Date Published: February 11, 2016
Publisher: Public Library of Science
Author(s): Qifang Bi, Andrew S. Azman, Syed Moinuddin Satter, Azharul Islam Khan, Dilruba Ahmed, Altaf Ahmed Riaj, Emily S. Gurley, Justin Lessler, Edward T. Ryan. http://doi.org/10.1371/journal.pntd.0004400
Abstract: Close interpersonal contact likely drives spatial clustering of cases of cholera and diarrhea, but spatial clustering of risk factors may also drive this pattern. Few studies have focused specifically on how exposures for disease cluster at small spatial scales. Improving our understanding of the micro-scale clustering of risk factors for cholera may help to target interventions and power studies with cluster designs. We selected sets of spatially matched households (matched-sets) near cholera case households between April and October 2013 in a cholera endemic urban neighborhood of Tongi Township in Bangladesh. We collected data on exposures to suspected cholera risk factors at the household and individual level. We used intra-class correlation coefficients (ICCs) to characterize clustering of exposures within matched-sets and households, and assessed if clustering depended on the geographical extent of the matched-sets. Clustering over larger spatial scales was explored by assessing the relationship between matched-sets. We also explored whether different exposures tended to appear together in individuals, households, and matched-sets. Household level exposures, including: drinking municipal supplied water (ICC = 0.97, 95%CI = 0.96, 0.98), type of latrine (ICC = 0.88, 95%CI = 0.71, 1.00), and intermittent access to drinking water (ICC = 0.96, 95%CI = 0.87, 1.00) exhibited strong clustering within matched-sets. As the geographic extent of matched-sets increased, the concordance of exposures within matched-sets decreased. Concordance between matched-sets of exposures related to water supply was elevated at distances of up to approximately 400 meters. Household level hygiene practices were correlated with infrastructure shown to increase cholera risk. Co-occurrence of different individual level exposures appeared to mostly reflect the differing domestic roles of study participants. Strong spatial clustering of exposures at a small spatial scale in a cholera endemic population suggests a possible role for highly targeted interventions. Studies with cluster designs in areas with strong spatial clustering of exposures should increase sample size to account for the correlation of these exposures.
Partial Text: Cholera is responsible for over 100,000 deaths each year and is endemic in many countries . In Bangladesh, approximately 450,000 cases of cholera are estimated each year . Cholera cases have been known to be tightly geographically clustered. For example, the scale of clustering of cholera cases in Matlab, Bangladesh was found to be around 250m in one study, and under 1km in another [3,4]. Close interpersonal contact likely drives clustering of secondary transmitted cases [5,6]. However, few studies have explored whether clustering in the risk factors themselves may explain clustering in disease.
Two hundred and thirty-nine households were recruited in 61 spatially matched-sets. Of those, 171 households (72%) in 48 matched-sets had complete household level exposure data, and were included in the present study (Fig 1). Eight hundred and sixty seven individuals from 239 households and 60 matched-sets were recruited, and were included for the individual level exposure analyses. The average geographic extent of the matched-sets was 42 meters (IQR = 22–55, range = 2–108), with an average of 3.6 households per matched-set (IQR = 3–4). We analyzed 10 household level and 5 individual level exposures from the spatially matched households (Table 1 and Table 2; See S1 Table for summary of exposures in the primary case households). We excluded one exposure that had almost no variability; 98.8% of households stored drinking water.
We found substantial clustering of single exposures and co-occurrence of different exposures suspected to modify cholera and diarrhea risk within groups of nearby households, and an overall trend of households nearer to one another being more likely to share risk factors. This suggests that even within a relatively compact neighborhood there can be substantial spatial dependence of risk for cholera and diarrheal diseases driven by spatial homogeneity in behavior or access to improved water and sanitation.