Cholera epidemic in Yemen – Author's reply
Authors: Anton Camacho, Malika Bouhenia, Andrew S Azman, Marc Poncin, Nevio Zagaria, Francisco J Luquero
Journal: The Lancet Global Health (2018)

Dureab and colleagues1Camacho A Bouhenia M Alyusfi R et al.Cholera epidemic in Yemen, 2016–18: an analysis of surveillance data.Lancet Glob Health. 2018; 6: e680-e690Summary Full Text Full Text PDF PubMed Scopus (100) Google Scholar concur with our call for urgent measures to enhance cholera preparedness and response in Yemen to stop the outbreak that has persisted for more than 2 years. However, they challenge our finding that the large second epidemic wave was associated with the start of the rainy season from April to May, 2017. They suggest that rainfall is unlikely to be a major risk factor for cholera transmission in Yemen because cholera and rainfall are spatially and temporally variable within the country, and because our analyses used modelled and not measured estimates of precipitation. We agree that the mechanistic relationship between cholera and and precipitation is non-linear and complex, and varies by context.2Moore SM Azman AS Zaitchik BF et al.El Niño and the shifting geography of cholera in Africa.Proc Natl Acad Sci USA. 2017; 114: 4436-4441Crossref PubMed Scopus (49) Google Scholar Therefore, we used a flexible modelling approach that accounted for spatial (ie, district-level) and temporal variability in cholera and precipitation, and allowed for heterogeneous effects of rain across the country while accounting for potential confounders. Across multiple models with different functional relationships between rainfall and cholera, the statistical association remained strong, suggesting a true association between accumulated rainfall and transmission. In view of no working weather stations in Yemen since 2015, the precipitation data are based on remote sensing estimates.3Funk C Peterson P Landsfeld M et al.The climate hazards infrared precipitation with stations—a new environmental record for monitoring extremes.Sci Data. 2015; 2: 150066Crossref PubMed Scopus (2250) Google Scholar However, we expect this to be analogous to non-differential misclassification bias, leading to weakening of the measured association between rainfall and transmission. While we found a statistical association between accumulated rainfall and transmission, our analyses did not reveal the causal mechanisms behind this association. As discussed in our original manuscript, the war in Yemen has disrupted water and sewage systems and increased fuel prices, making the use of unsafe water and food sources more likely, perhaps even more at the start of the rainy season when surface water is more readily available.4Naylor H In Yemen's grinding war, if the bombs don't get you, the water shortages will.The Washington Post. July 23, 2015; https://www.washingtonpost.com/world/middle_east/in-yemens-grinding-war-if-the-bombs-dont-get-you-the-water-shortages-will/2015/07/22/a0f60118-299e-11e5-960f-22c4ba982ed4_story.html?noredirect=on&utm_term=.9a7db654329dDate accessed: August 10, 2018Google Scholar In addition, our results do not exclude the possibility that other unmeasured factors have affected cholera transmission. Indeed, we found spatial heterogeneity in the relative risk of cholera transmission attributable to rainfall, with coastal arid districts being less at risk during the increasing phase of the second epidemic wave. More work is certainly needed to untangle the mechanistic drivers of cholera transmission across settings and to design tailored interventions, both for epidemic and endemic settings. Nevertheless, our analysis better characterises the spatiotemporal risk of cholera transmission in Yemen, which represents key information for action during the months (from April to August) most favourable for effective spread. Following the return of rain in April, 2018, a third epidemic wave of cholera started in Yemen (figure). Although reported cases are increasing slower than in the second wave, the 4000 cases reported in week 30 (ending July 29, 2018) represent a peak on the graph twice as high as the first wave. Despite the prevention and control efforts put in place by health authorities, including the cholera vaccine campaigns in the Governorates of Aden, Al-Hudaydah, and Ibb,5US Agency for International DevelopmentYemen—complex emergency fact Sheet #10, fiscal year (FY) 2018. Relief Web.https://reliefweb.int/report/yemen/yemen-complex-emergency-fact-sheet-10-fiscal-year-fy-2018Date: Aug 10, 2018Date accessed: August 10, 2018Google Scholar cholera risk remains high. Efforts to contain the third epidemic wave are compounded by attacks on cholera treatment centres.5US Agency for International DevelopmentYemen—complex emergency fact Sheet #10, fiscal year (FY) 2018. Relief Web.https://reliefweb.int/report/yemen/yemen-complex-emergency-fact-sheet-10-fiscal-year-fy-2018Date: Aug 10, 2018Date accessed: August 10, 2018Google Scholar Therefore we renew our urgent call to ensure health facilities remain safe and accessible and improve cholera risk reduction activities to bring the epidemic to an end. We declare no competing interests. Cholera epidemic in Yemen, 2016–18: an analysis of surveillance dataOur analysis suggests that the small first cholera epidemic wave seeded cholera across Yemen during the dry season. When the rains returned in April, 2017, they triggered widespread cholera transmission that led to the large second wave. These results suggest that cholera could resurge during the ongoing 2018 rainy season if transmission remains active. Therefore, health authorities and partners should immediately enhance current control efforts to mitigate the risk of a new cholera epidemic wave in Yemen. Full-Text PDF Open AccessCholera epidemic in YemenAnton Camacho and colleagues1 have reported data from the first two waves of the ongoing cholera epidemic in Yemen. They demonstrated that the small first wave in 2016 was followed by a much larger wave in 2017. Moreover, they showed a positive association between weekly rainfall and cholera incidence during the second wave, and published their concern that there might be another large wave associated with the 2018 rainy season. Full-Text PDF Open AccessYemen needs a concrete plan—nowMore than 3 years into the current conflict, the situation in Yemen has reached catastrophic proportions. These have been exacerbated by the collapse of the national economy and by intense fighting around Hodeidah, the country's principal port on the Red Sea where most commodities, including food and medicines, enter the largely import-based Yemeni market. The planned peace negotiations are dependent on the goodwill of two warring parties whose disregard for international law has already led to large civilian casualties and the destruction of essential infrastructure such as health facilities, water systems, roads, and markets. Full-Text PDF Open Access