Paragonimus westermani

Geographic Range

Paragonimus westermani is endemic to freshwater environments in far east and southeast Asia. This parasite has been found in countries ranging from China to Indonesia to India. Although P. westermani has been identified in individuals in non-Asian countries, these cases all involved immigrants from Asia. (Miyazaki and Habe, 1976; Nishimura and Hung, 1997; Tandon, et al., 2007; Terasaki, et al., 1995)


Paragonimus westermani inhabits a variety of freshwater environments where its host organisms reside. It is especially common in well-oxygenated flowing water areas, such as rivers and streams. Cercariae can be found crawling along the rocks on the river bed searching for a crustacean host, while adult worms will be generally located inside the lungs of carnivorous mammalian hosts that feed on crustaceans living in these bodies of water. (Kuntz, 1969; Roberts, et al., 2009)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools

Physical Description

Adult P. westermani are short in length and very thick. Their size and shape, combined with their reddish brown color has the appearance of a coffee bean. Oral and ventral suckers are present, and they are roughly the same size with the ventral sucker located preequatorially. Scale-like spines cover the tegument of the organism. Paragonimus westermani is hermaphroditic, and its ovary and testes are lobed. The testes are found in the posterior region of the body while the ovary is found slightly postacetabular (posterior to the acetebulum) and left of the midline. The genital pore is also postacetabular. The dimensions are from 7.5 to 12 mm by 4 to 6 mm. Defining characteristics of this species are singly-spaced spines and a six-lobed ovary, although variants with a five-lobed ovary have been discovered.

The eggs of P. westermani measure 80 μm to 118 μm by 48 μm to 60 μm. They are yellow-brown in color with an ovoid shape and an operculum structure. These eggs hatch and release a larval stage known as a miracidium.

Specific information regarding the morphology of the miracidium and sporocyst stages of P. westermani is not readily available. Digenean miracidia are small, ciliated organisms which often resemble protozoa. An apical papilla is located anteriorly, which contains duct openings that connect to penetration glands within the organism. The posterior half of the miracidia contains germ balls that will be used in a later stages to asexually produce offspring.

Miracidia enter a host and become sporocysts. Sporocysts lack cilia and many other external structures of miracidia. They have no mouth or digestive system, instead absorbing host nutrients through their skin. Sporocysts are sac-like in appearance and may be filled with redia, which make up the next stage of development.

The rediae of P. westermani are light yellow in color and elliptical in shape. They have no appendages, but they do have a mouth and well-developed pharynx and intestines. Sensory hairs can be found around the mouth. Their dimensions are approximately 460 to 850 μm by 170 to 360 μm.

Cercariae are oval in shape with a small tail. The tegument of the cercariae is covered in spines and sensory hairs. A mouth located ventrally on the anterior end of the organism surrounded by a muscular oral sucker. Oral stylets are present. Fourteen penetration gland cells can be found on the acetabulum, which is located ventrally and postequatorially (posterior and perpendicular). Dimensions were found to be 162.5 to 250 um by 67.5 to 100 um.

Metacercariae are whitish in color and may be round or oval in shape. They have a oral sucker located ventrally on the anterior end and a ventral sucker located slightly anterior of the mid-ventral line. Spines cover the skin, and papillae are located around the suckers and the ventro- and dorso-lateral sides of the organism. Size ranges from 335 to 389 μm with an average diameter of 352 μm. Spinal and papillae arrangement as well as the size of the cyst wall formed in the host may be used at this stage to distinguish P. westermani from similar species. (Higo and Ishii, 1987; Ishii, 1966; Roberts, et al., 2009; Tomimura, 1989)

  • Range length
    7.5 to 12 mm
    0.30 to 0.47 in


Paragonimus westermani hatches from its egg as a ciliated miracidium in a freshwater aquatic environment and searches for a snail that becomes its first intermediate host. The miracidium burrows into the soft tissues of the snail, loses its cilia, and develops into a sac-like sporocyst. As a sporocyst, P. westermani can asexually reproduce a number of redia, which are the next life stage of this organism. These redia are released from the sporocyst so that they can also asexually reproduce more daughter redia, which will then asexually produce cercariae. This initial period of development occurs over a period of about four months.

Cercariae will leave the snail host as free-swimming organisms and attempt to locate a crustacean, such as a crab. Paragonimus westermani will infect the crab either orally or percutaneously (under the skin). A cercaria will then develop into a metacercaria and encyst itself in the liver, gills, or muscular tissue of the crab while waiting for a predator to consume the crab. Paragonimus westermani has a wide range of carnivorous hosts, including birds, felines, and humans. Once in the intestines of the definitive host, P. westermani will excyst and migrate through the intestinal wall into the perotineal cavity. The parasite will travel through the abdominal wall and the diaphragm to the lungs, where it will develop into its final adult worm stage. The worm will encyst iteself in the lungs, mate, and produce more eggs which are released from the host in the sputum or feces if the sputum is swallowed by the host. These eggs will take sixteen days to several weeks in fresh water to produce fully-developed miracidia. (Kanazawa, et al., 1987; Kuntz, 1969; Noble, 1955; Roberts, et al., 2009; Tomimura, 1989; Yokogawa, et al., 1962)


Paragonimus westermani generally forms monogamous pairs. Two worms will locate each other in the lungs, form a cyst, and begin mating. The hermaphroditic worms will then exchange sperm and begin producing eggs. (Fan and Chiang, 1970; Roberts, et al., 2009; Song, et al., 2010)

Paragonimus westermani is capable of both sexual and asexual reproduction at certain stages of its life cycle. While living in the snail intermediate host, P. westermani can asexually produce many offspring in its sporocyst and redia stages.

Adult worms may produce viable eggs through either self- or cross-fertilization. Adults will crawl around the lung until they meet another worm, and then they will form a lung cyst. These worms will exchange sperm and produce fertilized eggs. However, cysts with only one worm have also been found to produce viable eggs, indicating self-fertilization as a alternative to cross-fertilization. Once the eggs are produced, they will be coughed out of the lungs into the trachea and swallowed. The eggs will then leave the body with the feces of the host. (Fan and Chiang, 1970; Noble, 1955; Roberts, et al., 2009; Tomimura, 1989)

There is no parental investment with Paragonimus westermani. Species survival in ensured by the production of large numbers of offspring. (Roberts, et al., 2009)

  • Parental Investment
  • no parental involvement


The life span of Paragonimus westermani is greatly dependent on whether or not it finds a host species. Studies on other members of the Order Digenea have shown that the miracidia will usually only survive for a few hours in the water unless it finds a host. Once inside the snail intermediate host, P. westermani will take approximately four months to develop cercariae. Although specific information on the lifespan of P. westermani cercariae was not available, studies in other digeneans have shown them to last for around one to four days without finding a host. Adult worms have been found to persist in human hosts for five to ten years, although occasionally infections have been found that last up to twenty years. It is reasonable to assume P. westermani can exist a similar duration of time in its other hosts as well. (Cross, et al., 2001; Johnson, et al., 1985; Noble, 1955; Oliver and Short, 1956)

  • Typical lifespan
    Status: wild
    5 to 20 years
  • Average lifespan
    Status: wild
    5-10 years


Paragonimus westermani utilizes a few different methods of locomotion during its life cycle. As a miracidium, it relies on cilia to propel itself. As a cercaria, it moves along in a fashion similar to an inchworm. In the final host, P. westermani has a curious migration route through the body. Once in the intestine, the metacercariae develop into juvenile worms which penetrate the intestinal wall withing thirty to sixty minutes. The worms move to the abdominal cavity in around three to six hours and then penetrate the abdominal wall. Paragonimus westermani undergoes a period of growth for six to ten days before reentering the abdominal cavity, tunneling through the diaphragm into the pleural cavity, and reaching the lungs. (Fan and Chiang, 1970; Roberts, et al., 2009; Yokogawa, et al., 1962)

Communication and Perception

Very little is known about the specific communication methods and sensory organs of Paragonimus westermani. Papillae are seen on several life stages of this organism, and they are presumed to have a sensory role. However, studies have not ascertained the exact function of these structures.

Various digeneans are known to have photoreceptor, chemoreceptor, tangoreceptor, and statoreceptor organs, so it is likely that P. westermani also has some of these adaptations. (Higo and Ishii, 1987; Roberts, et al., 2009; Sukhdeo and Sukhdeo, 2004)

Food Habits

Paragonimus westermani is a parasite that feeds on host tissues and proteins, including hemoglobin. The worm does this using a variety of enzymes and peptidases. (Song, et al., 2008)

  • Animal Foods
  • blood
  • body fluids


There is no information available about any anti-predator defenses in Paragonimus westermani.

Ecosystem Roles

Paragonimus westermani is a parasite of a wide variety of snails, crustaceans, and carnivorous mammals in its environment. It can also inhabit a number of paratenic hosts. (Blair, et al., 2001; Fan and Chiang, 1970; Miyazaki and Habe, 1976; Nishimura and Hung, 1997; Noble, 1955; Roberts, et al., 2009)

Species Used as Host

Economic Importance for Humans: Positive

Paragonimus westermani is a common parasite of humans, and it has been used for research purposes to help better understand trematode infections.

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

Paragonimiasis is a parasitic infection caused by Paragonimus westermani. The infection is often asymptomatic, but when it causes disease, P. westermani generally manifests as a lung infection due to its formation of cysts in lung tissue. An inflammatory response triggered by the worm causes the formation of granulomas, which can make the disease present like tuberculosis or other pulmonary infections. Symptoms can include chest pain, bloody sputum, bronchitis, and a bad cough. X-rays may reveal lung lesions similar to those produced by tuberculosis. Infected individuals can be definitively diagnosed by finding P. westermani eggs in feces or sputum. Occasionally, the infection can become disseminated if worms spread and encyst in other tissues besides the lungs. This can have serious complications if the worm encysts in the central nervous system or the heart. Cerebral paragonimiasis can cause meningitis, hemorrhaging, and calcification of the central nervous system. Common symptoms include headaches, seizures, or mental changes that may be similar to other diseases that affect the central nervous system. The disease can usually be effectively treated with praziquantel, an anthelminthic drug.

Paragonimiasis affects roughly twenty-two million people worldwide, but it is particularly prevalent in East Asia due to the practice of eating undercooked or pickled crustaceans that may be carrying the metacercariae of P. westermani. Humans can also become infected by paragonimiasis by eating raw boar or pig meat because these animals serve as paratenic hosts. The disease may also effect pets or livestock, including cats, dogs, and, as previously mentioned, pigs. (Johnson, et al., 1985; Miyazaki and Habe, 1976; Nishimura and Hung, 1997)

Conservation Status

Paragonimus westermani is not found on any endangered or threatened species lists.


David Fuller (author), University of Michigan-Ann Arbor, Heidi Liere (editor), University of Michigan-Ann Arbor, John Marino (editor), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.



living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map


reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal


uses smells or other chemicals to communicate


a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


Having one mate at a time.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


found in the oriental region of the world. In other words, India and southeast Asia.

World Map


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


development takes place in an unfertilized egg


reproduction that includes combining the genetic contribution of two individuals, a male and a female


mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


Blair, D., G. Davis, B. Wu. 2001. Evolutionary relationships between trematodes and snails emphasizing schistosomes and paragonimids. Parasitology, 123 (7): 229.

Cross, M., S. Irwin, S. Fitzpatrick. 2001. Effects of heavy metal pollution on swimming and longevity in cercariae of Cryptocotyle lingua (Digenea: Heterophyidae). Parasitology, 123 (5): 449.

Fan, P., C. Chiang. 1970. Exposure of kittens and puppies to single metacercariae of Paragonimus westermani from Taiwan. The Journal of Parasitology, 56 (1): 48-54.

Higo, H., Y. Ishii. 1987. Comparative studies on surface ultrastructure of newly excysted metacercariae of Japanese lung flukes. Parasitology Research, 73 (6): 541.

Ishii, Y. 1966. Differential morphology of Paragonimus kellicotti in North America. The Journal of Parasitology, 52 (5): 920-925.

Johnson, R., E. Jong, S. Dunning, W. Carberry, B. Minshew. 1985. Paragonimiasis: Diagnosis and the use of praziquantel in treatment. Reviews of Infectious Diseases, 7 (2): 200-206.

Kanazawa, T., H. Hata, S. Kojima, M. Yokogawa. 1987. Paragonimus westermani: a comparative study on the migration route of the diploid and triploid types in the final hosts. Parasitology Research, 73 (2): 140.

Kuntz, R. 1969. Biology of Paragonimus westermani (Kerbert, 1878) Braun, 1899: Infection in the crab host (Eriocheir japonicus de Haan) on Taiwan. Transactions of the American Microscopical Society, 88 (1): 118-126.

Lee, H. 1965. Paragonimus westermani infection in wild mammals and crustacean host in Malaysia. The American Journal of Tropical Medicine and Hygiene, 14 (4): 581.

Miyazaki, I., S. Habe. 1976. A newly recognized mode of human infection with the lung fluke, Parogonimus westermani (Kerbert 1878). The Journal of Parasitology, 62 (4): 648-648.

Nishimura, K., T. Hung. 1997. Current views on geographic distribution and modes of infection of neurohelminthic diseases. Journal of the Neurological Sciences, 145 (1): 5-14.

Noble, G. 1955. Three Play Host. BIOS, 26 (1): 15-22.

Oliver, J., R. Short. 1956. Longevity of miracidia of Schistosomatium douthitti. Experimental Parasitology, 5(3): 238-249.

Roberts, L., J. Janovy, Jr., G. Schmidt. 2009. Foundations of Parasitology. New York, NY: McGraw-Hill.

Song, S., J. Park, J. Kim, S. Kim, Y. Hong, H. Kong, D. Chung. 2008. Identification and characterization of Paragonimus westermani leucine aminopeptidase. Parasitology International, 57 (3): 334-341.

Song, S., J. Shin, J. de Guzman, J. Kim, H. Yu, B. Jha, H. Kong, Y. Hong, D. Chung. 2010. Paragonimus westermani: Identification and characterization of the fasciclin I domain-containing protein. Experimental Parasitology, 125 (2): 76-83. Accessed April 13, 2011 at

Sukhdeo, M., S. Sukhdeo. 2004. Trematode behaviours and the perceptual worlds of parasites. Canadian Journal of Zoology, 82 (2): 292.

Tandon, V., P. Prasad, A. Chatterjee, P. Bhutia. 2007. Surface fine topography and PCR-based determination of metacercaria of Paragonimus sp. from edible crabs in Arunachal Pradesh, Northeast India. Parasitology Research, 102 (1): 21-28.

Terasaki, K., S. Habe, L. Ho, H. Jian, T. Agatsuma, T. Shibahara, H. Sugiyama, K. Kawashima. 1995. Tetraploids of the lung fluke Paragonimus westermani found in China. Parasitology Research, 81 (7): 627.

Tomimura, T. 1989. Parasitological survey of the first intermediate host of Paragonimus westermani in Iga area of Mie Prefecture, Japan. The Japanese Journal of Veterinary Science, 51: 315.

Yokogawa, M., H. Yoshimura, M. Sano, T. Okura, M. Tsuji. 1962. The route of migration of the larva of Paragonimus westermani in the final hosts. The Journal of Parasitology, 48 (4): 525-531.