Abstract:
Background: Insecticide treated nets (ITNs) protect humans against bites from the
Anopheles mosquito vectors that transmit malaria, thereby reducing malaria morbidity
and mortality. It has been noted that ITN use leads to a switch from indoor to outdoor
feeding among these vectors. It might be expected that outdoor feeding would undermine
the effectiveness of ITNs that target indoors vectors, but data are limited. Methods: We
linked homestead level geospatial data to clinical surveillance data at a primary
healthcare facility in Kilifi County in order to map geographical heterogeneity in ITN
effectiveness and observed vector feeding behaviour using landing catches and CDC
light traps in six selected areas of varying ITN effectiveness. We quantified the
interaction between mosquitoes and humans to evaluate whether outdoor vector biting is
a potential explanation for the variation in ITN effectiveness. Results: We observed 37%
and 46% visits associated with positive malaria slides among ITN users and non-ITNusers, respectively; ITN use was associated with 32% protection from malaria (crude OR
= 0.68, 95% CI: 0.64, 0.73). We obtained modification of ITN effectiveness by
geographical area (p=0.016), and identified 6 hotspots using the spatial scan statistic.
Majority of mosquitoes were caught outdoor (60%) and were of the An. funestus group
(75%). The overall propensity to feed at times when most people were asleep was high;
the vast majority of the Anopheles mosquitoes were caught at times when most people
are indoors asleep. Estimates for the proportion of human-mosquito contact between the
first and last hour when most humans were asleep was consistently high across all
locations, ranging from 0.83 to 1.00. Conclusion: Our data do not provide evidence of an
epidemiological association between microgeographical variations in ITN effectiveness
and variations in the microgeographical distribution of outdoor biting.