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Taking aim at cholera

Monday, 14th of May 2018

The Lancet, Volume 391, No. 10133, p1868–1870, 12 May 2018

Comment

Taking aim at cholera

Eric MintzEmail the author Eric Mintz

Open Access

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DOI: https://doi.org/10.1016/S0140-6736(18)30543-9

In 1854, John Snows work on cholera in London immortalised the power of mapping as a tool for disease prevention and control.¹ Over 160 years later, a more ambitious effort to map cholera has been reported in The Lancet.² Forgoing so-called shoe leather epidemiology in favour of big data, Justin Lessler and colleagues² used 279 cholera datasets covering 2283 locations in 37 countries, and cluster-level maps of access to improved water and sanitation in 41 countries, to map cholera incidence across sub-Saharan Africa at a 20 km × 20 km grid scale. They merged this incidence grid with census data and subnational administrative borders to identify and rank districts by their mean annual cholera incidence. Based on these analyses, they estimate that 151 of 3751 districts (4·0%, 95% credible interval [CrI] 1·7 to 16·8) in sub-Saharan Africa, home to 87·2 million people (95% CrI 60·3 million to 118·9 million), can be classified as high incidence (>1 case per 1000 people), and that targeting just 35·3 million people (95% CrI 26·3 million to 62·0 million) in the highest incidence districts with proven interventions—safe water, sanitation, and cholera vaccination—could reduce mean annual cholera incidence in the entire region by 50%. Alternatively, targeting 50·8 million people (95% CrI 39·7 million to 62·8 million) in just five countries (Somalia, Nigeria, Democratic Republic of the Congo, Sierra Leone, and Ghana) could prevent over 38% of all regional cholera cases.

These encouraging results provide a region-wide geospatial roadmap to guide the actions described in WHO and partners Ending Cholera: a Global Roadmap to 2030,³ released by the Global Task Force on Cholera Control in 2017. Lessler and colleagues² show that high-burden cholera hot spots to target with interventions can be identified in each affected country. Furthermore, they suggest that this strategy will yield high returns on the resources invested. The latter is particularly important given constraints on the resources needed. Sustainable water supply, sanitation, and hygiene (WaSH) infrastructure is crucial for ending transmission of cholera and other diseases transmitted by the faecal–oral route, and is a Sustainable Development Goal 2030 target in its own right, but its construction is costly and time consuming. Oral cholera vaccines, which are effective at reducing cholera transmission in the short term (3–5 years), remain in short supply relative to global demand despite substantial success in increasing their production and accessibility.⁴

In sub-Saharan Africa—an area of around 24 million km² that is home to more than 1 billion people—cholera has been reported to WHO every year since 1971.⁵ It lags behind other regions in population coverage with improved water and sanitation,⁶ and populations in many African nations are besieged by other public health threats that increase the risk of cholera transmission and deaths, including drought or flooding;⁷ terrorism, civil conflict, or war;⁸ malnutrition;⁹ and acute economic crises or chronic poverty. Similar conditions afflict cholera-affected areas in the Middle East, Asia, and the Caribbean,^10, ¹¹ with refugees from nearby countries sometimes forced to flee to cholera-endemic areas.

Efforts to combat cholera in sub-Saharan Africa in the 21st century have been almost entirely reactive—focused on outbreak response through emergency WaSH measures and the occasional cholera vaccine campaign. Although cholera surveillance, reporting, and diagnostic capacity have benefited from investments in public health preparedness and training, and from a few cholera-specific projects, an overarching strategy to most effectively mobilise efforts and resources for cholera prevention has been sorely lacking.¹² As other disease control programmes have shown, the ability to identify and target high-incidence hot spots is a crucial element of successful strategies in resource-constrained environments.

Although essential, disease mapping alone is not sufficient to guarantee the success of a control programme. Continuous iteration will be needed to maintain accurate, reliable, and up-to-date disease maps and modelling. Additionally, disease maps can aid national and local governments, partners, and donors understanding and efforts in targeting interventions. Finally, an absence of dedicated resources for implementation of interventions beyond those available for outbreak response may hinder progress.

The work by Lessler and colleagues is relevant to many parallel efforts. In support of Sustainable Development Goal 6, mapping of WaSH data will be enhanced by 2030 with more information on water quality and an additional institutional focus on schools and health facilities. Also, through its initiative on so-called precision public health, the Institute for Health Metrics and Evaluation is mapping safe water and sanitation coverage at a fine scale and has mapped child death rates in Africa by 5 km² blocks. Recently, molecular microbiologists published a global history and geography of the seventh pandemic cholera in Africa, and suggested that extensive intercontinental spread has been accompanied by at least 11 introductions of a single expanded lineage from Asia.¹³ By combining epidemiological, environmental, and genomic information, maps could be generated that are capable of informing future disease prevention efforts in even more powerful ways than are currently possible. In the future, will we identify and intervene readily in high-transmission hot spots, which, like individual super-spreaders within populations, appear to amplify epidemic propagation through their social connectivity or other characteristics? Can cold spots within epidemic zones teach us anything about social behaviours, environmental conditions, or the presence of bacteriophage or other potential inhibitors of toxigenic Vibrio cholerae O1 survival in environmental reservoirs that would confer resistance to epidemic propagation? We inhabit not just a geographical space defined by distances but many different worlds within this space: the worlds of social interactions, behaviours, events, and climate, to name but a few. Disease and risk mapping help us to better understand, learn from, and more successfully navigate those worlds, and are necessary tools as we target an end to cholera.

I declare no competing interests. The views in this Comment are those of the author and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

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Taking aim at cholera

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