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. Intercity
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 host-seeking 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.
