Rethinking Indicators of Microbial Drinking Water Quality for Health Studies in Tropical Developing Countries: Case Study in Northern Coastal Ecuador

Abstract

Abstract. To address the problem of the health impacts of unsafe drinking water, methods are needed to assess microbiologic contamination in water. However, indicators of water quality have provided mixed results. We evaluate five assays (three for Escherichia coli and one each for enterococci and somatic coliphage) of microbial contamination in villages in rural Ecuador that rely mostly on untreated drinking water. Only membrane filtration for E. coli using mI agar detected a significant association with household diarrheal disease outcome (odds ratio = 1.29, 95% confidence interval = 1.02-1.65 in household containers and odds ratio = 1.18, 95% confidence interval = 1.02-1.37) in source samples. Our analysis and other published research points to the need for further consideration of study design factors, such as sample size and variability in measurements, when using indicator organisms, especially when relating water quality exposure to health outcomes. Although indicator organisms are used extensively in health studies, we argue that their use requires a full understanding of their purposes and limitations.

H2S as an Indicator of Water Supply Vulnerability and Health Risk in Low-Resource Settings: A Prospective Cohort Study

“…18 These variable results between study sites are not unexpected given regional differences in the non-fecal contributions to microbial indicator levels and in the relative importance of drinking water as a disease transmission pathway. 19 Our observation of low child diarrhea prevalence despite poor microbial water quality suggests that although the ground water sources in the study region are exposed to environmental microbial contamination, basic protection from fecal contamination may be sufficient to prevent the majority of water source-transmitted disease risk. 34 The consistent increase in household-level microbial contamination (Figure 1), however, indicates that water contamination during storage remains a potential transmission pathway for pathogens that could facilitate rapid disease spread during a diarrheal disease outbreak.…”

“…22,[38][39][40] In addition, culture-based assays for coliform species, particularly methods developed for field use, do not always produce comparable results. 19 It is likely that differences in the microbial ecology of water supplies and in the performance of coliform assays have both contributed to the variable associations reported between H 2 S and coliform test results. 22 Despite the challenges of evaluating H 2 S as a fecal indicator based solely on comparisons with coliform assays, our results show that in rural southern India, H 2 S was associated with higher levels of total coliform contamination ( Figure 2).…”

“…As shown in this and other studies, microbial water quality indicators are often poor surrogates for the actual health risks associated with drinking water. 2,14,19 Given the diversity of pathogens that cause diarrhea and the multiple exposure pathways to these pathogens (drinking water, food, hands, and other contaminated surfaces), the inconsistent associations between a limited set of microbial water quality indicators and disease risk is not surprising. It is likely that accurate evaluations of the health risks associated with a drinking water supply will require direct assessments of pathogen levels.…”

“…[10][11][12] These observations reflect the variable relationships observed between water quality indicators and actual fecal pathogens, 13,14 which may also contribute to the weak and inconsistent associations between water quality indicators and diarrheal disease. 2,13,[15][16][17][18][19] Most of the validated methods for quantifying total coliforms, thermotolerant coliforms, and E. coli in drinking water are costly and require both dedicated facilities and specialized training. To promote on-site microbial water testing in resource-poor settings, Manja and others 20 developed a simple presence/absence test for hydrogen sulfide (H 2 S)-producing bacteria, based on the observation that coliform bacteria in drinking water were consistently associated with H 2 S production.…”

“Improved” But Not Necessarily Safe: An Assessment of Fecal Contamination of Household Drinking Water in Rural Peru

“…Some research has shown an association between the consumption of highly contaminated water and diarrhea, 42,43 but other investigations have found no significant association between the level of fecal contamination in stored household drinking water and disease risk. [44][45][46][47] A recent systematic review and meta-analysis demonstrated an association between the detection-but not the amount-of E. coli contamination and diarrhea. 48 The considerable contamination of improved water sources observed in this and other studies demonstrates that the current MDG indicator for water quality should not be equated with safety.…”