Main » Infectious diseases » Snail-mediated Helminthiases » Schistosomiasis
The schistosomes provide a classical example of the life cycle of trematodes. Eggs passed in the faeces or urine hatch in water and the emerging miracidia invade susceptible snails. After reproduction through two generations of sporocysts, cercariae develop. These emerge from the snails and penetrate the skin of new human hosts who pass through the water. With the loss of the cercarial tail during penetration the parasite becomes a schistosomule which migrates actively through the body until it reaches the portal venous system of the liver. After copulating, the female lays eggs either in the vessels of the mesenteric plexus surrounding the large intestine (A. mansoni, Schistosoma japonicum and Schistosoma intercalatum), or the venous plexus of the bladder (5. haematobium). Spiny eggs penetrate the mucosa of these organs and can thus reach the exterior in faeces or urine. The use of molluscicides to control snail breeding, combined with mass chemotherapy is one of the best available means of combating schistosomiasis.
Distribution of Schistosoma mansoni. The distribution of schistosomiasis is regulated both by the presence of susceptible snail intermediate hosts and human sanitary habitSchistosoma Schistosoma mansoni occurs in Africa, the Middle East and in parts of South America. It was introduced from the Old World into the New World where potentially susceptible species of Biomphalaria were already present.
Schistosoma haematobium Schistosoma japonicum
Distribution of Schistosoma haematobium and Schistosoma japonicum. Schistosoma haematobium is found in Africa and the Middle East. Schistosoma japonicum is endemic in the Far East, South East Asia and the Philippines.
Photo 1. Miracidium in egg of Schistosoma mansoni. The three common species of schistosomes infecting man have easily recognisable eggs although those of Schistosoma haematobium may be confused with Schistosoma intercalatum. Miracidia can be seen inside mature eggSchistosoma (x600)
Photo 2. Living miracidium of Schistosoma mansoni hatching. These may readily be seen by immersing eggs separated from faeces in a large quantity of fresh water in which they hatch. (x 600)
Photo 3. Miracidium free in water.
Photo 4. Apical papilla of miracidium. This scanning electron micrograph shows the surface locomotor cilia and apical papilla with its 'mini-sucker' which helps it attach to a snail. (x3300)
Photo 5. Mother sporocyst in the hepatopancreas of a snail. The cycle in the snail is of variable duration depending on the species of parasite, host and environmental conditions, but it is usually only one month. Cercaria develop in the second generation ('daughter') sporocystSchistosoma The figure shows on the top several coils of the sporocyst dissected from the hepatopancreas. One of the cercaria has broken out and is seen at the bottom of the figure. (x 60)
Photo 6. Living bifurcate cercaria of Schistosoma mansoni. Once snails start to 'shed' cercariae they continue to do so during daylight hours for up to as much as 200 days. A heavily infected snail may shed 1500 to 2000 cercariae a day. (x 350)
Photo 7. Schistosomule of Schistosoma mansoni.
Photo 8. Living male and female Schistosoma mansoni in mesenteric vein of mouse. The slender female is seen within the gynecophoral groove of the male. (x 4.5)
Photo 9. Morphology of adults. Mature males and females live in copulating pairSchistosoma The common species are recognised by the characters shown in the figures. Testes of males: Schistosoma haematobium (9); Schistosoma mansoni (10); Schistosoma japonicum (11). Ovaries of females: Schistosoma haematobium (12); Schistosoma mansoni (13); Schistosoma japonicum (14). (x15)
Photo 10. Morphology of adults (Schistosoma mansoni).
Photo 11. Morphology of adults (Schistosoma japonicum).
Photo 12. Morphology of adults (Schistosoma haematobium).
Photo 13. Morphology of adults (Schistosoma mansoni).
Photo 14. Morphology of adults (Schistosoma japonicum).
Photo 15. Living intermediate hosts of schistosome. The snail intermediate hosts of Schistosoma mansoni are the species of Biomphalaria. Various species of Bulinus (16) are hosts of Schistosoma haematobium through its range. Oncomelania species (17) are hosts for Schistosoma japonicum in the Far East. Three examples are shown in the figure (see also Table). (x4)
Photo 16. Living intermediate hosts of schistosomes (Various species of Bulinus).
Photo 17. Living intermediate hosts of schistosomes (Oncomelania species).
Photo 18. Sampling snail populations. A scoopful of Biomphalaria sudanica, a host for Schistosoma mansoni in Lake Victoria.
Photo 19. Dermatitis from avian cercariae in Japanece patient. Penetration of the skin by cercariae may give rise to an itchy rash known as 'cercarial dermatitis'. This is occasionally seen in countries free of human schistosomiasis due to invasion by the cercariae of avian schistosomes which are otherwise non-pathogenic to man.
Photo 20. Cycle of human schistosomes.
The three common parasites of man, Schistosoma haematobium, S. japonicum and S. mansoni have a similar life cycle. Eggs passed in urine (S. haematobium A) or faeces (Y japonicum B, S. mansoni C), hatch in aggregations of water such as ponds, lake edges, streams and canals. From the eggs miracidia (D) hatch into the water where they penetrate into suitable snails. In the snails they develop two generations of sporocysts (not shown here) the second of which produces fork-tailed cercariae (E). These penetrate the skin (F) when a new host comes into contact with contaminated water. Once through the skin the cercariae shed their tails and become schistosomulae which migrate through the tissues until they reach the portal venous system of the liver (G). There males and females (H) copulate before settling down in pairs in the venous system of the liver. From there they migrate to the venous plexus of the bladder (I) or rectum (J) where spiny eggs are laid. These penetrate into the bladder or rectum from which they reach the exterior. Eggs laid by worms in the liver itself lead to local fibrotic changes and cirrhosis.
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» Schistosomes Found Uncommonly in Man
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