The seasonality of cholera in sub-Saharan Africa
Authors: Perez-Saez Javier , Lessler Justin , Lee Elizabeth C. , Luquero Francisco J. , Malembaka Espoir Bwenge , Finger Flavio , Langa José Paulo , Yennan Sebastian , Zaitchik Ben , Azman Andrew S 
Published: November 2021

Background

Cholera remains a major threat in Sub-Saharan Africa (SSA) where some of the highest case fatality risks are reported. Knowing in what months and where cholera tends to occur across the continent can aid in improving efforts to eliminate cholera as a public health concern; though largely due to lack of unified large-scale datasets, no continent-wide estimates exist. In this study we aim to estimate cholera seasonality across SSA.

Methods

We leverage the Global Task Force on Cholera Control (GTFCC) global cholera database with statistical models to synthesize data across spatial and temporal scale in order to infer the seasonality of excess suspected cholera occurrence in SSA. We developed a Bayesian statistical model to infer the monthly risk of excess cholera at the first and/or second administrative levels. Seasonality patterns were then grouped into spatial clusters. Finally, we studied the association between seasonality estimates and hydro-climatic variables.

Findings

The majority of studied countries (24/34) have seasonal patterns in excess cholera, corresponding to approximately 85% of the SSA population. Most countries (19/24) also had sub-national differences in seasonality patterns, with strong differences in seasonality strength between regions. Seasonality patterns clustered into two macro-regions (West Africa and the Sahel vs. Eastern and Southern Africa), which were composed of sub-regional clusters with varying degrees of seasonality. Exploratory association analysis found most consistent and positive correlations between cholera seasonality and precipitation, and to a lesser extent with temperature and flooding.

Interpretation

Widespread cholera seasonality in SSA offers opportunities for intervention planning. Further studies are needed to study the association between cholera and climate.

Funding

The NASA Applied Sciences Program and the Bill and Melinda Gates Foundation.

Research in Context

Evidence before this study

Previous studies were searched using PubMed on Nov 15, 2021, without language or date restrictions using the search terms [“cholera*” AND “season*” AND (“Africa*” OR “Angola” OR “Burundi” OR “Benin” OR “Burkina Faso” OR “Botswana” OR “Central African Republic” OR “Côte d’Ivoire” OR “Cameroon” OR “Democratic Republic of the Congo” OR “Republic of Congo” OR “Djibouti” OR “Eritrea” OR “Ethiopia” OR “Gabon” OR “Ghana” OR “Guinea” OR “Gambia” OR “Guinea-Bissau” OR “Equatorial Guinea” OR “Kenya” OR “Liberia” OR “Madagascar” OR “Mali” OR “Mozambique” OR “Mauritania” OR “Malawi” OR “Namibia” OR “Niger” OR “Nigeria” OR “Rwanda” OR “Sudan” OR “Senegal” OR “Sierra Leone” OR “Somalia” OR “South Sudan” OR “São Tomé and Príncipe” OR “Swaziland” OR “Chad” OR “Togo” OR “Tanzania” OR “Uganda” OR “South Africa” OR “Zambia” OR “Zimbabwe”)]. Two additional known articles not identified by our PubMed search were added, one on the seasonality of cholera in Kenya and one on the epidemiology of cholera in West Africa. Studies were included if they focused on the epidemiology of Vibrio cholerae O1 or O139, covering one or multiple countries in sub-Saharan Africa (SSA) for at least two years with reported cases. We identified a total of 140 studies, of which 36 met our inclusion criteria. Of these 4 were reviews, 3 were regional or global studies, and 29 focused on specific countries. Local-level seasonality studies were identified in Burundi (1), Côte d’Ivoire (1), Cameroon (2), Democratic Republic of the Congo (3), Ghana (1), Guinea-Bissau (1), Kenya (1), Mozambique (3), Nigeria (1), Senegal (1), Somalia (1), South Sudan (1), Togo (1), Tanzania (4), Uganda (2), South Africa (1), and Zambia (4). These local studies mainly consisted of epidemiological descriptions of cholera incidence at either the national or first administrative unit scale, covering between 2 and 31 years of data. Most of these studies found seasonality in cholera with different patterns between countries. Two regional studies in West Africa found seasonal patterns in cholera incidence in the second part of the calendar year, with evidence for synchrony with rainfall patterns. Finally one global study between 1975 and 2005 found evidence for a latitudinal gradient of seasonality strength with weaker seasonality around the equator.

Added value of this study

Although most local-level and regional studies have found evidence for cholera seasonal patterns in SSA, gaps remain for certain countries and there currently lacks a systematic and continental-scale investigation. By leveraging a large database of cholera incidence we evaluate and characterize cholera seasonality at sub-national scale in 34 countries of SSA for which sufficient data was available. We show that cholera is seasonal in the majority of countries (24/34), with sub-national heterogeneity in most of them (19/24). Results enable the description of cholera seasonality at the continental level and the exploration of associations with hydro-climatic variables.

Implications of all the available evidence

This work establishes the extent and strength of cholera seasonality in SSA, providing a basis on which to ground strategic decision on large-scale intervention allocation as well as future work on the climatic and non-climatic drivers of cholera seasonality in SSA.