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Paragonimiasis is a disease caused by lung flukes (Trematodes), Paragonimus. Several species present in tropical and subtropical countries affect about 20 million people worldwide. In South East Asia, the dominant species is P. westermani (P. heterotremus has also been reported in Thailand, Laos and in South China).


Light to moderate Paragonimus infections are usually asymptomatic. Heavy infections can cause pulmonary disease with inflammatory responses as the flukes become encapsulated. Severe pulmonary paragonimiasis has similar symptoms as those of pulmonary tuberculosis with chest pain, cough, night sweats, pleural effusion and haemoptysis.

Transmission and life cycle

Infection occurs by ingesting the flesh or juice of raw, undercooked or pickled fresh water crab or crayfish, which contains metacercariae. Shrimp may also be infected. Following ingestion, metacercariae excyst in the duodenum and young flukes penetrate the intestinal wall to migrate through the diaphragm into the pleural cavity and into the lungs, where they become mature flukes. Occasionally, the young flukes migrate to the liver, spleen, brain and other organs.

3 months after infection, eggs are produced and pass into the sputum and in faeces when swallowed. For the life cycle to be continued the eggs must reach water. In water, miracidia develops and hatch from the eggs. It then swims in the water in search of a suitable first intermediary host: snails. In the snail, the miracidia develops in sporocysts that produce cercariae. Development in the snail takes about 8 weeks. The cercariae are shed from the snail and encyst in the second intermediary host, crabs crayfish or shrimp to become metacercariae, the infective form. Humans and all crab and crayfish eating mammals such as dogs, cats, pigs, wild boar, deer and wild animals are the parasite reservoir. Consumption of raw or undercooked meat of animals, which feed on fresh water crustaceans, can transmit the parasite. For instance, few cases have been cited in Japan where raw boar meat was the source of human infection.


Because symptoms are similar to pulmonary tuberculosis, paragonimiasis is often misdiagnosed as pulmonary tuberculosis. The Ziehl-Neelsen method for microscopic detection of mycobacteria destroys the eggs of Paragonimus.

Patients with symptoms, residing in endemic areas and who have a history of eating the flesh or juice of raw, undercooked or pickled fresh water crab or crayfish should be investigated. Paragonimus eggs can be found in sputum and faeces (as sputum is often swallowed). For light infections, up to seven examinations are recommended. Both sputum and faeces should be examined to increase detection rate.

In heavy infection, the flukes become encapsulated (a fibrotic capsule forms around the worm). The cyst contains purulent fluid with “iron fillings” composed of brownish eggs. Many cysts perforate into the bronchioles and release their contents of eggs, necrotic debris and blood into the respiratory tract. It produces characteristic rusty colored sputum that contains rusty-brown particles (see picture 1; see picture 2 ). Such sputum should be examined microscopically for the presence of P.westermani.

*Note: In the sputum pictured here 185 eggs of P.westermani were found in a single wet preparation (portion of sputum transfered onto a slide and covered with a cover slide).

Description of the egg

Eggs are yellow-brown, oval and asymmetrical in shape; being slightly flattened on one side.
It measures 70 to 120 µm in length and 45 to 65 µm wide (see picture Obj X10). It is operculated (operculum) and opercular shoulders are well visible (see picture 1 Obj X40). Eggs have a dark golden brown shell moderately thick. The shell is distinctly thicker at the end opposite the operculum (see picture 2 Obj X40).


Praziquantel is the drug of choice. 25 mg/kg given orally 3 times per day for 2 consecutive days.
Triclabendazole can also be an alternative drug.

Prevention and control

  • Not eating raw, undercooked or pickled fresh water crabs, crayfish or shrimps. Consumption of raw meat from wild animals in endemic areas may also be a source of infection.
  • Not contaminating water with sputum or faeces. Construction and use of latrines away from riverbeds is helpful.
  • Detecting and treating infected persons in endemic areas.


Information provided here come from various sources. However, the main ones come from two excellent books:

  • “District Laboratory Practice in Tropical Countries” Part 1, Monica Cheesbrough. Cambridge university press.
  • “Practical Guide to Diagnostic Parasitology”, Lynne S. Garcia, American Society for Microbiology (ASM) press.


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