BACKGROUND : In April, 2004, chikungunya virus (CHIKV) re-emerged in Kenya and eventually spread to
the islands in the Indian Ocean basin, South-East Asia, and the Americas. The virus, which
is often associated with high levels of viremia in humans, is mostly transmitted by the urban
vector, Aedes aegypti. The expansion of CHIKV presents a public health challenge both
locally and internationally. In this study, we investigated the ability of Ae. aegypti mosquitoes
from three distinct cities in Kenya; Mombasa (outbreak prone), Kisumu, and Nairobi (no documented
outbreak) to transmit CHIKV.
METHODOLOGY/PRINCIPAL FINDINGS : Aedes aegypti mosquito populations were exposed to different doses of CHIKV (105.6±7.5
plaque-forming units[PFU]/ml) in an infectious blood meal. Transmission was ascertained
by collecting and testing saliva samples from individual mosquitoes at 5, 7, 9, and 14
days post exposure. Infection and dissemination were estimated by testing body and legs,
respectively, for individual mosquitoes at selected days post exposure. Tissue culture
assays were used to determine the presence of infectious viral particles in the body, leg,
and saliva samples. The number of days post exposure had no effect on infection, dissemination,
or transmission rates, but these rates increased with an increase in exposure dose
in all three populations. Although the rates were highest in Ae. aegypti from Mombasa at
titers 106.9 PFU/ml, the differences observed were not statistically significant (χ2 1.04,
DF = 1, P 0.31). Overall, about 71% of the infected mosquitoes developed a disseminated infection, of which 21% successfully transmitted the virus into a capillary tube, giving an
estimated transmission rate of about 10% for mosquitoes that ingested 106.9 PFU/ml
of CHIKV. All three populations of Ae. aegypti were infectious as early as 5±7 days post exposure. On average, viral dissemination only occurred when body titers were 104 PFU/
ml in all populations.
CONCLUSIONS/SIGNIFICANCE : Populations of Ae. aegypti from Mombasa, Nairobi, and Kisumu were all competent laboratory
vectors of CHIKV. Viremia of the infectious blood meal was an important factor in Ae.
aegypti susceptibility and transmission of CHIKV. In addition to viremia levels, temperature
and feeding behavior of Ae. aegypti may also contribute to the observed disease patterns.