Citation: Campbell SJ, Savage GB, Gray DJ, Atkinson J-AM, Soares Magalhães RJ, Nery SV, et al. (2014) Water, Sanitation, and Hygiene (WASH): A Critical Component for Sustainable Soil-Transmitted Helminth and Schistosomiasis Control. PLoS Negl Trop Dis 8(4): e2651. https://doi.org/10.1371/journal.pntd.0002651
Editor: Xiao-Nong Zhou, National Institute of Parasitic Diseases China CDC, China
Published: April 10, 2014
Copyright: © 2014 Campbell et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: SJC is funded by an Australian Postgraduate Award and a University of Queensland Advantage Scholarship, ACAC is an Australian National Health and Medical Research Council (NHMRC) Career Development Fellow (631619), RJSM is funded by a Post-doctoral Research Fellowship from the University of Queensland (41795457), JSM is an Australian National Health and Medical Research Council Practitioner Fellow, and DJG is an Australian Research Council (DECRA) Fellow. This work is funded by an NHMRC Partnership project in collaboration with WaterAid Australia. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Soil-transmitted helminths (STH) and schistosomes are parasites that affect the world's poorest people, causing losses of up to 39 million and 70 million disability adjusted life years (DALYs) respectively , . The World Health Organization (WHO) is at the forefront of developing policy for the control of STH and schistosomiasis, advocating for chemotherapy as the cornerstone of control, with the objective of reducing infection-associated morbidity , , . Global uptake of chemotherapy with albendazole or mebendazole for STH and praziquantel for schistosomiasis has significantly increased and remains the principal control strategy. It is cost-effective  and reduces STH  and schistosome  infections in human hosts.
However, a fundamental limitation of chemotherapy for STH and schistosomiasis control is that it does not kill immature worms and cannot prevent reinfection. Chemotherapy-based control programmes have a temporary effect on transmission . Indeed, studies have shown that infection prevalence and intensity can rapidly return to baseline levels soon after chemotherapy programmes are ceased. One factor is that the ability of helminth eggs and/or larvae to survive for extended periods in the environment  creates a source for rapid reinfection following chemotherapy . A second is that small sections of the population usually remain out of reach of chemotherapy programmes, subgroups that frequently have a disproportionately heavy burden of infection, thereby serving as a reservoir for reinfection. Thus, longer-term effectiveness of chemotherapy in interrupting transmission is dependent on maintenance of regular retreatment. Many helminth control programmes rely on donated drugs , so there is a degree of uncertainty around their sustainability in the long term. In endemic areas, once mass treatment is stopped, disease prevalence can return to pretreatment levels within 18–24 months –. For schistosomiasis, cessation of chemotherapy can also result in more severe rebound of immunopathology .
The most frequently used chemotherapeutic drug, albendazole, does not have 100% efficacy ; therefore, chemotherapy programmes will not cure all treated individuals. Additionally, helminth control programmes have predominantly focused on specific risk groups (primarily schoolchildren) rather than the whole community, despite evidence in many communities that prevalence may be high in other groups , for example, preschool children . A shift in approach to community-wide chemotherapy, or at least to include preschoolers as a target population, could potentially have a great impact on further reducing STH infections, particularly in settings where there is high prevalence in nonschool groups or where many children do not go to school.
Even where there are continuous control programmes, there is some evidence of declining uptake due to fear of treatment and poor communication about the chemotherapy process . There is also the potential that mass drug administration may result in drug resistance, as is occurring in livestock helminth control programmes –. Humphries et al. (2011) believe that, given the current treatment pressure, it will only be a matter of time before drug resistance is seen in STH species that infect humans . Controversially, recent reviews indicate that, on the basis of measures of infection-associated morbidity (such as improvements in nutrition, haemoglobin levels, school attendance, and school performance), there is insufficient reliable evidence to justify contemporary chemotherapy programmes , . We do, however, recognise that in developing country settings, where multiple disease and health-related interactions are likely to take place, it is difficult to associate nonspecific morbidity indicators to STH or schistosomiasis. Other issues that are not yet resolved with regards to chemotherapy include potential teratogenic effects of benzimidazole drugs and associations with eczema in children following maternal chemotherapy during pregnancy . Thus, whilst chemotherapy is necessary to rapidly reduce the burden and morbidity of helminth infections, we argue that by itself it is an unsustainable strategy for helminth control and for reaching control and elimination targets. This highlights the essential role of interventions aimed at reducing environmental exposure, which chemotherapy alone does not address.
The provision of access to WASH, being a safe water supply, appropriately constructed sanitation infrastructure that ensures safe disposal of human excreta, and the promotion of hygiene (defined as personal and household practices such as hand-washing, bathing, and management of stored water in the home, all aimed at preserving cleanliness and health), is critical. WASH is a necessary but undervalued tool for helminth prevention and control, aiming to provide long-term improvements in people's wellbeing. Interventions that include WASH have been shown to be highly effective in reducing the environmental exposure to, and transmission of, eggs and larvae for STH  and schistosomes . A 29% decrease in Ascaris lumbricoides prevalence and as much as a 77% reduction in schistosomiasis prevalence has been observed following implementation of improved water and/or sanitation facilities . A recent study in three African countries estimated that the population attributable fraction (PAF) of schistosomiasis due to no piped water was 47–71% .
Areas with poor sanitation coverage often experience a high burden of disease from STH and schistosomiasis (Figure 1). WASH implementation can be complex and comprised of a large set of “hardware” (e.g., toilets, latrines, sewage treatments, and provision of safe water)  and “software” (e.g., behaviour change promotion and community resource management) elements, many of which are, strictly speaking, outside the official service delivery remit of the health system. Challenges for implementing WASH can include cost, lack of health professional involvement , lack of local government involvement and local public-private partnerships for latrine and infrastructure development , lack of advocacy , inappropriate choice of technology, poor operation and maintenance, inadequate revenue collection, lack of adequate and equitable financial investment from both government and international donors , and the lack of perception in many rural communities of the importance of improved excreta disposal practices . This requires genuine cross-sectoral collaboration and political will; investment in WASH in developing countries contributes to practically all of the Millennium Development Goals (MDGs) , and should not be overlooked for helminth control simply because chemotherapeutic interventions exist that require a seemingly lower financial and logistical commitment.
Helminth Control Guidelines and the Neglect of WASH
For many years, authors ,  have argued that the effects of chemotherapy can only be sustainable if integrated with improvements in health promotion, hygiene, and sanitation. This has been recognised and advocated for in the World Health Assembly (WHA) resolutions on STH and schistosomiasis, as well as the recent resolution on NTDs. These foundational policy guidelines clearly highlight the importance of WASH as a fundamental component of helminth control and elimination –; however, as discussed below, WASH is not embraced in subsequent disease-specific control guidelines (e.g., STH and schistosomiasis). A longer-term view of effectiveness and sustainability of control efforts requires integrating interventions to reduce transmission and reinfection. Yet interventions such as WASH have been slow to be incorporated into control programmes. It is for this reason that the parties to the London Declaration on NTDs are seeking more coordinated access to clean water and basic sanitation, improved living conditions, vector control, health education, and stronger health systems in endemic areas .
The WHO published guidelines for the prevention and control of STH and schistosomiasis infections in 2002  and recently produced updated guidelines entitled “Helminth control in school-age children: a guide for managers of control programmes, 2nd edition” , specifically targeting STH and schistosomiasis. This second document acknowledges the importance of WASH and provides advice that helminth control programmes need to comprehensively include WASH, with the definitive statement, “The only definitive solution for eliminating schistosomiasis and STH infections is improvement in environmental conditions and a change in risk behaviours” . However, chemotherapy is prioritised as the “first-line rapid control measure,” while improved water and sanitation and health education should be only “implemented according to the epidemiological situation and the availability of resources” . No clear definition of what is meant by “epidemiological situation” in this context is provided. Our concern is that these last two statements will have the unintended effect of delaying action on WASH in favour of chemotherapy, without interrupting the vicious cycle of disease transmission. The guidelines could be enhanced by inclusion of comprehensive recommendations for implementing WASH hardware and software, citing methods and examples such as the Community-Led Total Sanitation (CLTS) approach, which has now been successfully implemented in over 20 countries , sanitation marketing, and other approaches that focus on creating demand for sanitation and changing unhealthy behaviours.
Of significant concern regarding the current WHO guidelines is that they contain no recommended control activities where prevalence of STH infection below 20% is identified at baseline . Instead, following the chemotherapy focus of the document, “Affected individuals should be treated [for STH] on a case-by-case basis” (Table 2.3 in ); however, no suggestions for identifying these individuals are proposed. Such an approach needs to be supported by rigorous epidemiological evidence that clearly demonstrates benefits to the community concerned and appropriate mitigation of the risk of cross-infection into uninfected individuals. STH and schistosomes are extremely difficult to eliminate in communities where poverty and inadequate water and sanitation prevail, due to their high transmission potential . Lack of specifying control activities in this scenario represents, at the very least, a missed opportunity for recommending WASH activities, particularly given the level of morbidity likely to be experienced in a community with 20% STH prevalence.
An additional area of the WHO guidelines that warrants close scrutiny are decision trees in the annexes, which recommend reducing frequency of chemotherapy after five to six years, based solely on measurements of prevalence. For prevalence of STH or schistosomiasis below 1%, the WHO guidelines indicate, “morbidity is under control with low risk of re-emergence,” although serology for schistosomiasis is recommended with positive cases continuing to receive chemotherapy . It is unclear whether serology is intended for all schoolchildren in this scenario, and additionally there is no evidence to indicate that risk of re-emergence of disease is not a problem at this threshold level, particularly if WASH is not adequate. We propose that WASH indicators be added to the decision trees, to provide sounder guidance for programme managers in their decision-making about helminth control programmes. It would also more comprehensively mitigate risk of resurgence of STH and schistosomes, as it would address necessary environmental improvements for control, as well as demonstrate longer-term, sustainable benefits to the communities concerned.
The WHO guidelines published in 2002  were the first such document of its kind. It admirably articulated a large volume of technical information to assist programme managers develop prevention and control strategies. The more recent version, however, does not seem to have progressed considerably from the earlier version. Rather, the recognition in the 2002 version that resources must not be diverted prematurely in countries where morbidity has been significantly reduced but transmission continues  mitigates risk more appropriately than the current second edition guidelines. We believe there is a strong justification for a further revision to be undertaken.
Getting the Indicators Right
The current WHO guidelines use prevalence of infection as the most emphasised indicator of the success of worm control programmes, whilst the “condition of latrines and the quality of water supplies in schools may also be monitored if their improvement is one of the objectives of the programme” . Use of prevalence is insufficient as it does not place emphasis on using interventions that have a more sustainable impact. Given the reinfection rate of STH and schistosomes, being guided by prevalence rates alone is high risk. As the WHO guidelines correctly point out, remaining “parasites maintain transmission capacity despite intense drug pressure, and this is predictive of a rapid return to high levels of prevalence if the [chemotherapy] intervention is interrupted” . Intensity of infection (as measured by number of eggs in stool/urine) is markedly different within various groups of the community, such as different age groups and sex . Thus, prevalence can easily mask the high transmission potential of a relatively small number of individuals. Hygiene activities are included with indicators for monitoring numbers of hygiene education programmes conducted, although these would not sufficiently measure hygiene behavioural change.
We recommend that, at the very least, corresponding WASH access indicators be included in any revised versions of WHO helminth control guidelines. These could include the MDG seven indicators of (i) proportion of the population using an improved drinking water source and (ii) proportion of the population using an improved sanitation facility , with “improved” water and sanitation defined by the WHO-UNICEF Joint Monitoring Programme for Water Supply and Sanitation . These are the most developed and consistently used WASH indicators. Many national health surveys are collecting data on some of these indicators; thus the addition of these indicators should not involve adding completely novel indicators into helminth control programmes. We acknowledge that there has been some criticism of the MDG indicators with regards to equity, specifically, that the MDGs target the richer proportions of each country's population, rather than those at greatest need. This has not been resolved, and there has been a general call to develop more equitable indicators beyond 2015 . However, based on current approaches, these indicators appear the most suitable at this time for ensuring that WASH is addressed in conjunction with chemotherapy.
There should also be guidance on appropriate implementation provided in the second edition WHO guidelines. Such guidance should encourage best-practice sanitation and hygiene promotion approaches relevant to the context in the programme location. The CLTS approach, which avoids the use of hardware subsidies and “latrinification” (construction of latrines for households without commensurate efforts to ensure safe sanitary practices and ownership and adequate maintenance of latrines) is one potential approach, alongside other emerging approaches such as sanitation marketing, which focuses on creation of demand for household investment sanitation hardware in order to allow progressive improvement away from basic latrines. Guidance should also specifically encourage improved coordination and planning across sectors, such as the participation of WASH agencies in national NTD task forces. It is known that sanitation does not become effective until it is used by a high percentage of the population , , with coverage of properly built, used, and maintained sanitation required to be 90% to have an effect on STH transmission . If insufficient proportions of people in a community have access to sanitation, even those who have latrines will still be at risk of infection , particularly if there is latrine access at local schools or institutions but not within the community, or vice versa. For this reason, we advocate for universal access to WASH to be considered in MDG planning beyond 2015. In the interim, setting WASH access indicators in any revised version of WHO helminth control guidelines is a crucial next step that will help to tackle the disease burden caused by STH and schistosomiasis. An additional and significant benefit of high community WASH access would be its impact on controlling other excreta-borne pathogens, including viruses, bacteria, and protozoa .
There is very little literature that indicates direct WASH impact on helminth control. We believe there is an urgent need to conduct epidemiological research, including appropriately structured intervention trials  and mathematical modelling studies , , to evaluate the effect of integrated interventions on helminth infections and infection-associated morbidity. Existing evidence is already strong enough to support complementing drug-based interventions with the provision of WASH for all , but more work can be done to determine intervention thresholds for the selected WASH indicators to be incorporated into decision trees such as those presented in the annexes of the WHO guidelines.
Progress towards achieving global control of helminths crucially depends on sustainable solutions that move beyond treating symptoms towards reducing exposure. With that in mind, it is necessary to augment chemotherapy with WASH and other interventions such as health promotion to achieve a cumulative impact of preventing reinfection and providing the greatest and most sustainable gains for helminth control and elimination. We believe that a strong justification exists to revise the WHO guidelines in the face of the abovementioned shortcomings. Such revision will result in a much-enhanced document that covers the full spectrum of short- and longer-term interventions for more holistic STH and schistosomiasis control. Impact indicators for WASH, in addition to disease-related indicators such as prevalence of infection, should define the success of a control programme and guide decisions as to when such programmes should cease. This would ensure current gains in helminth control are built upon beyond the current dependence on chemotherapy.
- 1. WHO (2002) Prevention and control of schistosomiasis and soil-transmitted helminthiasis: report of a WHO expert committee. Contract No.: 912. Geneva: World Health Organization.
- 2. King CH, Dangerfield-Cha M (2008) The unacknowledged impact of chronic schistosomiasis. Chronic Illn 4: 65–79.
- 3. WHO (2011) Helminth control in school age children: a guide for managers of control programmes. Second edition. Geneva: World Health Organization.
- 4. WHO (2012) Accelerating work to overcome the global impact of neglected tropical diseases - a roadmap for implementation. Geneva: World Health Organization.
- 5. Molyneux DH, Malecela MN (2011) Neglected tropical diseases and the millennium development goals: why the “other diseases” matter: reality versus rhetoric. Parasit Vectors 4: 234.
- 6. Utzinger J, Keiser J (2004) Schistosomiasis and soil-transmitted helminthiasis: common drugs for treatment and control. Expert Opin Pharmacother 5(2): 263–85.
- 7. Doenhoff MJ, Hagan P, Cioli D, Southgate V, Pica-Mattoccia L, et al. (2009) Praziquantel: its use in control of schistosomiasis in sub-Saharan Africa and current research needs. Parasitology 136: 1825–35.
- 8. Gray DJ, McManus DP, Li Y, Williams GM, Bergquist R, Ross AG (2010) Schistosomiasis elimination: lessons from the past guide the future. Lancet Infect Dis 10(10): 733–6.
- 9. Brooker S, Bethony J, Hotez PJ (2004) Human hookworm infection in the 21st century. Adv Parasitol 58: 197–288.
- 10. Clements AC, Bosque-Oliva E, Sacko M, Landouré A, Dembélé R, et al. (2009) A comparative study of the spatial distribution of schistosomiasis in Mali in 1984–1989 and 2004–2006. PLoS Negl Trop Dis 3: e431.
- 11. Doenhoff MJ, Cioli D, Utzinger J (2009) Praziquantel: mechanisms of action, resistance and new derivatives for schistosomiasis. Curr Opin Infect Dis 21: 659–67.
- 12. Ross AGP, Bartley PB, Sleigh AC, Olds GR, Li Y, et al. (2002) Schistosomiasis. N Eng J Med 346: 1212–19.
- 13. Bergquist R, Utzinger J, McManus DP (2008) Trick or treat: the role of vaccines in integrated schistosomiasis control. PLoS Negl Trop Dis 2: e244.
- 14. Horton J (2000) Albendazole: a review of anthelmintic efficacy and safety in humans. Parasitology 121: S113–S132.
- 15. Anderson RM, Truscott JE, Pullan RL, Brooker SJ, Hollingsworth TD (2013) How effective is school-based deworming for the community-wide control of soil-transmitted helminths? PLoS Negl Trop Dis 7: e2027.
- 16. Albonico M, Montresor A, Crompton DW, Savioli L (2006) Intervention for the control of soil-transmitted helminthiasis in the community. Adv Parasitol 61: 311–48.
- 17. Parker M, Allen T (2012) Will mass drug administration eliminate lymphatic filariasis? Evidence from Northern Coastal Tanzania. J Biosoc Sci Sep 27: 1–29.
- 18. Albonico M (2003) Methods to sustain drug efficacy in helminth control programmes. Acta Trop 86: 233–242.
- 19. Albonico M, Engels D, Savioli L (2004) Monitoring drug efficacy and early detection of drug resistance in human soil-transmitted nematodes: a pressing public health agenda for helminth control. Int J Parasitol 34: 1205–1210.
- 20. Jackson F, Coop RL (2000) The development of anthelminthic resistance in sheep nematodes. Parasitology 120 Suppl: S95–107.
- 21. Humphries D, Mosites E, Otchere J, Twum WA, Woo L, et al. (2011) Epidemiology of hookworm infection in Kintampo North Municipality, Ghana: patterns of malaria coinfection, anemia, and albendazole treatment failure. Am J Trop Med Hyg 84: 792–800.
- 22. Taylor-Robinson DC, Maayan N, Soares-Weiser K, Donegan S, Garner P (2012) Deworming drugs for soil-transmitted intestinal worms in children: effects on nutritional indicators, haemoglobin and school performance. Cochrane Database Syst Rev 11: CD000371.
- 23. Humphries D, Nguyen S, Boakye D, Wilson M, Cappello M (2012) The promise and pitfalls of mass drug administration to control intestinal helminth infections. Curr Opin Infect Dis 25: 584–589.
- 24. Ndibazza J, Mpairwe H, Webb EL, Mawa PA, Nampijja M, et al. (2012) Impact of anthelminthic treatment in pregnancy and childhood on immunisations, infections and eczema in childhood: a randomised controlled trial. PLoS ONE 7: e50325.
- 25. Esrey SA, Potash JB, Roberts L, Shiff C (1991) Effects of improved water supply and sanitation on ascariasis, diarrhoea, dracunculiasis, hookworm infection, schistosomiasis, and trachoma. Bull World Health Organ 69: 609–621.
- 26. Kosinski KC, Adjei MN, Bosompem KM, Crocker JJ, Durant JL, et al. (2012) Effective control of Schistosoma haematobium infection in a Ghanaian community following installation of a water recreation area. PLoS Negl Trop Dis 6: e1709.
- 27. Soares Magalhaes RJ, Barnett AG, Clements AC (2011) Geographical analysis of the role of water supply and sanitation in the risk of helminth infections of children in West Africa. Proc Natl Acad Sci U S A 108: 20084–20089.
- 28. Bartram J, Cairncross S (2010) Hygiene, sanitation, and water: forgotten foundations of health. PLoS Med 7: e1000367.
- 29. Cairncross S (2003) Sanitation in the developing world: current status and future solutions. Int J Environ Health Res 13: S123–S131.
- 30. Cairncross S, Valdmanis V (2006) Water supply, sanitation, and hygiene promotion. In: Jamison DT, Breman JG, Measham AR, Alleyne G, Claeson M, et al.., editors. Disease control priorities in developing countries. 2nd edition. Washington (DC): World Bank. pp. 771–792.
- 31. OECD (2000) Global trends in urban water supply and waste water financing and management: changing roles for the public and private sectors. Paris: Organisation for Economic Co-operation and Development.
- 32. Esrey SA, Feachem RG, Hughes JM (1985) Interventions for the control of diarrhoeal diseases among young children: improving water supplies and excreta disposal facilities. Bull World Health Organ 63: 757–772.
- 33. WHO (2001) WHA54.19 Schistosomiasis and soil-transmitted helminth infections. Available: http://www.who.int/neglected_diseases/mediacentre/WHA_54.19_Eng.pdf. Accessed 29 July 2013.
- 34. WHO (2012) Schistosomiasis strategy. Available: http://www.who.int/schistosomiasis/strategy/en/. Accessed 29 July 2013.
- 35. WHO (2013) WHA66.12 Neglected tropical diseases. Available: http://www.who.int/neglected_diseases/mediacentre/WHA_66.12_Eng.pdf. Accessed 19 August 2013.
- 36. Department for International Development (2012) London declaration on neglected tropical diseases. Available: http://www.dfid.gov.uk/Documents/publications1/NTD%20Event%20-%20London%20Declaration%20on%20NTDs.pdf. Accessed 20 December 2012.
- 37. Institute of Development Studies (2011) The website for Community Led Total Sanitation (CLTS). Available: http://www.communityledtotalsanitation.org/page/clts-approach. Accessed 11 July 2012.
- 38. Hotez PJ, Bundy DAP, Beegle K, Brooker S, Drake L, et al.. (2006) Helminth infections: soil-transmitted helminth infections and schistosomiasis. In: Jamison DT, Breman JG, Measham AR, Alleyne G, Claeson M, et al.., editors. Disease control priorities in developing countries. 2nd edition. New York: Oxford University Press and The World Bank. pp. 467–82.
- 39. Brooker S, Peshu N, Warn PA, Mosobo M, Guyatt HL, et al. (1999) The epidemiology of hookworm infection and its contribution to anaemia among pre-school children on the Kenyan coast. Trans R Soc Trop Med Hyg 93: 240–246.
- 40. United Nations Statistics Division (2008) Official list of MDG indicators. Available: http://mdgs.un.org/unsd/mdg/Resources/Attach/Indicators/OfficialList2008.pdf. Accessed 29 July 2013.
- 41. WHO/UNICEF Joint Monitoring Programme for Water Supply and Sanitation (2009) Types of drinking-water sources and sanitation. Available: http://www.wssinfo.org/definitions-methods/watsan-categories/. Accessed 29 July 2013.
- 42. Cairncross S, Bartram J, Cumming O, Brocklehurst C (2010) Hygiene, sanitation, and water: what needs to be done? PLoS Med 7: e1000365.
- 43. Appleton CC, Mosala TI, Levin J, Olsen A (2009) Geohelminth infection and re-infection after chemotherapy among slum-dwelling children in Durban, South Africa. Ann Trop Med Parasitol 103: 249–261.
- 44. Asaolu SO, Ofoezie IE (2003) The role of health education and sanitation in the control of helminth infections. Acta Trop 86: 283–294.
- 45. Chongsuvivatwong V, Pas-Ong S, McNeil D, Geater A, Duerawee M (1996) Predictors for the risk of hookworm infection: experience from endemic villages in southern Thailand. Trans R Soc Trop Med Hyg 90: 630–633.
- 46. Kilama WL (1989) Sanitation in the control of ascariasis. In: Crompton DWT, Nesheim MC, Pawlowski ZS, editors. Ascariasis and its prevention and control. London: Taylor and Francis. pp. 289–300.
- 47. Clasen T, Boisson S, Routray P, Cumming O, Jenkins M, et al. (2012) The effect of improved rural sanitation on diarrhoea and helminth infection: design of a cluster-randomized trial in Orissa, India. Emerg Themes Epidemiol 9: 7.
- 48. Basanez MG, McCarthy JS, French MD, Yang GJ, Walker M, et al. (2012) A research agenda for helminth diseases of humans: modelling for control and elimination. PLoS Negl Trop Dis 6: e1548.
- 49. Williams GM, Sleigh AC, Li Y, Feng Z, Davis GM, et al. (2002) Mathematical modelling of schistosomiasis japonica: comparison of control strategies in the People's Republic of China. Acta Trop 82: 253–262.
- 50. Cairncross S, Hunt C, Boisson S, Bostoen K, Curtis V, et al. (2010) Water, sanitation and hygiene for the prevention of diarrhoea. Int J Epidemiol 39(Suppl 1): i193–205.
- 51. WHO/UNICEF (2012) Progress on drinking water and sanitation: 2012 Update. New York: UNICEF and World Health Organization.