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Schistosomiasis, also known as bilharzia, is a disease caused by parasitic worms. Although the worms that cause schistosomiasis are not found in the United States, more than 200 million people are infected worldwide. In terms of impact this disease is second only to malaria as the most devastating parasitic disease. Schistosomiasis is considered one of the Neglected Tropical Diseases (NTDs).

The parasites that cause schistosomiasis live in certain types of freshwater snails. The infectious form of the parasite, known as cercariae, emerge from the snail, hence contaminating water. You can become infected when your skin comes in contact with contaminated freshwater. Most human infections are caused by Schistosoma mansoni, S. haematobium, or S. japonicum.


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 Babesia microti

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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