|Agricultural Development and Vector-Borne Diseases (FAO - HABITAT - UNEP - WHO, 1996, 91 p.)|
|Topic B: Relevant disease vectors|
Slide B.3 The life cycle of schistosome parasites
The life cycle of the Schistosoma parasite is complex, and involves various transformations, adapting the parasites to different micro-environments. The adult parasites, belonging to the group of trematodes (flukes), live as male/female pairs in the bloodstream of humans, either in the blood vessels of the mesenteric plexus surrounding the large intestine (S. mansoni and S. japonicum) or of the venous plexus of the bladder (S. haematobium and S. intercalatum).
The eggs they produce have to pass through the tissue to reach the intestinal tract or me bladder. In this process they cause damage, and many eggs get trapped and calcify - the accumulated effect is what causes the illness. Eggs leave the body of their host with either feces or urine.
In fresh water the eggs hatches and a free-living larval stage, called miracidium, starts its search for an appropriate intermediate snail host. In an internal organ of the snail (equivalent to the liver and pancreas combined) the miracidium transforms into a cercaria (the infective larval stage) in about a month.
The shedded cercariae will attach to and penetrate the skin of humans in touch with water and once in the human bloodstream the larva will reach its final destination (described above) via a complicated route, while undergoing a transformation into an adult worm.
While the details of the life cycle are the domain of parasitologists, the non-expert working in the area of agricultural development should remember the two crucial stages in the Life cycle where he/she can exert influence:
The sanitation aspect: if communities have adequate sanitation and permanent health education keeps them aware of the need to use this sanitation and to teach their children to use it, then contamination of water bodies can be minimized.
The water contact aspect: if communities have access to safe water (for drinking and for laundry) then (re-)infection can be avoided. Children and adolescents will always remain a vulnerable group and deserve special monitoring and treatment Men and women working in irrigated agricultural production are also at risk, depending on their water contact patterns, and need special attention, as well.