Aedes-borne disease outbreaks in West Africa: A call for enhanced surveillance
Graphical Abstract
Aedes-borne disease occurrence in West Africa. Arboviral disease occurrences were extracted from previous compilations, systematic review of literature, and outbreak reports. Data are organized by timing of outbreak with panels showing oldest (bottom panel) to most recent (top panel) observations.
Introduction
There is growing recognition that West Africa is at risk for outbreaks of dengue virus (DENV), chikungunya virus (CHIKV) and Zika virus (ZIKV). (Amarasinghe et al., 2011, Stoler et al., 2014, Meda et al., 2016, Weetman et al., 2018, Jaenisch et al., 2014) In 2019, the World Health Organization noted a sharp increase in cases of arboviruses worldwide, including in the Republic of Congo, Tanzania and Cote d'Ivoire in the African Region. (WHO 2019) A recent systematic review estimated 18% seroprevalence against DENV in sub-Saharan Africa, (Simo et al., 2019) and there is growing evidence of changing vector distributions. However, there remain large gaps in our understanding of the current distribution of these arboviral diseases as vector-borne disease surveillance in the region is focused primarily on malaria.
These gaps in our understanding of emerging arboviral disease in Africa have important public health implications. Undetected arboviral disease may lead to inappropriate clinical treatment and unnecessary health care expenditures, as clinical management of febrile illness defaults to malaria. (D'Acremont et al., 2009) Moreover, major gaps in our understanding of the epidemiology of arboviral transmission in the region make it difficult to design effective surveillance and control strategies: there is a need for up-to-date documentation of the epidemiology of arboviral transmission in the region, including identification of relevant vectors and areas of greatest transmission risk.
Here, we focus on DENV, CHIKV, and ZIKV in West Africa. West Africa, for the purpose of this review, is comprised of the countries in the northwestern region of Africa, with Mauritania as the northernmost, Niger the farthest east, and Gabon the farthest south. The region is ecologically diverse, including the Sahelian desert as well as tropical rainforests. West Africa is particularly vulnerable to arboviral disease transmission as this rapidly urbanizing region has suitable vector habitat, channels of pathogen import due to increasing globalization, as well as endemic yellow fever virus (YFV), indicating pre-existing arboviral transmission suitability. Recent evidence points to ongoing transmission of DENV, ZIKV, and CHIKV in West Africa: travelers to the region returning home with DENV (Amarasinghe et al., 2011) and ZIKV infection, (Foy et al., 2011) abnormally high rates of microcephaly, (Olusanya, 2012) and high seroprevalence against DENV and CHIKV in retrospective surveys. (Stoler et al., 2015, Simo et al., 2019, de Araújo Lobo et al., 2016) Moreover, the rate of malaria diagnosis in West Africa outpaces the estimated true rate of clinical malaria, and some have hypothesized that a substantial proportion of these mis-diagnosed fevers may actually be attributable to arboviral infections. (Amarasinghe et al., 2011, Stoler et al., 2014, Crump et al., 2013) Models predicting the global distribution of arboviral diseases consistently find that YFV, ZIKV, DENV, and CHIKV have high environmental suitability for current circulation in West Africa. (Bhatt et al., 2013, Campbell et al., 2015, Brady et al., 2012, Bogoch et al., 2016) Despite the risks of arboviral transmission in West Africa, surveillance for DENV, ZIKV, and CHIKV in the region is lacking, and the epidemiology of these infections in the region is poorly documented. (Amarasinghe et al., 2011, Stoler et al., 2014, Meda et al., 2016, Weetman et al., 2018) We note that considerable arboviral risk also exists in East Africa, as demonstrated by outbreaks in Reunion Island and Kenya, (Gérardin et al., 2008, Sergon et al., 2008) prompting research on arboviral diseases in these areas. (Ochieng et al., 2015, Hertz et al., 2012) Given the diverse ecology and sociocultural environments across the African continent, we posit that there is value in regionally focused information on arboviral transmission.
We conducted a review of the literature to evaluate evidence of DENV, ZIKV, and CHIKV and the distribution of their Aedes mosquito vectors in West Africa. This review updates prior estimates of Aedes mosquito and arbovirus distribution in West Africa, (Kraemer et al., 2015, Nsoesie et al., 2016, Messina et al., 2016, Messina et al., 2014) providing a current, region-specific synthesis of this rapidly evolving public health challenge. Based on these findings, we characterize the potential contributions of major demographic and environmental changes occurring in the region in the observed and future disease trends and articulate a set of recommendations regarding the most urgent priorities in addressing arboviral disease in West Africa.
Section snippets
Review strategy
We compiled reports of Aedes-borne arboviral disease and Aedes mosquito observations in West Africa by reviewing the published literature. Sources included previous compilations, (Kraemer et al., 2015, Nsoesie et al., 2016, Messina et al., 2016, Messina et al., 2014) the peer-reviewed literature, and outbreak reports indexed in ProMed or the World Health Organization Disease Outbreak News published previous to October 2018. Pubmed and Embase were searched for the terms “Aedes aegypti” OR “Aedes
Evidence of Aedes vector populations and arboviral disease outbreaks in West Africa
Aedes-borne virus outbreaks appear to have increased in frequency and magnitude in West Africa in recent years (Figures 1 & 2) although it is difficult to accurately quantify increases in cases due to improvements in arbovirus detection methods as well as increased awareness and testing. There have been multiple large epidemics of DENV and CHIKV in West Africa in the past decade, with DENV outbreaks occurring in urban areas of Burkina Faso (Promed 2017) and Cote d'Ivoire, (Promed 2017) and
What is driving recent changes in arboviral transmission in West Africa?
This evidence suggests that arboviral transmission is currently emerging in West Africa in a manner that is distinct from sylvatic YFV outbreaks that have previously occurred in the region. Recent large-scale arbovirus outbreaks in West Africa have been primarily focused in urban areas, characterized by widespread mosquito-to-human transmission. Aedes albopictus has played an important role in several West African outbreaks, and Ae. aegypti, long documented in the region, remains a wide-spread
Urbanization in West Africa and Aedes-borne disease
The global growth of urban centers is a primary driver of Aedes-borne disease outbreaks globally. (Ebi and Nealon, 2016, Mayer et al., 2017, Dengue, 2011) Aedes aegypti and Ae. albopictus mosquitoes thrive in urban and peri-urban environments as they feed on humans (Delatte et al., 2010, Kamgang et al., 2012, Richards et al., 2006, Ponlawat and Harrington, 2005) and prefer oviposition in artificial containers common in urban settings, including discarded plastics, used tires, flower pots, and
Climate change in West Africa and Aedes-borne disease
Aedes-borne viruses are highly sensitive to meteorological conditions due to the ecological requirements of both the vectors and the viruses. (Niang et al., 2014, Mordecai et al., 2017, Tesla et al., 2018, Butterworth et al., 2016) Climate change is projected to increase mean annual temperature by more than 2˚C on the African continent within the century, and increase periods of extreme precipitation variability including drought and flooding. (Niang et al., 2014) In fact, the effects of
Priorities for the Future
There is strong evidence that transmission of arboviral diseases including CHIKV, ZIKV, and DENV is occurring in West Africa and that the nature of transmission is distinct from sylvatic transmission of YFV that has historically been present in the region. Since the date when we completed this review (October 2018), major outbreaks of DENV have occurred in urban areas of Cote d'Ivoire and Senegal. (Promed 2019, Promed 2019) Despite these risks, the region is ill prepared to respond to this
Conclusions
There has been a profound shift in the epidemiology of arboviral diseases in West Africa. Large urban outbreaks of DENV, CHIKV and ZIKV have recently occurred, and seroprevalence surveys suggest the true burden of disease is far greater than case-reports indicate. Recent outbreaks indicate both Ae. aegypti and Ae. albopictus are important vectors, and risk appears to be concentrated in urban areas. Rapid urbanization in the region threatens to exacerbate arboviral disease risk, while warming
Authors’ contributions
AGB and EJC conceived of the study and lead the writing of the paper. AGB conducted the literature search, interpreted the findings and prepared the figures. All authors contributed to the writing and editing of the manuscript.
Role of the Funding Source
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
The authors have no conflict of interest to declare.
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