In East Africa, Yellow fever (YF) has been characterized by focal periodic outbreaks spaced by lengthy inter-epidemic periods. In Kenya the first ever outbreak occurred in 1992/93 where human cases were limited to sylvatic intermediate transmissions with no urban transmission detected. This outbreak was controlled by a mass vaccination in 1993 and while this was repeated ten years later in 2003; routine vaccination has now been instituted only in the five districts that were affected. Recent reports of outbreaks of YF in the region, including Kenya (1992-95), Sudan (2003; 2005; 2010) and western Uganda (2011) constitute a worrying trend and a great public health concern to regional public health authorities. Because of the long inter- epidemic periods associated with YF, only a few studies have investigated its ecology in East Africa despite the region's recognized growing threat of arbovirus transmissions and outbreaks. The YF endemicity, maintenance mechanism, vector ecology and competence, and potential epizootics and outbreaks remain unresolved in Kenya. It is the aim of this study to identify enzootic cycles and assess transmission risk in part of Kenya bordering outbreak areas and at risk of receiving viremic travelers in order to establish and maintain surveillance activities in such areas and to recommend a vaccination strategy that is affordable and based on scientific evidence. With dengue and YF sharing a niche in the ecosystem there is much to be gained in working in both agents at the same time. To achieve this, specific locations bordering areas that have reported outbreaks in the region will be selected and spatial and relative abundance of known YF vectors determined and their vector potential evaluated in the laboratory, determine the presence and movement of non-human primate reservoirs to identify existence of enzootic cycles of transmission and determine possibility of human involvement will be determined by serological survey of the human population in the selected border areas. In addition, the risk of urban transmission in major citie in Kenya will be determined by experimental evaluation of the competence of the circulating vector species in sustaining transmission should viremic cases arrive in these cities. Results from this study will provide further evidence of the potential for future sylvatic intermediate and urban epidemic outbreaks of YF and related arbovirus Dengue, that share the same vectors, hosts, and ecological niche. Improvement of surveillance tools for monitoring key vectors will be developed that will be valuable in vector population monitoring for further risk assessments. Control efforts could then be focused on areas with proven risk saving the country much needed scarce resources. Improvement of surveillance tools valuable in monitoring vector population will be developed for further risk assessment.

Public Health Relevance

There is renewed vigor of transmission of yellow fever (YF) in East Africa with reports of outbreaks in Kenya, south Sudan and Uganda which have become a major concern to public health authorities in Kenya. YF outbreaks are normally controlled using relatively safe and effective YF live attenuated vaccine but instituting routine vaccination for YF control in Kenya is considered a financial burden due to other competing public health priorities, hence the authorities consider it prudent to institute such programs only in areas considered to be at risk based on scientific evidence. Each time outbreaks occur in the neighboring country, authorities debated at length on whether mass vaccination should be conducted in the border districts but these decisions are shelved due to lack of evidence that transmission could be sustained in the region which underscores the need for this research that will evaluate risk of transmission in sites bordering outbreak areas and urban cities to bring about better understanding of YF ecology in Kenya.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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Special Emphasis Panel (ZRG1)
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Costero-Saint Denis, Adriana
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International Centre of Insect Physiolog
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Agha, Sheila B; Tchouassi, David P; Bastos, Armanda D S et al. (2017) Assessment of risk of dengue and yellow fever virus transmission in three major Kenyan cities based on Stegomyia indices. PLoS Negl Trop Dis 11:e0005858
Agha, Sheila B; Tchouassi, David P; Bastos, Armanda D S et al. (2017) Dengue and yellow fever virus vectors: seasonal abundance, diversity and resting preferences in three Kenyan cities. Parasit Vectors 10:628
Agha, Sheila B; Chepkorir, Edith; Mulwa, Francis et al. (2017) Vector competence of populations of Aedes aegypti from three distinct cities in Kenya for chikungunya virus. PLoS Negl Trop Dis 11:e0005860