Schistosoma japonicum, blood fluke

Taxonomic lineage

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Acoelomata - Platyhelminthes - Trematoda - Digenea - Strigeidida - Schistosomatoidea - Schistosomatidae - Schistosoma - Schistosoma japonicum

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General information

• Schistosoma japonicum is digenic trematode ("digenic" means that its lifecycle includes two hosts - definitive and intermediate) of the superfamily Schistosomatoidea.
• Intermediate host of S. japonicum are snails of the genus Onchomelania hupensis spp.
• S. japonicum occurs in Southeast Asia and western Pacific countries (including China, the Philipines and Indonesia).
• Apart from humans S. japonicum infect a wide range of animals including cattle, dogs, pigs, and rodents.

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Life cycle of Schistosoma japonicum

• egg female discharges 500-3,5500 eggs per day into mesenteric venule of definitive host (human); the eggs eventually obstruct blood flow in the venule causing a partial necrosis in the intestinal wall; hence, the eggs are dropped of into the lumen of the intestine, and passed to feces; eggs measure ~80 X 60 µm in diameter, are oval to round in shape with subterminal spine
• miracidium a free-swimming larva; once the egg is released into environment, miracidium hatches immediately and starts swimming; if it happened to swim into a snail of species Oncomelania, it enters the snail and starts its first life as a parasite
• sporocyst a sac-like secondary larval stage; miracidium transforms into a primary (mother) sporocyst; germ cells within the primary sporocyst begin dividing to produce secondary (daughter) sporocysts, which migrate to the snail hepatopancreas; once at the hepatopancreas, germ cells within the secondary sporocyst begin to divide again, this time producing thousands of new parasites, known as cercariae, which are the larvae capable of infecting mammals
• cercarium an infectious form of Schistosoma which infects their hosts by direct skin penetration; cercariae emerge daily from the snail host; they are highly motile, alternating between vigorous upward movement and sinking; cercarium attaches to the human skin and secretes proteolytic enzymes helping it to enter into cuteneous capillary vessel; upon the penetration the cercarium sheds its tail and transforms into schistosomulum
• schistosomulum a tailless cercarium; after penetration schistosomula migrate to the lungs (in 3-4 days), and after passing through the pulmonary capillaries, enter the systemic circulation and, eventually, are carried to the mesenteric vein of the host; there they mature into adult schistosomes in a month; male schistosome holds female between its gynecophoral canal; at this time, the infection of the final host is complete and sexual reproduction of the parasite begins
• adult the adult worms can live for years; male and female are always hugged together; up to half the eggs released by the worm pairs become trapped in the mesenteric veins, or will be washed back into the liver, where they will become lodged; trapped eggs mature normally, secreting antigens that elicit a vigorous immune response; schistosomiasis is a chronic desease characterized by abdominal pain, fever, anemia, hepatosplenomegaly; acute schistosomiasis (Katayama fever) may occur weeks after initial infection

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References

• Jones MK et al. Correlative and dynamic imaging of the hatching biology of Schistosoma japonicum from eggs prepared by high pressure freezing. PLoS Negl Trop Dis. 2008;2(11):e334.
  

  Hatching of the egg of S. japonicum, light microscopy.
  A. Activated larva prior to translocation in the shell. The egg on the left hand side contains an activated larva. Here the miracidium lies longitudinally in the egg and its terebratorium is clearly evident.
  B. The same larva has turned so that it lies across the egg.
  C. Egg immediately after shell rupture. The outer envelope and vacuoles are swelling and obscure the now smaller shell.
  D. The outer envelope is now markedly expanded and the miracidium swims actively within its increasingly viscous matrix. The shell lies to the bottom left of the figure, and is slightly out of focus. Abbreviations: OE- outer envelope; Mir- miracidium; S-shell; T-terebratorium.

• Major topic "Schistosoma japonicum" (free full-text articles in PubMed)

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