Heavy rainfall events and diarrhea incidence: the role of social and environmental factors

Abstract

The impact of heavy rainfall events on waterborne diarrheal diseases is uncertain. We conducted weekly, active surveillance for diarrhea in 19 villages in Ecuador from February 2004 to April 2007 in order to evaluate whether biophysical and social factors modify vulnerability to heavy rainfall events. A heavy rainfall event was defined as 24-hour rainfall exceeding the 90th percentile value (56 mm) in a given 7-day period within the study period. Mixed-effects Poisson regression was used to test the hypothesis that rainfall in the prior 8 weeks, water and sanitation conditions, and social cohesion modified the relationship between heavy rainfall events and diarrhea incidence. Heavy rainfall events were associated with increased diarrhea incidence following dry periods (incidence rate ratio = 1.39, 95% confidence interval: 1.03, 1.87) and decreased diarrhea incidence following wet periods (incidence rate ratio = 0.74, 95% confidence interval: 0.59, 0.92). Drinking water treatment reduced the deleterious impacts of heavy rainfall events following dry periods. Sanitation, hygiene, and social cohesion did not modify the relationship between heavy rainfall events and diarrhea. Heavy rainfall events appear to affect diarrhea incidence through contamination of drinking water, and they present the greatest health risks following periods of low rainfall. Interventions designed to increase drinking water treatment may reduce climate vulnerability.

Keywords: Ecuador; climate; diarrhea; heavy rainfall events; rain; social vulnerability; water treatment.

Figure 1.

Map of the region in northern coastal Ecuador included in a study of heavy rainfall events and diarrhea incidence, showing the location of study villages and rainfall monitors, 2004–2007.

Figure 2.

Diarrhea incidence (A) and rainfall (B and C) in 19 study villages in a study of heavy rainfall events and diarrhea incidence, Ecuador, February 2004–April 2007. A) Weekly diarrhea incidence (cases per 1,000 person-weeks) across all villages. B) Maximum 24-hour rainfall in a 1-week period for each village. The graph shows heavy rainfall events occurring following periods of low (black dots), moderate (gray dots), and high (white dots) 8-week rainfall. The horizontal line shows the 90th percentile value (56 mm), used to define heavy rainfall events. C) Total rainfall in the previous 8 weeks for each village. Horizontal lines indicate the 33rd (426 mm) and 66th (605 mm) percentile values, used to delineate low, moderate, and high 8-week rainfall. Aug, August; Feb, February; Nov, November.

Figure 3.

Estimated association between heavy rainfall events and diarrhea incidence at different levels of community drinking-water treatment when rainfall during the previous 8 weeks was low (A), moderate (B), or high (C), Ecuador, 2004–2007. The incidence rate ratio (IRR; solid line) and 95% confidence interval (CI; dashed lines) are shown for the 10th through 90th percentiles of water treatment values observed in this study. Water treatment was defined as filtration, boiling, or chlorination of drinking water.