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.
Morphology
In size, shape, and color, Paragonimus westermani resembles a coffee bean when alive. Adult worms are 7.5 mm to 12 mm long and 4 mm to 6 mm wide. The thickness ranges from 3.5 mm to 5 mm. The skin of the worm (tegument) is thickly covered with scalelike spines. The oral and ventral suckers are similar in size, with the latter 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. Inspection of the tegumental spines and shape of the metacercariae may distinguish between the 30-odd species of Paragonimus spp. but the distinction is sufficiently difficult to justify suspicion that many of the described species are synonyms.
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. Cercaria (not shown): Cercariae are often indistinguishable between species. There is a large posterior sucker, and the exterior is spined. Metacercaria: Metacercariae are usually encysted in tissue. The exterior is spined and has two suckers Adults: Adult flukes are typically reddish-brown and ovoid, measuring 7 to 16 mm by 4 to 8 mm, similar in size and appearance to a coffee bean. They are hermaphroditic, with a lobed ovary located anterior to two branching testes. Like all members of the Trematoda, they possess oral and ventral suckers.
Discovery Paragonimus westermani was discovered when two Bengal tigers died of paragonimiasis in zoos in Europe in 1878. Several years later, infections in humans were recognised in Formosa (present-day Taiwan). P. westermani was discovered in the lungs of a human by Ringer in 1879 and eggs in the sputum were recognized independently by Manson and Erwin von Baelz in 1880. Manson proposed the snail as an intermediate host and various Japanese scientists detailed the whole life cycle in the snail between 1916 and 1922. The species name P. westermani was named after Pieter Westerman (1859–1925), a zookeeper who noted the trematode in a Bengal tiger in an Amsterdam Zoo[Artis].
Life cycle Unembryonated eggs are passed in the sputum of a human or feline. Two weeks later, miracidia develop in the egg and hatches. The miracidia 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 freshwater. The crawling cercariae penetrate freshwater crabs and encyst in its muscles becoming metacercaria. Humans or felines then eat the infected crabs raw. Once eaten, the metacercaria excysts and penetrates the gut, diaphragm and lung where it becomes an adult worm in pairs. The first intermediate hosts of the Paragonimus westermani are freshwater snails:
Koreoleptoxis amurensis Semisulcospira calculus Semisulcospira cancellata Semisulcospira extensa Semisulcospira gottschei Semisulcospira libertina – synonym: Semisulcospira toucheana Semisulcospira mandarina – synonym: Semisulcospira wegckiangensis Semisulcospira multicincta Semisulcospira nodiperda Semisulcospira nodiperda quinaria Semisulcospira paucincta Semisulcospira peregrinomum For many years Tarebia granifera was believed to be an intermediate host for the Paragonimus westermani, but Michelson showed in 1992 that this was erroneous. Paragonimus has a quite complex life-cycle that involves two intermediate hosts as well as humans. Eggs first develop in water after being expelled by coughing (unembryonated) or being passed in human feces. In the external environment, the eggs become embryonated. In the next stage, the parasite miracidia hatch and invades the first intermediate host such as a species of freshwater snail. Miracidia penetrate its soft tissues and go through several developmental stages inside the snail but mature into cercariae in 3 to 5 months. Cercariae next invade the second intermediate host such as crabs or crayfish and encyst to develop into metacercariae within 2 months. Infection of humans or other mammals (definitive hosts) occurs via consumption of raw or undercooked crustaceans. Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite. The metacercariae excyst in the duodenum, penetrate through the intestinal wall into the peritoneal cavity, then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults. The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. However, when this takes place completion of the life cycles is not achieved, because the eggs laid cannot exit these sites.
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