Laura Braun, Imperial College London, 21/2/2018
Since the first full description of the schistosome lifecycle in 1908, the focus of control programs has shifted between different interventions (1). Initially, control measures focused on mollusciciding and preventing contact with contaminated water (2). The enthusiasm for snail control led to the development of molluscicides (3). The research on water treatment and snail control was slowed by the development of effective orally-administered drugs in the late 1970’s (3, 4). Ever since, chemotherapy has been the focus of schistosomiasis control programs (5). Although praziquantel is effective, safe, and inexpensive, the treatment does not prevent subsequent re-infection. As schistosomiasis control targets become more ambitious and move towards elimination, interest is again increasing in the potentially complementary roles of water, sanitation, and hygiene (WASH) interventions which may disrupt transmission of the parasite, thereby reducing reinfection following treatment.
The options for providing safe water (safe meaning free of viable schistosome cercariae) are either treating the water, or providing an alternate safe source of water (such as boreholes or rainwater). In endemic regions, there is often no safe alternative water source, and hence water treatment is required to provide a safe water supply. Water infrastructure must provide water for domestic activities such as washing, laundry, and bathing - activities which are associated with long water-contact periods and therefore risk of exposure to cercariae. Exposure while drinking water is not the main concern when it comes to schistosomiasis, though of course safe drinking water must also be provided in many cases.
There are numerous challenges with implementing water treatment infrastructure. The first is that there are no robust, evidence-based guidelines on how to effectively design water treatment processes to remove or inactivate cercariae. A recently completed systematic review confirmed that there is no consistent, complete data that could be used to design a reliable water treatment infrastructure. The review focused on five water treatment methods – storage, heating, chlorination, filtration and ultraviolet disinfection, all of which have been previously used as water treatment in developing countries and which have shown to be effective against schistosome cercariae to varying degrees, though these previous studies used different experimental protocols for the application of the water treatment processes and for measuring cercaria viability. Our WISER project aims to generate robust, reproducible water treatment data that can be developed into guidelines, for example to say what minimum sand grain size and bed depth effectively filter out cercariae from water.
Although water treatment infrastructure can provide safe water for many household activities, it will not prevent all contact with contaminated water. Occupational activities such as fishing rely on contact with natural water bodies, and exposure to contaminated water will not be affected by the introduction of a safe water supply. Even with access to safe water, the community may continue to visit transmission sites for reasons including overcrowding or lack of privacy at the safe water source. Therefore, for the water infrastructure to be effective in preventing exposure to contaminated water bodies, it will require that risk awareness campaigns accompany the infrastructure implementation (see Objective 3 of WISER). While completely eliminating contact with infested water bodies may be unachievable, strategies should at least be targeted at reducing this contact during and after drug administration programmes, to minimise reinfection and hence maximise the value of the chemotherapy investment, thus hopefully allowing a move towards complete elimination of infection in the community.
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2. Great Britain Army Medical Services. Memoranda on medical diseases in the tropical and sub-tropical war areas. London: 1919.
3. King CH, Bertsch D. Historical perspective: Snail control to prevent schistosomiasis. PLoS Negl Trop Dis. 2015;9(4):e0003657.
4. Sandbach FR. The history of schistosomiasis research and policy for its control. Med Hist. 1976;20(3):259-75.
5. Sturrock RF. Schistosomiasis epidemiology and control: how did we get here and where should we go? Mem Inst Oswaldo Cruz. 2001;96 17-27.