Clinical Overview of Paragonimiasis

What to know

Paragonimiasis is an infection caused by a parasitic lung fluke (flat worm). It is transmitted by eating raw or undercooked crabs and crayfish. It usually causes a lung infection in the affected person. The infection is typically diagnosed by identification of Paragonimus eggs in sputum and sometimes in stool samples.

How it spreads

Human paragonimiasis is acquired through ingestion of raw or undercooked crabs or crayfish and is usually a lung infection. After ingestion, metacercariae excyst in the small intestine and release larvae that penetrate the duodenal wall and enter the peritoneal cavity. The larvae migrate for approximately one week, then penetrate the diaphragm, enter the pleural cavity, and migrate directly through lung tissue to reach the bronchi. There they form cystic cavities and develop into adult worms in 5 – 6 weeks. The adult parasites are reddish brown and ovoid, measuring 7.5 – 12 mm by 4 – 6 mm. Adult worms induce an inflammatory response in the lungs, generating a fibrous cyst that contains a purulent, bloody effusion and eggs released by the flukes which are passed into the environment via expectoration, or may be swallowed and passed with feces. When deposited in fresh water, eggs hatch to release miracidiae, which then invade specific snail hosts. Thousands of cercariae are later released from the infected snail, which encyst (as metacercariae) in the gills, muscles, legs, and viscera of freshwater crustaceans (crabs or crayfish).


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.


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.

Treatment and recovery

Praziquantel is the drug of choice: adult or pediatric dosage, 25 mg/kg given orally three times per day for 2 consecutive days. For cerebral disease, a short course of corticosteroids may be given with the praziquantel to help reduce the inflammatory response around dying flukes.

Alternative: Triclabendazole, 2 doses of 10 mg/kg given 12 hours apart in patients 6 years of age and older.


Treatment in pregnancy

Praziquantel is a pregnancy category B drug. There are no adequate and well-controlled studies in pregnant women. However, the available evidence suggests no difference in adverse birth outcomes in the children of women who were accidentally treated with praziquantel during mass drug administration (MDA) campaigns compared with those who were not. In MDA campaigns for which the World Health Organization (WHO) has determined that the benefit of treatment outweighs the risk, WHO encourages the use of praziquantel in any stage of pregnancy. For individual patients in clinical settings, healthcare providers should consider the risk of treatment in infected pregnant women with the risk of disease progression in the absence of treatment.

Pregnancy Category B: Either animal-reproduction studies have not demonstrated a fetal risk plus there are no controlled studies in pregnant women, or animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in the first trimester (and there is no evidence of a risk in later trimesters).

Treatment during lactation

Praziquantel is excreted in low concentrations in breast milk. According to WHO guidelines for MDA campaigns, the use of praziquantel during lactation is encouraged. For individual patients in clinical settings, healthcare providers should consider the risk of treatment in infected breastfeeding women with the risk of disease progression in the absence of treatment.

Treatment in pediatric patients

The safety of praziquantel in children aged less than 4 years has not been established. WHO now recommends treating children at least 2 years of age with praziquantel during MDA campaigns for schistosomiasis control, citing evidence that praziquantel is safe in this age group. For individual patients in clinical settings, healthcare providers should consider the risk of treatment in children younger than 4 years old with the risk of disease progression in the absence of treatment.

Triclabendazole, a benzimidazole compound active against immature and adult Fasciola parasites, is the drug of choice for treatment of fascioliasis. In February 2019, the U.S. Food and Drug Administration (FDA) approved triclabendazole for treatment of fascioliasis in patients at least 6 years of age.

  • As with all medications, use of triclabendazole should be individualized.
  • Take Triclabendazole orally, with food, to improve absorption. According to the FDA-approved product label, the recommended dosage regimen (for patients at least six years of age) is two doses of 10 mg/kg given 12 hours apart.
  • There have been documentations of Triclabendazole resistance, particularly in infected animals but also in some infected humans.

Additional perspective about therapy

Based on limited data, nitazoxanide might be effective therapy in some patients. Patients take the drug orally, with food. The dosage regimen for adults is 500 mg po bid (twice a day) for seven days.

Praziquantel, which is active against most trematodes (flukes), typically is not active against Fasciola parasites and therefore not recommended. In some patients who have biliary tract obstruction, manual extraction of adult flukes (e.g., via endoscopic retrograde cholangiopancreatography [ERCP]) may be indicated.

Treatment in pregnancy

There are no available data on the use of triclabendazole in pregnant women to inform a drug-associated risk for major birth defects, miscarriage, or adverse maternal or fetal outcomes. See additional perspective in the product label.

Treatment during lactation

According to the product label, there are no data on the presence of triclabendazole in human milk, the effects on the breastfed infant, or the effects on milk production. Published animal data indicate that triclabendazole is detected in goat milk when administered as a single dose to one lactating animal. When a drug is present in animal milk, it is likely that the drug will be present in human milk. The developmental and health benefits of breastfeeding should be considered along with the mother's clinical need for triclabendazole and any potential adverse effects on the breastfed infant from the medication or from the underlying maternal condition.

Treatment in pediatric patients

According to the product label, which addresses treatment of fascioliasis, triclabendazole is safe and effective for pediatric patients ages 6 years and older (The FDA has approved the treatment for this age group, but not for younger patients).