Abstract:
Background: The transmission patterns of dengue (DENV) and yellow fever (YFV)
viruses, especially in urban settings, are influenced by Aedes (Stegomyia) mosquito
abundance and behavior. Despite recurrent dengue outbreaks on the Kenyan coast, these
parameters remain poorly defined in this and other areas of contrasting dengue
endemicity in Kenya. In assessing the transmission risk of DENV/YFV in three Kenyan
cities, we determined adult abundance and resting habits of potential Aedes (Stegomyia)
vectors in Kilifi (dengue-outbreak prone), and Nairobi and Kisumu (no dengue outbreaks
reported). In addition, mosquito diversity, an important consideration for changing
mosquito-borne disease dynamics, was compared.
Methods: Between October 2014 and June 2016, host-seeking adult mosquitoes were
sampled using CO2-baited BG-Sentinel traps (12 traps daily) placed in vegetation around
homesteads, across study sites in the three major cities. Also, indoor and outdoor resting
mosquitoes were sampled using Prokopack aspirators. Three samplings, each of five
consecutive days, were conducted during the long-rains, short-rains and dry season for
each city. Inter-city and seasonal variation in mosquito abundance and diversity was
evaluated using general linear models while mosquito-resting preference (indoors vs
outdoors) was compared using Chi-square test.
Results: Aedes aegypti, which comprised 60% (n = 7772) of the total 12,937 hostseeking mosquitoes collected, had comparable numbers in Kisumu (45.2%, n = 3513)
and Kilifi (37.7%, n = 2932), both being significantly higher than Nairobi (17.1%, n =
1327). Aedes aegypti abundance was significantly lower in the short-rains and dry
season relative to the long-rains (P < 0.0001). Aedes bromeliae, which occurred in low
numbers, did not differ significantly between seasons or cities. Mosquito diversity was
highest during the long-rains and in Nairobi. Only 10% (n = 43) of the 450 houses
aspirated were found positive for resting Ae. aegypti, with overall low captures in all
areas. Aedes aegypti densities were comparable indoors/outdoors in Kilifi; but with
higher densities outdoors than indoors in Kisumu and Nairobi.
Conclusions: The presence and abundance of Ae. aegypti near human habitations and
dwellings, especially in Kilifi/Kisumu, is suggestive of increased DENV transmission
risk due to higher prospects of human vector contact. Despite low abundance of Ae.
bromeliae suggestive of low YFV transmission risk, its proximity to human habitation as
well as the observed diversity of potential YFV vectors should be of public health
concern and monitored closely for targeted control. The largely outdoor resting behavior
for Ae. aegypti provides insights for targeted adult vector control especially during
emergency outbreak situations.