| dc.description.abstract | Vector control remains central to the fight against malaria with insecticide-treated bednets
(ITNs) and indoor residual spraying (IRS) being a central component of the World Health
Organization (WHO) global strategy. The recent upsurge of insecticide resistance in various
parts of the world has led to fears that control efforts may be compromised. The frequency of
insecticide resistance in western Kenya has not been mapped in a large scale and the effect it
is likely to have on ability of vectors to survive exposure to ITNs is unknown. With malaria
control programs embarking universal coverage every 3 years, it is important to determine
whether ITNs are able to protect person using them once they have holes. The effect massive
vector control have on vector age structure needs to be evaluated if successful transmission
reduction is to be achieved, hence the need to evaluate age grading tools. Anopheles
mosquitoes were sampled from clusters (sub-locations) then susceptibility to permethrin or
deltamethrin determined. A subset of An. arabiensis samples from Bondo and Nyando
underwent gene-expression profiling using whole genome microarray. Cross-sectional
surveys of ITNs were conducted in houses in Gem and Bungoma in May 2013 where ITN
condition, number and species of mosquitoes resting inside, and insecticidal activity against
susceptible and resistant mosquitoes were quantified. Children under 5 years of age were
recruited into a case control study at the local health facility and followed back to their homes
and their ITNs taken for evaluation. A questionnaire detailing household characteristics and
net use was administered. Ages of wild mosquitoes collected at different times were
estimated using the Near Infra-Red Spectrometry (NIRS). Mortality to deltamethrin ranged
from 45-100%, and to permethrin from 30-100% and varied between An. arabiensis and An.
gambiae s.s. although this was not statistically significant. There was no correlation between
mortality to permethrin and deltamethrin in An. arabiensis (Z=2.9505, P=0.2483). Microarray
results revealed that CYP6M2 (the mutant allele) was consistently expressed at higher levels
in Bondo. A total of 552 An. gambiae s.l., 5 An. funestus s.l. and 137 Culex spp. were found
resting inside nets in Bungoma. The ITNs from Bungoma retained strong activity against a
susceptible laboratory (>90% mortality) but not against first filial generation (Fl) offspring
of field-collected An. gambiae s.s. «60% mortality). There was no difference in the number
and size of holes in ITN s used by cases versus controls thus there was no association of the
presence of malaria infection with sleeping under a holed ITN. Roof type and number of
people that slept inside the households was associated with malaria infection (P=O.013;
P=O.006). Higher resistance was observed in older mosquitoes compared to the younger ones.
Insecticide resistance is heterogeneous within small geosraphical areas. The CYP6M2,
identified in this study is associated with resistance to -multiple insecticide classes. In areas
with pyrethroid resistant vectors, mosquitoes are able to enter ITNs and survive. In an area of
high insecticide resistance, ITN s may not offer protection against malaria once they have
holes. Age was observed to select for resistance in mosquitoes such that there was an increase
in insecticide resistance with increasing mosquito age. Results from this study will inform
resistance monitoring programs on the need for multiple sampling within a district. The
finding of mosquitoes inside ITN s indicates the need for integrated vector management for
example IRS with a non-pyrethroid. There is a need for prompt replacement of ITNs
especially those used by children once they start acquiring holes. The observation of older
mosquitoes having higher insecticide resistance means these mosquitoes may survive
insecticide exposure in ITNs and thus the need for insecticide resistance mitigation strategies. | en_US |
