Environmental factors associated with increased cholera cases in low-income districts in Zambia, 2017-2018

Nyambe Sinyange^1,X, Tadatsugu Imamura^2,3,X, Orbrie Chewe¹, Paul Msanzya Zulu¹, Fred Kapaya¹, Raymond Hamoonga¹, Mazyanga Lucy Mazaba¹, Muzala Kapina¹, Nathan Kapata¹, Edgar Simulundu⁴, Victor Mukonka^1,&

 

^XThese authors contributed equally to this paper.

 

¹Zambia National Institute of Public Health, Lusaka, Zambia, ²Japan International Cooperation Agency, Tokyo, Japan, ³National Center for Child Health and Development, Tokyo, Japan, ⁴The University of Zambia, Lusaka, Zambia

 

 

^&Corresponding author
Victor Mukonka, Zambia National Public Health Institute, Lusaka, Zambia

 

 

 

 

Cholera is an acute diarrheal infection caused by the bacterium toxigenic Vibrio cholerae (V.cholerae), which remains a major public health issue in Africa [1]. Zambia has been affected by repeated epidemics of cholera since 1977, and the nationwide epidemic in 2017-2018 caused more than 5,900 cases and 110 deaths [2,3]. The capital city Lusaka was reported as one of the hot spots for cholera outbreaks, and residents in some of the low-income resident districts in the city were suggested as being at high risk for cholera infections [2,4]. Notably, one of those districts in a peri-urban area of Lusaka (i.e., Kanyama sub-district) was reported as the origin of the recent major outbreaks in 2005-2006, 2016, and 2017-2018 [4,5]. A case-control study for cholera cases and their neighbours in Lusaka during the 2017-2018 outbreak has revealed poor sanitation and hygiene practices as potential risk factors for acquiring cholera infections at individual levels [6]. Conversely, despite the significant roles of the Kanyama sub-district for the onset and spread of cholera outbreaks in Lusaka, environmental factors associated with the increased number of cholera outbreaks in the area have not been clarified. In the study, we determined the environmental factors associated with the increased number of cholera cases in the Kanyama sub-district by using geocoordinate data collected during the outbreak in 2017-2018.

 

 

Study setting

 

The study was conducted using data collected during the cholera outbreak response which was implemented by the Zambia National Public Health Institute (ZNPHI) and the Zambia Ministry of Health (MoH) in 2017 and 2018.

 

Case definition and study population

 

During the cholera outbreak, patients presenting three or more events of acute “rice water” diarrhoea within 24 hours were defined as suspected cholera cases, and those who tested positive for V. cholerae O1 by culture using Thiosulfate-citrate-bile salts-sucrose (TCBS) agar and serotyping was defined as confirmed cholera cases [7]. In the study, suspected cholera cases identified in Kanyama sub-district, which is located on the west outskirts of Lusaka, Zambia, between 1 October 2017 and 31 May 2018 were categorized as cholera cases.

 

Cholera cases with available geocoordinate data of their houses were included for analysis. Geo-coordinate data of houses of cholera cases, water sources, toilets, waste collection sites, and flooded wetlands (i.e., areas covered by water after heavy rainfall) were collected by using global positioning system (GPS) during the physical visits of public health workers, which was implemented by the ZNPHI and the MoH [3].

 

Data analysis

 

A digital map of the Kanyama sub-district was developed using QGIS version 3.10 A Coruna (https://qgis.org/en/site/ ). Boundaries were drawn along the streets to divide the Kanyama sub-district into small zones with 1-10 small blocks, by which a total of 218 zones were generated (Figure 1A ). The number of cholera cases, water sources, toilets, and waste collection sites, as well as the area of flooded wetlands per zone, were plotted using QGIS.

 

Statistical analysis was carried out by using R version 3.5.0 (R Foundation for Statistical Computing, Vienna, Austria). A Wilcoxon rank sum test was used to explore the distribution of environmental factors between zones with at least one cholera case (i.e., zones with cases) and those without any cases (i.e., zones without cases). Spearman´s rank correlation coefficient was calculated to explore the relationship between the number of cholera cases and the independent factors (number of water sources, toilets, waste collection sites, and the area of flooded wetlands per zone). The correlation coefficient which was higher than +0.3 or lower than -0.3 was regarded as being correlated [8]. A p-value less than 0.05 was considered statistically significant.

 

Ethical Consideration

 

Ethical clearance for the use of patients´ data collected as part of the public health response of ZNPHI and MoH for scientific analysis and publication was obtained from the National Health Research Authority, Zambia (reference number NHRA000054/30/03/2022). Geo-coordinate data of the household was handled carefully as it was private information of the cases. Data was deidentified and data analysis was conducted on password-protected computers.

 

 

A total of 405 cholera cases with geocoordinate data of their houses were identified in the Kanyama sub-district between October 2017 and May 2018. One or more cholera cases were identified in 136 zones (62%, 136/218) (Figure 1B ). The median number (interquartile range; IQR)of water tanks were significantly smaller in zones with cases than those without cases (0, 0-21; vs 0.5, 0-19; p=0.021) (Table 1). Among the 6 types of toilets, the number (median, IQR) of toilets outside houses (56.5, 0-256; vs 35.5, 0-151; p=0.0001) and pit latrines (51, 0-194; vs 28; 0-117; p<0.001) were significantly larger in zones with cases than those without cases (Table 1). Among the 4 types of waste collection sites, none of them showed significant differences in the distribution of cholera cases (Table 1). The area of the flooded wetlands per zone was not significantly different between zones with cases and those without them (Table 1).

 

In addition, we explored the associations between the increased number of cholera cases and the distribution of environmental factors (i.e., the number of water sources, toilets, and waste collection sites, and the area of flooded wetlands per zone). Among the 6 types of toilets, the number of toilets outside houses (rho=0.307, p<0.001) and pit latrines (rho=0.354, p<0.001) were positively correlated with the number of cases per zone (Table 2). The number of any types of water sources and waste collection sites, and the area of flooded wetlands did not show significant correlations with the number of cases per zone (Table 2).

 

 

We explored the environmental factors for the increased number of cholera cases in the Kanyama sub-district; a low-income resident district, which has been the origin of the recent cholera outbreaks in Lusaka, Zambia. Distributions of toilets outside houses and pit latrines were significantly correlated with the identification of cases and the increased number of cases in each zone. It suggested that a lack of access to sanitary toilet facilities was associated with an increased risk for transmission of cholera in the Kanyama sub-district. Our results were in line with a previous study which reported that households without latrines were at higher risk for cholera infections in Lusaka [9]. Our study further demonstrated that a lack of access to sanitary toilet facilities was also associated with the increased number of cholera cases in the origin place of the whole outbreaks in Lusaka.

 

The effectiveness of improving sanitation systems in reduction of the cholera incidence has been demonstrated in previous studies in different parts of the world [10]. Despite the intensive public health interventions to improve the sanitation systems that have been made in Lusaka, a lack of access to those systems was reported as one of the potential risk factors for cholera outbreaks in the study, which was suggestive of the difficulties in maintaining those systems and eliminating the risks of cholera outbreaks in those areas. Regular monitoring and improvement of the sanitation systems might be necessary especially for those low-income resident districts to prevent the onset and spread of cholera outbreaks.

 

Meanwhile, the distribution of water tanks, which were built as part of the public health response during the outbreaks in 2017-2018, was negatively associated with the number of cholera cases in the area. Our results highlighted the importance of safe water supplies to prevent the transmission of cholera in low-income resident districts.

 

Limitations of our study include the study design being an ecological study with a limited number of cholera cases with their household locations available. Analysis with an increased number of cases with geo coordinates might help understand the overall geographical distribution patterns of cases and spatial risk factors in the Kanyama sub-district. In addition, the effect of the methodology of manually creating boundaries on the analysis output cannot be fully ruled out. However, it was assumed to be minimal as we obtained similar results using multiple types of boundaries generated through the same methodologies.

 

 

A lack of access to sanitary toilet facilities was shown to be associated with the increased number of cholera cases in a low-income resident district (i.e., Kanyama sub-district), which has been the origin place of the recent cholera outbreaks in Lusaka, Zambia. Further studies using additional data (e.g., an increased number of cases with geocoordinates, and water source test results) are required to fully define the risk factors for the spread of cholera outbreaks in those low-income resident districts to suppress future cholera outbreaks.

 

 

 

 

The authors declare no competing interest.

 

 

N.S. and T.I. were the principal investigators. N.S., O.C., P.Z., F.K., R.H., M.L., M.K., N.K., and V.M. designed the study. O.C. and F.K. collected the data. N.S., T.I., and E.S. analyzed the data. N.S. and T.I. wrote the manuscript.

 

 

We thank the Lusaka City Council for kindly providing geocoordinate data collected during the chorea outbreak response in 2017-2018. We thank the Zambia Ministry of Health, Centers for Disease Control and Prevention, the United States, and World Health Organization for supporting the cholera outbreak response in Zambia.

 

 

Table 1: Characteristics of zones with cases of Vibrio cholerae O1 in Kanyama sub-district

Table 2: The relationship between the number of cholera cases and associated environmental factors in Kanyama sub-district, Lusaka, Zambia

Figure 1: Distribution of cholera cases in Kanyama sub-district between October 2017 and May 2018

 

 

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