Study links higher enteric disease risk to E. coli in high-risk water supplies

A nationwide New Zealand study suggests that while most treated public water systems were not linked to higher overall disease risk, E. coli in vulnerable supplies and during heavy rainfall was tied to greater risk of bacterial enteric illness.

Study: Drinking water quality and enteric disease: a nationwide case-crossover study (2015–2019) in New Zealand. Image Credit: Ezume Images / Shutterstock

In a recent study published in the Journal of Exposure Science & Environmental Epidemiology, a group of researchers examined how microbiological indicators of drinking water quality influence the risk of enteric diseases in New Zealand.

Global Burden of Waterborne Disease Risk

What if the water you drink every day quietly increases your risk of illness? Contaminated drinking water is responsible for an estimated 505,000 annual deaths worldwide, and it remains a significant public health threat. In developed countries, including New Zealand, the Havelock North outbreak of 2016 showed how easily public water systems can fail.

Microbiological indicators such as Escherichia coli (E. coli) and total coliforms are the most common indices used to assess whether water is safe to drink, but their association with individual disease risk is poorly understood because they depend on environmental factors. It is crucial to understand these links in order to enhance prevention strategies, and further research is required to clarify them.

Case-Crossover Study Design and Data Sources

This nationwide study applied a case-crossover design to evaluate short-term associations between drinking water quality and enteric disease. The dataset for this analysis included all 46,020 notified enteric disease cases between 2015 and 2019 linked to a public water supply from a water distribution zone (WDZ). People who depend on private or domestic self-supplies were excluded due to a lack of water quality data.

Compliance monitoring led to the provision of daily data on microbiological indicators like E. coli, total coliforms, turbidity, and free available chlorine. Daily environmental variables, such as rainfall and temperature, were integrated into the analysis from national climate datasets. Missing symptom onset dates were estimated using disease-specific median intervals between reporting and onset.

Exposure Assessment and Statistical Modeling Approach

Exposure was defined as the period before the onset of disease, specifically the 2 weeks prior. Controls were available for use throughout the remainder of the two-week period prior to and after the onset of disease. 

The researchers applied conditional logistic regression models to examine the relationship between the water quality indicators and the risk of developing enteric disease. Analyses were conducted based on demographic variables, source of water (ground vs surface), and characteristics of treatment capabilities, including the ability to reduce protozoa.

The researchers also examined interactions between E. coli and precipitation to assess combined environmental risks.

Overall Associations Between Water Indicators and Disease

The analysis included 30,858 cases with data for at least one direct indicator in both case and control periods, providing a large national assessment of drinking water and enteric disease in New Zealand. Overall, enteric disease risk was not statistically associated with most water quality indicators, including total coliforms, turbidity, or free available chlorine. Likewise, E. coli did not show a significant overall association with enteric disease when all cases were combined.

Increased Risk in Surface Water Systems

Under certain environmental and infrastructural circumstances, the presence of E. coli correlated with a higher risk. For example, in surface water-drawn water supplies, detection of E. coli was associated with a 23% increased risk of enteric bacterial disease. This was in contrast to groundwater systems, where no such association was observed, likely reflecting the greater protection groundwater can provide against microbial contamination, though this depends on factors such as bore depth, soil permeability, and borehead maintenance.

Impact of Water Treatment and Contamination Risk

Water systems with known source contamination risks, particularly those requiring a 4-log (99.99%) protozoa reduction, showed even stronger associations. In these systems, E. coli presence corresponded to a 28% increased risk of bacterial infection.

Interestingly, systems with protozoan barriers also showed elevated risk, possibly reflecting their placement in higher-risk environments rather than the treatment failing.

Rainfall Interaction and Environmental Exposure Effects

An independent association was found between higher rainfall levels and the risk of enteric disease, with the highest rainfall tertile associated with a 7% increase in risk, indicating that environmental factors play a major role. 

More importantly, when E. coli was detected in conjunction with heavy rainfall, the risk increased by 27%, suggesting that rainfall may facilitate contamination through runoff, particularly from agricultural sources, although the study noted that rainfall could also reflect alternative transmission pathways beyond drinking water itself.

Demographic Differences in Enteric Disease Risk

Individuals aged 0-22 years showed a 44% higher risk when exposed to E. coli in high-risk water systems. People in less deprived areas and of European ethnicity also had a higher risk. This might be due to more access to healthcare and testing rather than biological susceptibility. 

Another explanation given was seasonal variations, suggesting that there might have been a greater risk in winter months due to the potential for prolonged pathogen persistence at lower temperatures.

Study Limitations and Data Constraints

The authors also noted several important limitations. Notifiable disease data likely capture only a subset of all enteric infections, especially more severe cases that come to medical attention. The study also could not rule out non-waterborne transmission routes such as foodborne or person-to-person spread.

In addition, water supply characteristics were based on 2024 system data, so the analysis assumed these characteristics were broadly similar during the 2015-2019 study period. 

The authors also cautioned that the registered public supplies included in the study accounted for approximately 85% of New Zealand’s population, so the findings may not fully reflect risks in smaller, nonregistered, or domestic rural supplies.

Public Health Implications for Water Safety

This study shows that drinking water contamination is not only responsible for large outbreaks but may also contribute to some sporadic cases of enteric disease. Heavy rains can increase the risk of E. coli contamination in water, particularly in higher-risk supplies such as surface water systems or those with known source-water risks.

As this information makes clear, areas vulnerable to environmental impacts from heavy rain need more effective monitoring and treatment of water supplies. Communities need to be made aware that treated water may not always be safe for use. However, the findings do not suggest that all treated public water is broadly unsafe. 

Rather, they indicate that targeted improvements in monitoring and treatment may help reduce risk in more vulnerable systems. Improved surveillance and remediation of environmental hazards will help protect public health.