Epidemiology and transmission of diarrhoeal disease and soil-transmitted helminth infection
Despite advances in prevention and treatment, diarrhoea and soil-transmitted helminth (STH) infections still represent a large disease burden, particularly in low-income countries. Diarrhoeal diseases account for an estimated 1.4 million deaths worldwide and rank fourth globally for leading causes of years of life lost due to premature mortality (Lozano 2013). Among children under the age of five, diarrhoea kills more than 700,000 children annually, making it the second leading cause of mortality after pneumonia (Walker 2013). Over five billion people worldwide, including one billion school-aged children (aged five to 14 years), are at risk of infection with at least one STH species (Pullan 2012). The three STHs responsible for most infections are Ascaris lumbricoides, Trichuris trichiura and hookworms (Ancylostoma duodenale or Necator americanus), with 819 million, 464.6 million and 438.9 million people infected in 2010 respectively (Pullan 2014).
The pathogens that cause diarrhoea are mainly transmitted via the faecal-oral route (Byers 2001). Pathogens from contaminated faeces can be passed on to a new susceptible host via contaminated hands, drinking water, soil, flies, or by ingesting contaminated food (Wagner 1958). The settings, pathogens and their prevalence in different populations will determine the importance of each transmission route (Brown 2013). The symptoms of diarrhoea and course of disease vary with age, nutritional and immune status of the infected person, and the causative pathogens (Clasen 2010). The main characteristics of infection are changes in stool consistency, increases in volume or fluidity, and increased frequency of defecation (Thapar 2004). The three clinical presentations of diarrhoea are: (1) acute watery diarrhoea lasting several hours or days, (2) acute bloody diarrhoea (dysentery) and (3) persistent diarrhoea lasting 14 days or more (Heymann 2008). The direct threat from acute watery diarrhoea is dehydration, loss of fluids and electrolytes. Severe dehydration can result in death if untreated (Keusch 2006).
STHs are transmitted via ingestion of STH eggs (A. lumbricoides and T. trichiura) or larvae (A. duodenale), or via penetration of third stage larvae (hookworms) (Bethony 2006). The larvae go through several developmental stages in the human host and depending on the species, the adult parasites can settle in different parts of the gastrointestinal (GI) tract, where they can live several years, mating and producing eggs that are passed in the faeces (Bethony 2006). The eggs (A. lumbricoides and T. trichiura) and larvae (hookworm) can survive in the soil for several months or several weeks, respectively, depending on the environmental conditions, including humidity, soil moisture and temperature (Brooker 2006). Morbidity caused by STHs is linked to the intensity of infection, which is the number of worms per human host measured by the number of eggs per gram of faeces (Bethony 2006). STHs infections can have several clinical features, which can be classified into acute manifestations linked to larval migrations through the skin and intestines, and acute and chronic manifestations associated with parasite presence in the GI tract (Bethony 2006).
In addition to the direct health consequences of diarrhoeal diseases and STHs infections, they have longer term impacts on human development due to malabsorption and malnutrition (resulting in stunting and chronic anaemia), and on capacity (via lower cognition, school absenteeism and inability to work); which in turn can have impacts on development and poverty (Harhay 2010). STHs are believed to be one of the main causes of physical and intellectual growth retardation in the world (Bethony 2006).
Sanitation and disposal of child faeces
As the aetiological agents associated with diarrhoea and STHs are transmitted through faeces, the safe collection and disposal of human excreta has the potential to significantly reduce exposure and disease. When readers of the British Medical Journal were asked to vote on the "greatest medical advance" since 1840, they chose the sanitary revolution (the introduction of clean water and sewage disposal) over antibiotics, anaesthesia, vaccines and germ theory (Ferriman 2007). Large scale efforts have been made to increase coverage of improved sanitation, most recently as part of the Millennium Development Goal (MDG) sanitation target of halving the proportion of the population without access to basic sanitation by 2015 (United Nations 2013). However, this target is far from being met; 2.5 billion people were still without improved sanitation by the end of 2011 including more than one billion people who were practicing open defecation (WHO/UNICEF 2013).
A series of published systematic reviews have consistently concluded that sanitation interventions are effective in preventing diarrhoea and STH infections. Esrey 1991 reported a 22% median reduction in diarrhoea from 11 observational studies and 36% from the five rigorous studies. They also reported reduction in Ascaris and hookworm from water supply and sanitation interventions, especially on the reduction in disease intensity (egg counts). Fewtrell 2005 reported a pooled estimate risk on diarrhoea of 0.68 (95% confidence interval (CI) 0.53 to 0.87) from two intervention studies. Waddington 2009 reported a pooled estimate of 0.63 (95% CI 0.43 to 0.93) from six controlled studies among children. Clasen 2010 found a consistent protective effect against diarrhoea among 13 intervention studies but noted that nearly all involved water or hygiene interventions in addition to sanitation. Norman 2010 reported that sewerage led to a 30% reduction in diarrhoea (RR 0.70, 95% CI 0.58 to 0.85) among 17 observational studies. Ziegelbauer 2012 reported that sanitation interventions were protective against Ascaris, Trichuris and hookworm.
All of these reviews, however, focused on interventions to improve coverage, use or functionality of sanitation facilities; none specifically addressed the disposal of child faeces, another source of exposure even among households with improved sanitation. Actually, the unsafe disposal of child faeces may represent a more significant health risk than that of adults. This is because young children have the highest incidence of enteric infections (Walker 2012), and their faeces are most likely to contain infectious agents (Feachem 1983). Young children are more likely to defecate in places where susceptible children could be exposed (Lanata 1998). This exposure is worse for other young children due to the amount of time they spend on the ground and their exploratory behaviours including putting fingers and fomites in their mouths, and common behaviours such as geophagia (intentional consumption of earth) (Moya 2004; Ngure 2013; Young 2011). Perhaps for these reasons, World Health Organization (WHO) and United Nations Children’s Fund (UNICEF) Joint Monitoring Programme for Water Supply and Sanitation (JMP), which is charged with assessing progress toward the MDG sanitation target, treats disposal of child faeces that are not deposited in a latrine or buried as unsanitary (WHO/UNICEF 2006).
An additional risk of contamination of the environment with faeces, including those of children, is that it may result in extended exposure of children to faecal pathogens which may lead to enteropathy, a disorder of the small intestine that is characterised by villous atrophy, crypt hyperplasia, inflammatory cell infiltrate, increased permeability and malabsorption (Humphrey 2009). Enteropathy is thought to lead to under nutrition and growth faltering (Humphrey 2009; Lin 2013).
We are unaware of any published, peer-reviewed study summarizing the evidence on the impact of child faeces disposal on human health. In an unpublished review and meta-analysis of 10 observational studies published between 1987 and 2001, Gil 2004 found that child faeces disposal behaviours considered risky (open defecation, stool disposal in the open, stools not removed from soil, stools seen in household soil and children seen eating faeces) were associated with a 23% increase in risk of diarrhoeal diseases (RR 1.23, 95% CI 1.15 to 1.32); on the other hand, behaviours considered safe (use of latrines, nappies, potties, toilets, washing diapers) were borderline protective (RR 0.93, 95% CI 0.86 to 1.00). In a study in rural Bangladesh, it was found that the disposal of child faeces in closed spaces such as pit latrines resulted in a 35% reduction in helminthiasis in children under two compared with disposal in open space (Roy 2011) indicating that safe disposal of child faeces may also play a role in the control of STH infections.