Risk assessment of urban yellow fever virus transmission in Kenya: is Aedes aegypti an efficient vector?

ABSTRACT

The absence of urban yellow fever epidemics in East Africa remains a mystery amidst the proliferation of Aedes aegypti in this region. To understand the transmission dynamics of the disease, we tested urban (Mombasa, Kisumu, and Nairobi) Aedes mosquito populations in Kenya for their susceptibility to an East African yellow fever virus (YFV) genotype. Overall, 22% (n = 805) of the Ae. aegypti that were orally challenged with an infectious dose of YFV had a midgut infection, with comparable rates for Mombasa and Kisumu (χ²= 0.35, df = 1, P = 0.55), but significantly lower rates for Nairobi (χ² ≥ 11.08, df = 1, P ≤ 0.0009). Variations in YFV susceptibility (midgut infection) among Ae. aegypti subspecies were not associated with discernable cytochrome c oxidase subunit 1 gene haplotypes. Remarkably, no YFV dissemination or transmission was observed among the orally challenged Ae. aegypti populations. Moreover, Ae. aegypti mosquitoes that were intrathoracically inoculated with YFV failed to transmit the virus via capillary feeding. In contrast, dissemination (oral exposure) and transmission (intrathoracic inoculation) of YFV was observed among a few peri-domestic Ae. bromeliae mosquitoes (n = 129) that were assessed from these urban areas. Our study highlights an inefficient urban Ae. aegypti population, and the potential for Ae. bromeliae in sustaining an urban YFV transmission in Kenya. An assessment of urban Ae. aegypti susceptibility to other YFV genotypes, and vector potential of urban Ae. bromeliae populations in Kenya is recommended to guide cost-effective vaccination.

KEYWORDS:

• Aedes aegypti
• Aedes bromeliae
• vector competence
• yellow fever virus
• urbanization
• East Africa
• transmission barrier

Acknowledgment

Special thanks to the members of the Githogoro, Kanyarkwar, Kajulu, and Rabai communities for their support during sample collection. We appreciate also the support of John Gachoya (Kenya Medical Research Institute), Francis Mulwa, James Wauna, and Mwaura Kageche (icipe) in field sample collection.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The financial support through a scholarship to SBA by the German Academic Exchange Service (DAAD); a Wellcome Trust International Intermediate Fellowship [grant number 222005/Z/20/Z] to DPT; National Institutes of Health (NIH) [grant number 1R01AI099736-01A1] to RS; and the project, Combatting Arthropod Pests for better Health, Food and Climate Resilience (Project number: RAF-3058 KEN-18/0005) funded by Norwegian Agency for Development Cooperation (Norad) are acknowledged. We gratefully acknowledge the financial support for this research by the following organizations and agencies: Swedish International Development Cooperation Agency (Sida), Swiss Agency for Development and Cooperation (SDC), Federal Democratic Republic of Ethiopia and the Government of the Republic of Kenya. The views expressed herein do not necessarily reflect the official opinion of the donors.