The roles of water, sanitation and hygiene in reducing schistosomiasis: a review

Abstract

Schistosomiasis is a disease caused by infection with blood flukes of the genus Schistosoma. Transmission of, and exposure to, the parasite result from faecal or urinary contamination of freshwater containing intermediate host snails, and dermal contact with the same water. The World Health Assembly resolution 65.21 from May 2012 urges member states to eliminate schistosomiasis through preventive chemotherapy (i.e. periodic large-scale administration of the antischistosomal drug praziquantel to school-aged children and other high-risk groups), provision of water, sanitation and hygiene (WASH) and snail control. However, control measures focus almost exclusively on preventive chemotherapy, while only few studies made an attempt to determine the impact of upgraded access to safe water, adequate sanitation and good hygiene on schistosome transmission. We recently completed a systematic review and meta-analysis pertaining to WASH and schistosomiasis and found that people with safe water and adequate sanitation have significantly lower odds of a Schistosoma infection. Importantly though, the transmission of schistosomiasis is deeply entrenched in social-ecological systems, and hence is governed by setting-specific cultural and environmental factors that determine human behaviour and snail populations. Here, we provide a comprehensive review of the literature, which explores the transmission routes of schistosomes, particularly focussing on how these might be disrupted with WASH-related technologies and human behaviour. Additionally, future research directions in this area are highlighted.

Figure 1

Flow diagram demonstrating the roles of human water contact, and immunological and physiological factors, in determining schistosome infections. Point 1 demonstrates that since water from safe sources should be free of cercariae, provision of such water should prevent schistosome infections. However, as shown at point 2, the provision of safe water often does not prevent all contact with infested water. Point 3 shows another barrier to schistosome infections, namely the host’s immune system and physiology, which may kill invading cercariae before they can develop into adult schistosomes and cause pathology. Despite the host’s immunological and physiological defences, some cercariae successfully develop into adult worms (point 4). The relative importance of the water contact versus immunology and physiology, in preventing schistosome infections, is poorly understood.

Figure 2

Flow diagram demonstrating how schistosome transmission may persist despite the use of adequate sanitation. Schistosome eggs hatch upon entry into freshwater, and release miracida. These miracidia cannot survive for long without infecting an intermediate host, so miracidia in adequate sanitation systems are unlikely to contribute to transmission (point 1). However, it is possible that some eggs may enter freshwater as a result of washing off the bodies or soiled clothing of those infected (point 2). Reservoir hosts provide another potential source of miracidia (point 3).

Figure 3

Flow diagram demonstrating how sanitation may increase or decrease snail numbers, depending on the organic pollution of the water. By containing excreta, and keeping it away from water bodies, sanitation will reduce organic pollution. However, this may be either detrimental, or under certain circumstances, beneficial to intermediate host snails, that thrive under conditions of mild (but not low or high) organic pollution.

Figure 4

Flow diagram demonstrating how sanitation may increase or decrease cercarial production depending on the setting. Reducing snails’ exposure to miracidia may reduce the number of snails infected and shedding cercariae, or under certain circumstances, by reducing the number of miracidia infecting each snail, it may increase snail longevity and cercarial output. Which effect is stronger depends on how frequently snails encounter miracidia, which varies between settings.

Figure 5

A world map of the 138 primary field studies cited in this review. The remaining 81 papers, being reviews, laboratory studies and commentaries, are not displayed on this map. Note that (i) South Sudan and Sudan are shown together since the studies from this region were all conducted before South Sudan became independent in 2011, (ii) Zanzibar is counted as a part of Tanzania, and (iii) studies’ countries were mutually exclusive but their topics of study (i.e. water supplies, sanitation and hygiene) were not.