The micro-hotspots of cholera in Kano State, Nigeria, 2010-2019—analysis of patient characteristics, Spatio-temporal patterns and contextual determinants at the ward level
Authors: Ngwa Moise Chi , Ihekweazu Chikwe , Okwor Tochi Joy , Yennan Sebastian , Williams Nanpring , Elimian Kelly , Karaye Nura Yahaya , Oche James Agada , Bello Imam Wada , Sack David A. 
Published: August 2021

Cholera is endemic in Nigeria, and Kano State reports outbreaks yearly with a case fatality rate (CFR) of 3.3% from 2010 to 2019. The lack of data at ward level has enabled the disease to evade focused interventions. The goal of this study was to describe the geographic distributions, care-seeking behaviors, Spatio-temporal cluster patterns of the micro-hotspots (hotspots wards) linked with suspected and confirmed cases and deaths of cholera in Kano State.

Suspected and confirmed cholera morbidity and mortality at the ward level from 2010-2019 were acquired from the Nigeria Centre for Disease Control. Population and waterbody data were obtained from the Nigeria Expanded Program on Immunization and online, respectively. Data analysis used SaTScan and methods recommended by the Global Task Force on Cholera Control.

During these ten years, 18,483 suspected and confirmed cases (617 deaths) were reported with 67.7% of the cases and 72% of the deaths from rural wards. The ages of the cases ranged from 1 month to 100 years with a distribution skewed to the older years. CFRs were statistically higher in the <5-year olds compared to those >14 years (p-value = 0.0005). For 2010-2019, gender was statistically associated with cholera outcome (survived/died) (p-value = 0.0006), and women in the rural setting disproportionately died from cholera than women in the urban area (p-value = 0.003). Cholera severity, as measured by hospitalization and death, was higher in the urban (77.4%) compared with the rural (53.4%) setting with the highest severity (84.7%) registered among those >14 years. Rapid Diagnostic Tests (RDT) were performed in 1.3% (249) samples of all suspected cases and ranged from 0.7% among the 5-14 year-olds in the rural to 3.5 % among the < 5-year-olds in the urban areas. Of the stool samples collected, 62.7% tested positive for V. cholerae using RDT. The positivity rate was least in the urban setting amongst the <5 years (41.2%) while care-seeking-behavior ranged from 52.1% in the urban to 82.7% in the rural settings. Seasonal patterns of disease often differed between urban and rural settings with outbreaks occurring in both the dry and rainy seasons, but with more intense transmission occurring during the rainy season from week 22 (early June) to week 40 (late September). A Spatio-temporal clustering analysis detected 168 micro-hotspots out of 404 wards, with a population of 4,876,254, having a significantly higher risk (relative risk 1.01-18.73) compared to the State as a whole. While 79 micro-hotspots with a population of 2,119,974 had a RR ≥ 2. The micro-hotspots tended to cluster around waterbodies. SaTScan and GTFCC methods generally agreed in micro-hotspots detection.

This study shows the epidemiology of cholera in Kano State differs between urban and rural settings and that hotspot maps at the ward level, not hotpots maps at the Local Government Area level, are best suited for targeting interventions including vaccines. Appropriate studies are needed to further delineate the urban and rural divide of outbreaks but targeting interventions to the identified high-priority micro-hotspots will facilitate cholera elimination from the state.

Author summary

Cholera is endemic in Nigeria since 1970, and Kano State reports outbreaks almost every year. From 2010-2019, Kano State reported 18,483 cases and 617 deaths, for a case fatality rate of 3.3%. Focusing interventions at the Local Government Area instead of the ward (local) level contributed to the continuous threat from cholera in Kano State. When we divided the state into its two geographic areas (urban and rural), there were very different epidemiology as well as Spatio-temporal patterns of clustering of wards with elevated relative risk (micro-hotspots). Nearly two-thirds of the cases and deaths were reported from rural wards. The ages of the cases ranged from 1 month to 100 years with case fatality ratios higher in the <5-years olds compared to those >14 years (p-value = 0.0005). Women in the rural area not only had more cases but also were more likely to die from cholera than women in the urban area (p-value = 0.003). The hospitalization rate was higher in the urban than rural setting whereas care-seeking behavior was higher in rural than urban areas. Rapid Diagnostic Tests to confirm cholera was very low overall. Seasonal patterns of disease differed between urban and rural areas with outbreaks occurring in both the dry and rainy seasons, but with more cases from June to September, during the rainy season. A population of 4,876,254 live in the 168 micro-hotspots in which cholera risks were 1.01 to 18.73 times higher compared to the State as a whole. Following the Global Task Force on Cholera Control recommendations, interventions should focus on these identified micro-hotspots for cholera elimination from Kano State.