Assessment of risk of dengue and yellow fever virus transmission in three major Kenyan cities based on Stegomyia indices

Abstract

Dengue (DEN) and yellow fever (YF) are re-emerging in East Africa, with contributing drivers to this trend being unplanned urbanization and increasingly adaptable anthropophilic Aedes (Stegomyia) vectors. Entomological risk assessment of these diseases remains scarce for much of East Africa and Kenya even in the dengue fever-prone urban coastal areas. Focusing on major cities of Kenya, we compared DEN and YF risk in Kilifi County (DEN-outbreak-prone), and Kisumu and Nairobi Counties (no documented DEN outbreaks). We surveyed water-holding containers for mosquito immature (larvae/pupae) indoors and outdoors from selected houses during the long rains, short rains and dry seasons (100 houses/season) in each County from October 2014-June 2016. House index (HI), Breteau index (BI) and Container index (CI) estimates based on Aedes (Stegomyia) immature infestations were compared by city and season. Aedes aegypti and Aedes bromeliae were the main Stegomyia species with significantly more positive houses outdoors (212) than indoors (88) (n = 900) (χ2 = 60.52, P < 0.0001). Overall, Ae. aegypti estimates of HI (17.3 vs 11.3) and BI (81.6 vs 87.7) were higher in Kilifi and Kisumu, respectively, than in Nairobi (HI, 0.3; BI,13). However, CI was highest in Kisumu (33.1), followed by Kilifi (15.1) then Nairobi (5.1). Aedes bromeliae indices were highest in Kilifi, followed by Kisumu, then Nairobi with HI (4.3, 0.3, 0); BI (21.3, 7, 0.7) and CI (3.3, 3.3, 0.3), at the respective sites. HI and BI for both species were highest in the long rains, compared to the short rains and dry seasons. We found strong positive correlations between the BI and CI, and BI and HI for Ae. aegypti, with the most productive container types being jerricans, drums, used/discarded containers and tyres. On the basis of established vector index thresholds, our findings suggest low-to-medium risk levels for urban YF and high DEN risk for Kilifi and Kisumu, whereas for Nairobi YF risk was low while DEN risk levels were low-to-medium. The study provides a baseline for future vector studies needed to further characterise the observed differential risk patterns by vector potential evaluation. Identified productive containers should be made the focus of community-based targeted vector control programs.

Spatial modelling of the infestation indices of Aedes aegypti: an innovative strategy for vector control actions in developing countries

“…Spatial analysis has been used to show correlations between entomological indicators with the climate, disease transmission and socioeconomic conditions [26][27][28]; however, the strategy had not yet been used to analyse contiguous, or adjoining, neighbourhoods in order to better define areas of similar risk and vulnerability. Data from the literature highlight that the results of scientific research have not impacted the vector control practices in many countries, including Brazil [3,8] However, the methodological approach and results of the present study are promising and can be easily replicated in future entomological surveillance services.…”

“…A similar study conducted in three cities in Kenya in order to assess the potential risk for the transmission of dengue and yellow fever, considered both BI and CI indices, as well as the seasonality of these regions. Based on the established vector index thresholds, the results have shown low to medium risk levels for urban yellow fever and high risk for dengue in Kilifi and Kisumu, while for Nairobi the yellow fever risk was lower and dengue risk levels were low to medium [28].…”

Seroprevalence of yellow fever, dengue, West Nile and chikungunya viruses in children in Teso South Sub-County, Western Kenya

“…Kenya is endemic for these arboviruses. It lies in the tropical belt, and competent vectors such as Aedes, Culex,and Anopheles mosquitoes thrive throughout the country (Agha et al, 2017;Baba et al, 2016;Mease et al, 2011;LaBeaud et al, 2011). Anopheline mosquitos that transmit malaria and O'nyong-nyong virus are located in the same environment as arbovirus vectors, and these infections often co-exist.…”

Research agenda for preventing mosquito-transmitted diseases through improving the built environment in sub-Saharan Africa

“…It is clear though that dengue is being transmitted in the region and Ae. aegypti, is common in African towns and cities (Agha et al 2017). As urban areas expand it is highly likely that Aedes-borne diseases will become more common in SSA, unless action is taken to reduce the aquatic habitats of the immature mosquitoes (Abílio et al 2018) and reduce the number of adult biting mosquitoes, since there is no dengue vaccine suitable for programmatic use or any specific treatment.…”