Definitions

scalelike

Paragonimus westermani

Paragonimus westermani is a lung fluke and is most prominent in Asia and South America. It was discovered from two Bengal tigers that died in zoos in Europe in 1878. Several years’ later, infections in humans were found in Formosa.

Morphology

size, shape, and color resembles a coffee bean when alive. Adult worms are 7.5 mm to 12 mm long and 4 mm to 6mm wide. The thickness ranges from 3.5 mm to 5 mm. The skin of the worm (tegument) is heavily covered with scalelike spines. The oral and ventral suckers are similar in size, with the later placed slightly pre-equatorially. The excretory bladder extends from the posterior end to the pharynx. The lobed testes are adjacent from each other located at the posterior end, and the lobed ovaries are off-centered near the center of the worm (slightly postacetabular). The uterus is located in a tight coil to the right of the acetabulum, which is connected, to the vas deferens. The vitelline glands, which produce the yolk for the eggs, are widespread in the lateral field from the pharynx to the posterior end. By viewing the tegumental spines and shape of the metacercariae, one could distinguish between the ~30 species of Paragonumus spp.

Life cycle

Unembryonated eggs are passed in the sputum of a human or feline. Two weeks later, miracidum develop in the egg and hatches. The miracidum penetrate its first intermediate host (snail). Within the snail mother sporocyst form and produce many mother rediae, which subsequently produce many daughter rediae which shed crawling cercariae into fresh water. The crawling cercariae penetrate fresh water crabs and encyst in its muscles becoming metacercaria. Humans or felines then eat the infected crabs raw. Once eaten, the metacerciaria excysts and penetrates the gut, diaphragm and lung where it becomes an adult worm in pairs.

Epidemiology

Reservoir hosts of Paragonumus spp. include numerous species of carnivores including felids, canids, viverrids, mustelids, some rodents and pigs. Humans only become infected after eating raw fresh water crabs that have been encysted with the metacerciaria. Southeast Asia is more predominately more infected because of lifestyles. Raw seafood is popular in theses countries. Crab collectors string raw crabs together and bring them miles inland to sell in Taiwan markets. These raw crabs are then marinated or pickled in vinegar or wine to coagulate the crustacean muscle. This process of cooking does not kill the metacercariae, consequently infecting the host. Smashing rice-eating crabs in rice paddies, splashing juices containing metacercariae, can also transmit the parasite. Or using juices strained from fresh crabs to medicinal uses. This parasite is easily spread because it is able to infect other animals (zoonosis). An assortment of mammals and birds can be infected and act as paratenic hosts, ingestion of the paratenic host can lead to infection of this parasite.

Pathology

Once in the lung or ectopic site, the worm stimulates an inflammatory response that allows it to cover its self in granulation tissue forming a capsule. These capsules can ulcerate and heal over time. The eggs in the surrounding tissue become pseudotubercles. If the worm becomes disseminated and gets into the spinal chord, it can cause paralysis; capsules in the heart can cause death. The symptoms are localized in the pulmonary system, which include a bad cough, bronchitis, and blood in sputum (hemoptysis).

Diagnosis and treatment

To properly diagnosis this parasite infection is by looking at the sputum and finding the eggs. Sometimes eggs are shed in the feces. Radiological methods can be used to X-ray the chest cavity and look for worms. This method is easily misdiagnosed, because pulmonary infections look like tuberculosis, pneumonia, or spirochaetosis. A lung biopsy can also be used to diagnose this parasite. An assay that detects worm antigens using monoclonal antibody can also be used to diagnosis. The drug of choice is praziquantel. Not eating raw fresh water crabs can dramatically reduce the chance of infection.

References

  • “Foundations of Parasitology” Larry S. Roberts and John Janovy, Jr., seventh edition McGraw Hill 2005, pages 279-283
  • “Lung Flukes: Paragonumus westermani” Power Point slides, Professor Ravinder Sehgal Biol 453 General Parasitology Spring 08
  • “Emerging and Reemerging Helminthiases and the Public Health of China” Peter J. Hotez, Feng Zheng, Xu Long-qi, Chen Ming-gang, Xiao Shu-hua, Liu Shu-xian, David Blair, Donald P. McManus, and George M. Davis. http://www.cdc.gov/ncidod/eid/vol3no3/hotez.htm

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