Paragonimus westermani (Japanese lung fluke or oriental lung fluke) is the most common species of lung fluke that infects humans, causing paragonimiasis. Human infections are most common in eastern Asia and in South America. Paragonimiasis may present as a sub-acute to chronic inflammatory disease of the lung. It was discovered by Dutch zoologist Coenraad Kerbert in 1878.

Causative agent

More than 30 species of trematodes (flukes) of the genus Paragonimus have been reported to infect animals and humans. Among the more than 10 species reported to infect humans, the most common is Paragonimus westermani, the oriental lung fluke.

thumb|Egg of Paragonimus westermani

  • Eggs: Paragonimus westermani eggs range from 80 to 120 μm long by 45 to 70 μm wide. They are yellow-brown, ovoid or elongate, with a thick shell, and often asymmetrical with one end slightly flattened. At the large end, the operculum is clearly visible. The opposite (abopercular) end is thickened. The eggs are unembryonated when passed in sputum or feces.
  • Semisulcospira calculus P. westermani has been increasingly recognized in the United States because of the increase of immigrants from endemic areas such as Southeast Asia

Transmission

Transmission of the parasite P. westermani to humans and mammals primarily occurs through the consumption of raw or undercooked seafood. In Asia, an estimated 80% of freshwater crabs carry P. westermani.

Diagnosis

Diagnosis is based on microscopic demonstration of eggs in stool or sputum, but these are not present until 2 to 3 months after infection. However, eggs are also occasionally encountered in effusion fluid or biopsy material. Furthermore, you can use morphologic comparisons with other intestinal parasites to diagnose potential causative agents. Finally, antibody detection is useful in light infections and in the diagnosis of extrapulmonary paragonimiasis. In the United States, detection of antibodies to Paragonimus westermani has helped physicians differentiate paragonimiasis from tuberculosis in Indochinese immigrants.

Public health and prevention strategies

Prevention programs should promote more hygienic food preparation by encouraging safer cooking techniques and more sanitary handling of potentially contaminated seafood. The elimination of the first intermediate host, the snail, is not tenable due to the nature of the organisms habits.

Further reading

  • Center for Disease Control paragonimiasis web article
  • The Human Lung Fluke – Paragonimus westermani at Cambridge Schistosomiasis Research Group