Drinking water quality and enteric disease: a nationwide case-crossover study (2015-2019) in New Zealand.

BACKGROUND

Despite the well-known effects of microbiological contamination of drinking water on enteric disease, there is limited epidemiological research investigating the relationship with drinking water quality direct indicators.

OBJECTIVE

To investigate the association between drinking water direct indicators and enteric disease.

METHODS

This study used a nationwide case-crossover study design of 46,020 cases of enteric disease between 2015 and 2019. Cases were successfully linked to a public water supply and exposure to E. coli, total coliforms, turbidity and free available chlorine quantified for case and control periods.

RESULTS

There were no statistically significant associations found between all enteric, bacterial-only or protozoan-only enteric disease notifications and water quality direct indicators. The presence of E. coli was associated with increased risk of enteric disease in supplies served by surface water (OR 1.23: 95% CI 1.04, 1.46), with known source water risks (OR 1.28: 95% CI 1.08, 1.52) and where the case was exposed to the highest tertile of rainfall (OR 1.27: 95% CI 1.01, 1.58).

SIGNIFICANCE: Major outbreaks of enteric disease can be caused by contamination of public drinking water supplies. Our findings suggest that this mechanism might also be responsible for sporadic cases of enteric disease caused by zoonotic bacterial pathogens.

IMPACT: This nationwide case-crossover study investigates the relationship between microbiological water quality and enteric disease in New Zealand. Linking over 46,000 cases (2015-2019) to public water supplies, we found that E. coli presence, particularly in surface water supplies with known source risks and during high rainfall, was significantly associated with increased enteric disease risk. These findings provide novel evidence that routine water quality indicators may predict sporadic enteric infections, reinforcing the need for enhanced microbial monitoring and treatment, especially in high-risk supply zones.