Consistently high estimates for the proportion of human exposure to malaria vector populations occurring indoors in rural Africa

Abstract

Background Insecticide-treated nets (ITNs) and indoor residual spraying (IRS) are highly effective tools for controlling malaria transmission in Africa because the most important vectors, from the Anopheles gambiae complex and the A. funestus group, usually prefer biting humans indoors at night.Methods Matched surveys of mosquito and human behaviour from six rural sites in Burkina Faso, Tanzania, Zambia, and Kenya, with ITN use ranging from 0.2% to 82.5%, were used to calculate the proportion of human exposure to An. gambiae sensu lato and An. funestus s.l. that occurs indoors (πi), as an indicator of the upper limit of personal protection that indoor vector control measures can provide. This quantity was also estimated through use of a simplified binary analysis (πiB) so that the proportions of mosquitoes caught indoors (Pi), and between the first and last hours at which most people are indoors (Pfl) could also be calculated as underlying indicators of feeding by mosquitoes indoors or at night, respectively.Results The vast majority of human exposure to Anopheles bites occurred indoors (πiB = 0.79–1.00). Neither An. gambiae s.l. nor An. funestus s.l. strongly preferred feeding indoors (Pi = 0.40–0.63 and 0.22–0.69, respectively), but they overwhelmingly preferred feeding at times when most humans were indoors (Pfl = 0.78–1.00 and 0.86–1.00, respectively).Conclusions These quantitative summaries of behavioural interactions between humans and mosquitoes constitute a remarkably consistent benchmark with which future observations of vector behaviour can be compared. Longitudinal monitoring of these quantities is vital to evaluate the effectiveness of ITNs and IRS and the need for complementary measures that target vectors outdoors.

Habitat Partitioning of Malaria Vectors in Nchelenge District, Zambia

“…As a result, we cannot overlook the possibility of indoor and/or outdoor exposure to these two vectors prior to being under a bed net that may also contribute to malarial transmission. [49][50][51][52][53][54] Each vector's role in transmission was further defined by determining the SIR for both An. funestus and An.…”

A second chance to tackle African malaria vector mosquitoes that avoid houses and don't take drugs

“…All estimates of π i for vector populations in Kenya, Tanzania, Zambia, and Burkina Faso were obtained directly from a recently published analysis. 24 For Anopheles gambiae in Equatorial Guinea, an approximate value was derived from published estimates 25 of the proportion of mosquitoes caught indoors (P i ) and the proportion caught during the first and last hours that the majority of residents were considered to be asleep indoors (P fl ):…”

Variation in the effectiveness of insecticide treated nets against malaria and outdoor biting by vectors in Kilifi, Kenya

“…Outdoor biting is currently not a major factor influencing residual malaria transmission since 95% of the population are indoors at the peak biting period for malaria vector mosquitoes. Human behaviour is the primary driver of when and where exposure occurs and is far more variable than the mosquito behaviour that matter within a single vector species [3].…”

“…Q2) The most important data clearly missing from the characterization of the study scenario are (a) spoorozoite rates (mentioned in the methods but not the results) and EIR estimates, to confirm that Anopheles funestus group mosquitoes are the most important vectors of malaria in this area, (b) quantitative estimates of where and when humans are exposed to these two major vector taxa (not species unless PCR data are added) that weight the biting estimates by surveys of human behaviour [2][3][4][5]. These are increasingly common calculations applied to data from all over the tropics [6][7][8][9][10][11][12][13], and vitally important to conduct before making any quantitative statements about proportional contributions of outdoor biting exposure.…”

“…There are uncertainties in global estimates [1][2][3] ; however in 2015, the World Health Organization estimated global deaths due to malaria to be 438,000 (range: 236,000-635,000) and the burden of febrile illness at 214 million cases (range: 149-303 million) 4 . Modelling studies suggest that approximately 1.4 billion of the world's population live at risk of stable malaria and ~1.1 billion at risk of unstable malaria 5 .…”