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Schistosoma japonicum

By Sarah Green

Geographic Range

Schistosoma japonicum is found in China, Taiwan, the Philipines and Southeast Asia. (Roberts and Janovy 2000, WHO 1996)

Habitat

This species must have snail and vertebrate hosts to survive. The snails it uses live in warm freshwater streams, ponds, and ditches.

Physical Description

These parasitic worms have a complex life cycle with many stages.

The first stage, the egg, is round or oval and very small (about 80 by 60 micrometers), with a very small spur on one end.

The first stage that hatches from the egg is called a miracidium, and is not much bigger then the egg. It is free-swimming,ovoid, covered with cilia, and could easily be mistaken for protozoan. It has a complex of glands at the front end that are used to penetrate the skin of a snail host.

Once inside a snail, the miracidium sheds its skin and cilia, and metamorphoses into a different shape called a sporocyst. This stage has no mouth or gut, it takes its food directly from the snail it lives in. Each sporocyst reproduces asexually -- creating daughter sporocysts. These live and grow inside the snail host, completing another round of asexual reproduction, but this time the offspring have a different structure, and are called cercaria.

S. japonicum cercaria look like tiny flatworms with fishtails. The body is about 200 by 70 micrometers, the long thin tail is another 220 micrometers long, and has two "flukes" called furci that are each about 80 micrometers long. The front end is equipped with an array of glands and other structures for entering a new host. This stage leaves the snail and enters a bird or mammal (including humans) host and then transforms again, shedding it's tail and moving through the bloodstream.

The worm finally matures inside the intestinal veins of a mammal. It grows to about 15 mm in length. Males are shorter and stouter than females, and have a long groove on their underside in which the female (should one arrive) attaches herself. Both sexes have a strong sucker around the mouth, and another called an acetabulum, a little ways further down on the ventral side. The skin of the worms is coated with tiny spines, ridges and sensory organs that are probably involved in helping resist and avoid the host immune system.

Reproduction

This species reproduces sexually: a male and female worm must mate in the veins of the host before the female can lay eggs (many other flatworms can produce eggs without mating first). Female worms produce enormous numbers of offspring: a single mated pair may release 3,000 eggs per day, and live for over 20 years. Through a complex process involving secretions from the egg and the hosts' immune system, the eggs are moved through the wall of the vein and into the gut or bladder, where they are excreted by the host. Once they are exposed to freshwater the eggs hatch, and the miracidium emerges. This stage lives for only a few hours, and must find a snail in the genus Oncomelania to tunnel into or it will die. Inside the snail it becomes a sporocyst (larva). While in the snail the sporocysts reproduce asexually and become daughter sporocysts. The daughter sporcysts then reproduce asexually again, this time producing cercaria. It is in this stage that they enter humans and other vertebrates. The cercaria emerge from their snail host and swim in the water. If they contact a host, they quickly stick to the skin and shed their tail. Then they secrete digestive compounds that allow them to penetrate the skin of the host and enter the hosts' circulatory system. They transform again, this time into adult worms that live in the veins of the small intestine (all sources)

  • Parental Investment
  • no parental involvement

Behavior

See other sections for life-cycle information. This species has an array of chemical sensing organs that help locate hosts. The free-swimming stages (miracidium and cercaria) swim vigorously, struggling to find a host. Cercaria tend to swim to the surface of the water they are in, sink down, and then swim up again (Roberts and Janovy 2000).

Food Habits

The source of the worm's nutrition is the host. The sporocyst stage absorbs nutrition through its skin, taking it directly from the tissues of its snail host. Adult worms feed on blood in the intestinal veins where they live (Roberts and Janovy 2000)

Economic Importance for Humans: Positive

This animal does not benefit humans.

Economic Importance for Humans: Negative

S. japonicum is a parasite that uses humans as a primary host, and is a major cause of disease in the regions where it lives. Most of the harm it causes is due to the massive number eggs released. These trigger immune responses that in turn cause many health problems. As the eggs move through the host tissues, they cause fevers, weakness, liver and kidney damage, blood in the urine, and abdominal pain. In a few cases the eggs drift in the circulatory system and end up in other parts of the body, including the brain, causing even more severe damage. In places where this species occurs, many rural people are afflicted with this condition, called schistosomiasis (All Sources).

Conservation Status

There are currently no conservation efforts for this animal. Because of its negative effects on humans most efforts are aimed at destruction and containment. The World Health Organization is working on "...controlling morbidity...health education and provision of safe water." (WHO pg 3) The climate in which the flatworm is found dictates the types of control that we are able to exert in an effort to contain and eradicate it.

Other Comments

This species is one of three (the others are S. haematobium and S. mansoni) that cause schistosomiasis. They are a major cause of human illness and suffering in the tropical regions of the world.

Schistosome infection can be diagnosed by checking for eggs in the feces, by biopsy of the liver or bladder, and by serological tests for schistosome antigens. Chemical treatment using the drug praziquantel has a high degree of success. Unfortunately millions of people are infected with schistomes every day due to exposure to contaminated water. Efforts to control this disease are ongoing, but it has proved difficult to eliminate. At this time there is no vaccine for schistosomes. (see Roberts and Janovy, Maszle, Toyoda, WHO below).

Contributors

Sarah Green (author), Fresno City College, Jerry Kirkhart (editor), Fresno City College.

Glossary

Palearctic

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

World Map

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

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.

ectothermic

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

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

native range

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

oriental

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

World Map

parasite

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

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

scrub forest

scrub forests develop in areas that experience dry seasons.

References

Committee on Infectious Diseases American Academy of Pediatrics, 2000. Red Book. Elk Grove Village, Il: American Academy of Pediatrics.

Hickman, C., L. Roberts, A. Larson. 1997. Integrated Principles of Zoology. New York: McGraw-Hill Higher Education.

Maszle, D. 1996. "Schistosoma Japonica Overview" (On-line). Accessed 3 April 2001 at http://ehs.sph.berkeley.edu/china/sjap.htm.

Roberts, L., J. Janovy Jr.. 2000. Gerald D. Schmidt & Larry S. Roberts' Foundations of Parasitology, Sixth Edition. Burr Ridge, Illinois: McGraw Hill.

Toyoda, A. "Schistosoma japonicum -their nature and aspects-" (On-line). Accessed 26 March, 2001 at http://www2.ttcn.ne.jp/~akky/parasite/sjmain.htm.

World Health Organization, 1996. "Schistosomiasis" (On-line). Accessed Ocotober 24, 2000 at http://www.who.int/inf-fs/en/fact115.html.

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