Water-based interventions for schistosomiasis control

Abstract

Mass drug administration with praziquantel is the mainstay of programs for the control of schistosomiasis morbidity. However, there is a growing recognition that treatment alone will not be sufficient for eventually effecting elimination and that additional measures will be required to interrupt transmission. In the absence of a safe and an effective vaccine for human schistosomiasis, the strategies to reduce infection levels will necessarily involve some interventions that affect the water-related stages of the schistosome life cycle: by reducing exposure to infectious water, by moderating availability of the intermediate snail host, or by decreasing contamination of water with egg-containing excreta. While much research on the importance of water on schistosomiasis has been performed, advances in these areas have perhaps languished with the ready availability of a cost-effective treatment. As some endemic areas near a shift to an elimination goal, a better understanding of water-based interventions that can be used alone or in concert with treatment will be needed. Reinvigoration of laboratory, field, and human behavioral aspects of this research now will ensure that the appropriate strategies are available by the time their implementation becomes necessary.

Keywords: Control,; Elimination,; Schistosomiasis,; Snail,; Transmission.; Water,.

Figure 1

Eggs are eliminated with feces or urine (1). Under optimal conditions, the eggs hatch and release miracidia (2), which swim and penetrate specific snail intermediate hosts (3). The stages in the snail include two generations of sporocysts (4) and the production of cercariae (5). Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host (6), and shed their forked tail, becoming schistosomulae (7). The schistosomulae migrate through several tissues and stages to their residence in the veins (8, 9). Adult worms in humans reside in the mesenteric venules in various locations, which are specific for each species (10). For instance, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine A, and S. mansoni occurs more often in the superior mesenteric veins draining the large intestine B. S. haematobium most often occurs in the venous plexus of bladder C, but it can also be found in the genital tract or rectal venules. The females deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S. mansoni and S. japonicum) and of the bladder and ureters (S. haematobium), and are eliminated with feces or urine, respectively (1) (source: http://www.cdc.gov/dpdx/schistosomiasis/index.html).

Figure 2

Putative waster-based interventions for schistosomiasis control. Measures could be employed individually or in concert but success of any intervention will be dependent on its acceptability to the endemic population and should be developed with local culture and customs in mind.