Schistosomiasis: Part 2

Lifecycle

S. haematobium: Eggs (about 140 um) escape from human host via micturation and are are deposited into fresh water. The eggs develop into rounded miracidia (180 um) and find a snail host (intermediate host). Asexual reproduction occurs in the intermediate host and the Miracidia develop into many cercariae (800 um) and swim in search of a human host. These cercariae are long, thin and have a forked tail, and bulbous superior glands. Once they find a host, they infect their hosts by directly penetrating the body surface with the help of their penetration glands. Once they burrow into the skin, they lose their forked tails, convert to anaerobic respiration and transform into schistosomula, which enter the vasculature and migrate into the portal system where they mature into adult worms. The adults then move against the flow of blood to the mesenteric or vesicular veins, depending on the species (John, Petri). At this point they are diecious and must find a mate of the opposite sex in order to produce eggs. Eggs are released by the female parasites within the vasculature, then they cross the endothelium and basement membrane of the vein, and enter the basement membrane and epithelium of the bladder. Eggs are released from the body in the urine 2 .

S. japonicum and S. mansoni have the same life cycle as S. haematobium except that they infect the basement membrane and epithelium of the intestine rather than the bladder and exit the body in the stool rather than the urine 2 .

Morphology

Adult human schistosomes are diecious (male and female worms are separate organisms), and the sexes have different morphologies. The adult worms are bilaterally symmetrical and have both a digestive system and oral and ventral suckers for attachment and stabilization. Male worms are .6-2.2 centimeters in length and rather thick. They possess a structure known as a gynecophoral canal running the length of the body in which the 1.2-2.6 centimeter-long female remains during much of the life cycle 3 .

The thinner female separates from her mate to migrate to the venules bordering the intestine or bladder in order to deposit eggs 3 . These eggs are responsible for the clinical manifestations of schistosomiasis, and the eggs of each species are easily-distinguishable.

S. haematobium produces rather ovoid eggs with a straight terminal spine.

S. intercalatum eggs are ovoid as well, and have a curved terminal spine.

S. japonicum eggs are more circular, and have no spine.

Intermediate Host

Intermediate Hosts: Three genera of freshwater snails host miracidia of the important human schistosomes.

Bulinus (S. haematobium)

Biomphalaria (S. mansoni)

Oncomelania (S. japonicum)

Reservoirs

S. mansoni has been found to rarely parasitize monkeys and some rodents, but does not have any genuinely established non-human hosts .

S. haematobium has no known animal reservoirs.

S. japonicum finds competent reservoirs in many domesticated animals including “cats, dogs, cattle, horses and pigs.” Wild mammalian reservoirs are also known for this species 3 .

Transmission

The perpetuation of schistosome infection within a population relies on several factors. First,there must be humans (or in the case of S. japonicum, animals) and snails living in close proximity and moving through the same aqueous environments. Additionally, infected humans must excrete their feces or urine into or nearby the snail-infested water. These are the conditions necessary to maintain the multi-stage life cycle.

Because multiple settlements often use the same or adjoining water sources the level of intervillage connectivity in particular places may cause varying rates of transmission. In areas with high levels of connectivity, geographic clustering of parasites may result, producing a greater schistosome burden in the water sources and an increased likelihood of local infection. The degree of schistosomiasis presence upstream has an impact on the degree of infection in villages downstream and using the same water source 4 .

The demands of growing populations for water and electricity has often led to increased levels of transmission as a result of irrigation and the building of dams. The slow-moving water created by dams and irrigation canals is ideal habitat for the host snails in many areas. 5 Additionally, in South Asia, rice production contributes greatly to schistosomiasis transmission, as the rice fields are essentially flooded plots often fertilized with animal or human waste 6 .

Index

References

2 Pearce, Edward J and Andrew S. MacDonald. 2002. "The Immunobiology of Schistosomiasis". Nature Reviews 2: 499-511.

3 John, David T and William A. Petri, Jr. Markell and Voge's Medical Parasitology. 9th ed. United States: Saunders Elsevier, 2006. 181-196.

4 Xu, Bing, Gong, Peng, Seto, Edmund, Liang, Song, Yang, Changhong, Wen, Song,Qiu, Dongchuan, Gu, Xueguang & Spear, Robert (2006) A Spatial-Temporal Model for Assessing the Effects of Intervillage Connectivity in Schistosomiasis Transmission. Annals of the Association of American Geographers 96 (1); 31-46.

5 Chitsulo, L., D. Engels, A. Montresor, L. Savioli. 2000. The global status of schistosomiasis and its control. Acta Tropica; 77 41–51.

6 Spear, Robert. Telephone interview. 18 May 2006.

img2 Corniss, Michael Patrick. The 'Scope. Schistosoma page. 27 Aug. 2001. 17 May 2006. (sporocyst from life cycle)

img3 Beran, Bernadett, Sebastian Brachs, Wlfried Haas and Tina Loy. Parasitology. Department of Zoology, University Erlangen. 17 May 2006 http://www.biologie.uni-erlangen.de/parasit/contents/research/scercar.html (cercaria from life cycle)

img4DPDx Laboratory Identification of Parasites of Public Health Concern. 2002. Centers for Disease Control and Prevention. 17 May 2006 http://www.dpd.cdc.gov/dpdx/HTML/Schistosomiasis.asp?body=Frames/S-Z/Schistosomiasis/body_Schistosomiasis_mic1.htm(eggs from life cycle)

img5Opperdoes, Fred. Schistosomiasis page. 27 Nov. 2002. 17 May 2006 http://www.icp.ucl.ac.be/~opperd/parasites/schisto1.html ( Adult S. haematobium Courtesty of WHO/TDR)

img6“Schistosomiasis.” DPDx Laboratory Identification of Parasites of Public Health Concern. 2002. Centers for Disease Control and Prevention. 17 May 2006 http://www.dpd.cdc.gov/dpdx/HTML/Schistosomiasis.asp?body=Frames/S-Z/Schistosomiasis/body_Schistosomiasis_mic1.htm (S.haematobium egg)

img7“Schistosomiasis.” DPDx Laboratory Identification of Parasites of Public Health Concern. 2002. Centers for Disease Control and Prevention. 17 May 2006 http://www.dpd.cdc.gov/dpdx/HTML/Schistosomiasis.asp?body=Frames/S-Z/Schistosomiasis/body_Schistosomiasis_mic1.htm (S.intercalatum egg)

img8“Schistosomiasis.” DPDx Laboratory Identification of Parasites of Public Health Concern. 2002. Centers for Disease Control and Prevention. 17 May 2006 http://www.dpd.cdc.gov/dpdx/HTML/Schistosomiasis.asp?body=Frames/S-Z/Schistosomiasis/body_Schistosomiasis_mic1.htm(S.japonicum egg)

img9 NERCRMP. North Eastern Region Community Resource Management Project for Upland Areas. Apr 2003. Rice Plantation. 17 May 2006. http://www.necorps.org/photo%20gallery/pages/rice_plantation_00.htm (Rice Fields)

All other images created by Laura Davis