragonimus westermani and several  other species are found throughout eastern, southwestern, and southeast Asia;  (including China, the Philippines, Japan, Vietnam, South Korea, Taiwan, and  Thailand). P. africanus is found  in Africa, and P. mexicanus in Central and South America. P.  kellicotti is found in the mid-western and southern United States living in  crayfish. Some cases of infection have been associated with eating raw crayfish  on river raft trips in the Midwest. P. kellicotti or other species have  caused paragonimiasis after ingestion of raw freshwater crabs in sushi. There  are several species of Paragonimus in  other parts of the world that can infect humans.


How is Paragonimus  transmitted?

The infection is transmitted by eating crab or crawfish that is either, raw, partially cooked, pickled, or salted. The larval stages of the parasite  are released when the crab or crawfish is digested. They then migrate within  the body, ending up in the lungs. In 6-10 weeks the larvae mature into adult  flukes.


What are the signs and symptoms?

Adult flukes living in the lung cause lung  disease that may never be diagnosed or is thought to be tuberculosis. After 2-15 days, the initial signs and symptoms may be  diarrhea and abdominal pain. This may be followed several days later by fever, chest pain, and fatigue. The symptoms may also  include a dry cough initially, which later often becomes productive with  rusty-colored or blood-tinged sputum on exertion.


How is Paragonimus infection diagnosed?

The diagnosis is usually made by identifying Paragonimus eggs in the sputum or  sometime in the stool (from ingesting after coughing up).


Is Paragonimus infection contagious?

No. Paragonimus is not contagious.


Is there treatment?

Yes, there is treatment. Several drugs are available through  your physician after being accurately diagnosed.


How can I prevent  Paragonimus infection?

Never eat raw freshwater crabs or crayfish. Cook crabs and crayfish for to at least 145°F (~63°C). Travelers should be advised to avoid traditional meals containing undercooked freshwater crustaceans.



Several species of Paragonimus cause most infections; the most important is P.  westermani, which occurs primarily in Asia including China, the  Philippines, Japan, Vietnam, South Korea, Taiwan, and Thailand. P. africanus causes infection  in Africa, and P. mexicanus in Central and South America. Specialty dishes in which shellfish are consumed  raw or prepared only in vinegar, brine, or wine without cooking play a key role  in the transmission of paragonimiasis. Raw crabs or crayfish are also used in  traditional medicine practices in Korea, Japan, and some parts of Africa.

Although rare, human paragonimiasis from P.  kellicotti has been acquired in the United States, with multiple  cases from the Midwest. Several cases have been associated with ingestion of  uncooked crawfish during river raft float trips in Missouri.



Causal Agent:

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 Paragonimus westermani

The eggs are excreted unembryonated in the sputum, or alternately they are swallowed and passed with stool.   In the external environment, the eggs become embryonated, and  miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues.   Miracidia go through several developmental stages inside the snail: sporocysts,   rediae, with the latter giving rise to many   cercariae , 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   host.   Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that  harbor metacercariae of the parasite.   The metacercariae excyst in the duodenum, 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 adults. 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.

Life cycle image and information courtesy of DPDx.



Paragonimiasis is an acute infection with cough, abdominal pain, discomfort, and low-grade fever that may occur 2 to15 days after infection. The infection usually resolves without treatment. Persons with light infections may have no symptoms. Symptoms of long-term infection may mimic bronchitis or tuberculosis, with coughing up of blood-tinged sputum.



The infection is usually diagnosed by identification of Paragonimus eggs in sputum. The eggs are sometimes found in stool samples (coughed-up eggs are swallowed). A tissue biopsy is sometimes performed to look for eggs in a tissue specimen.

Specific and sensitive antibody tests based on P. westermani antigens are available through CDC, and serologic tests using a variety of techniques are available through commercial laboratories.



Never eat raw freshwater crabs or crayfish. Cook crabs and crayfish for to at least 145°F (~63°C). Travelers should be advised to avoid traditional meals containing undercooked freshwater crustaceans.

For more information view the source:Center for Disease Control

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