Schistosoma

Part of the class Trematoda, Schistosoma —commonly known as the blood-fluke— is “the most important human parasite from a world health perspective” (Gilbertson, 1999. It has four main species: Schistosoma mansoni or else known as Manson’s blood fluke or swamp fever, found AFrica, Brazil, Venezuela, Surian, the lesser Antilles, Puerto Rico, and the Dominican Republic. S. japonicum whose common name is simply blood fluke, found widely spread in Eastern Asia and the southwestern Pacific. In Taiwan, this specie only affects animals, not humans. S. mekongi is related to the S. japonicum, in which affect both superior and inferior mesenteric veins. S. mekongi differs in that it has smaller eggs, a different intermediate host, and longer prepatent period in the mammalian host. Finally S .haematobium that is referred to as the bladder fluke, originally founf in Africa, the Near East, and the Mediteranean basin, was introduced into India during World War II. S. indicum, S. nasale, S. leiperi are parasites of ruminants. Trematoda is one of the four classes of the phylum Platyhelminthes; the others being Turbelleria, Monogenea and Cestoda. Members of the phylum Platyhelminthes are soft bodied animals with no body cavity (aceolomates) characterized by bilateral symmetry, cephalization, and are triploblastic. Flatworms are also hermaphroditic in wchi case a single animal is both a male and a female, producing sperms and eggs at the same time. Because of their thin bodies, flatworms don’t need any circulatory or respiratory systems; these functions are carried on by simple diffusion through their body wall. Also, they possess “protonephredia, [nephridial] organs that function in osmoregulation and disposal of metabolic wastes.” (Solomon 2002). In her book The Management of Schistosomiasis, Patricia L. Rosenfield reports that in the case of S. haematobium, the adult worm lives in the venules on the wall of the human bladder (and rectum), while the adult S. mansoni and S. japonicum inhabit the blood vessels of the human large intestine (p.1), in the venules of the colon. Unlike all the other trematodes however, schistosomes are diecious in that the sexes are separate, and “the smaller female lies in the canal of the male where they are in permanent copulation.” (Gilbertson, 1999).

• Their life cycle can be portrayed as follow:
  1. Diagnostic stage:
     “In this stage, the adult blood fluke lay eggs that develop and leave the human host through the feces and urine. Miracidia (ciliate larvae) hatch from the eggs in freshwater and each miracidium enters appropriate species of snail, who will be the intermediate host. This miracidium will form a mother sporocyst that will produce many daughter sporocysts. The daughters enclose many developing larvae called cercariae (see above picture) that will break out of the daughter cell to leave the snail and become a free-swimming cercariae. Then, it will enter the next stage of the life-cycle.”(Leventhal and Cheadle, 2002).
  2. Infective stage:
     “The stage begins when the cercariae penetrates the skin of a human being; it leaves its tail behind and become a schistosomule.”(Leventhal and Cheadle, 2002). “This schistosomule will enter the bloodstream to migrate through the blood vessels to the appropriate site. The schistosomule will migrate to the lungs then to the liver, where the adults will mate, and in the case of S. japonicum and S. mansoni migrate to mesenterics and live in the mesenteric venules; as for S. haematobium, the adults will migrate to the bladder, where they will live in the venous plexus.”(Despommier, 1995.

And the cycle goes round in a vicious circle. The tissues are severly damaged by the presence of schistosomes (adult and eggs) in the veins for they will “hemorrhage, become inflamed, fibrotic and will be unable to function normally. The damage to the small intestine (or urinary bladder) and liver accumulate over time and result in a chronic, disabling disease that can be fatal.” (http://www.biosci.ohio-state.edu/~parasite/home.html). “In addition, severe complication such as cirrhosis of the liver, cancer of the bladder, and central nervous system disorders may occur in humans affected with high worm loads.” (Rosenfield, 1979)

• Treatment

Chemotherapy has been both difficult and dangerous, however, drugs have become available that promise effective conrtol. Praziquantel, for example, with a 1-day oral administration is well tolerated and reportedly attains 95% cure of all schistosomes of humans at any stage of development.

“Adult schistosomes have a basic bilateral symmetry; suckers; a body covering which is a syncytial integument with the sunken cytons; a blind alimentary system consisting of mouth, oesophagus and bifurcated caeca; the area between the integument and alimentary canal filled with a loose cellular network; musculature of circular, oblique, longitudinal and special fibers; and an excretory or osmoregulation system based on flame cells (protonephridia).” (Stirewalt, 1973). "The three types of schistosomes feed on hemoglobin that they split apart with the help of enzymes. They digest the hemoglobin portion, and regurgitate the heme moiety as pigment into the bloodstream. They absorb low-molecular weight nutrients such as free amino acids and sugars by active transport through the tegumental surface and store glycogen as a reserve source of energy. Their energy metabolism is facilitated largely through glycolysis.” (Despommier, 1995)

Their Taxonomy levels are as follow:

• Kingdom: Animalia; Eukaryotes, multicellular heterotrophs, differentiated cells, complex organ systems, nervous sytem. (ie: mammals, sponges, insects)
  • Phylum: Platyhelminthes; Flatworms, bilateral symmetry, tripoblastic, soft body, acoelomates, 20000 species in 4 classes
    • Subphylum: Euplatyhelminthes
      • Superclass: Rabdotiphora; 2 hosts, eudoparasites
        • Class: Trematoda; monoecious or dioecious
          • Subclass: Digenea
            • Order: Strigeiformes
              • Genus: Schistosoma; Dioecious, blood flukes
                • Species: S.japonicum, S. mansoni, S. haematobium, S. mekongi