Correlations Between Household Occupancy and Malaria Vector Biting Risk in Rural Tanzanian Villages: Implications for High-resolution Spatial Targeting of Control Interventions.

Fine-scale targeting of interventions is increasingly important where epidemiological disease profiles depict high geographical stratifications. This study verified correlations between household biomass and mosquito house-entry using experimental hut studies, and then demonstrated how geographical foci of mosquito biting risk can be readily identified based on spatial distributions of household occupancies in villages. A controlled 4 × 4 Latin square experiment was conducted in rural Tanzania, in which no, one, three or six adult male volunteers slept under intact bed nets, in experimental huts. Mosquitoes entering the huts were caught using exit interception traps on eaves and windows. Separately, monthly mosquito collections were conducted in 96 randomly selected households in three villages using CDC light traps between March-2012 and November-2013. The number of people sleeping in the houses and other household and environmental characteristics were recorded. ArcGIS 10 (ESRI-USA) spatial analyst tool, Gi* Ord Statistic was used to analyse clustering of vector densities and household occupancy. The densities of all mosquito genera increased in huts with one, three or six volunteers, relative to huts with no volunteers, and direct linear correlations within tested ranges (P < 0.001). Significant geographical clustering of indoor densities of malaria vectors, Anopheles arabiensis and Anopheles funestus, but not Culex or Mansonia species occurred in locations where households with highest occupancy were also most clustered (Gi* P ≤ 0.05, and Gi* Z-score ≥1.96). This study demonstrates strong correlations between household occupancy and malaria vector densities in households, but also spatial correlations of these variables within and between villages in rural southeastern Tanzania. Fine-scale clustering of indoor densities of vectors within and between villages occurs in locations where houses with highest occupancy are also clustered. The study indicates potential for using household census data to preliminarily identify households with greatest Anopheles mosquito biting risk

A Modified Experimental Hut Design for Studying Responses of Disease-Transmitting Mosquitoes to Indoor Interventions: The Ifakara Experimental Huts

Differences between individual human houses can confound results of studies aimed at evaluating indoor vector control interventions such as insecticide treated nets (ITNs) and indoor residual insecticide spraying (IRS). Specially designed and standardised experimental huts have historically provided a solution to this challenge, with an added advantage that they can be fitted with special interception traps to sample entering or exiting mosquitoes. However, many of these experimental hut designs have a number of limitations, for example: 1) inability to sample mosquitoes on all sides of huts, 2) increased likelihood of live mosquitoes flying out of the huts, leaving mainly dead ones, 3) difficulties of cleaning the huts when a new insecticide is to be tested, and 4) the generally small size of the experimental huts, which can misrepresent actual local house sizes or airflow dynamics in the local houses. Here, we describe a modified experimental hut design - The Ifakara Experimental Huts- and explain how these huts can be used to more realistically monitor behavioural and physiological responses of wild, free-flying disease-transmitting mosquitoes, including the African malaria vectors of the species complexes Anopheles gambiae and Anopheles funestus, to indoor vector control-technologies including ITNs and IRS. Important characteristics of the Ifakara experimental huts include: 1) interception traps fitted onto eave spaces and windows, 2) use of eave baffles (panels that direct mosquito movement) to control exit of live mosquitoes through the eave spaces, 3) use of replaceable wall panels and ceilings, which allow safe insecticide disposal and reuse of the huts to test different insecticides in successive periods, 4) the kit format of the huts allowing portability and 5) an improved suite of entomological procedures to maximise data quality

Diagrammatic illustration of eave trap and window trap.

<p>Panel A and B shows the dimensions and materials used to construct these traps, while panel C and D shows the eave and window traps fitted onto an Ifakara experimental hut during collection.</p

Mean daily wind speeds and cumulative rainfall outside Ifakara experimental huts between May and October 2010.

<p>Mean daily wind speeds and cumulative rainfall outside Ifakara experimental huts between May and October 2010.</p

^a Geometric Mean (GM) number and the 95% confidence intervals (CI) of mosquitoes caught in the Ifakara experimental huts. whenever baffles were used compared to when no baffles were used.

a<p>The Relative Rate (RR) and 95% confidence intervals (CI) of the mosquito counts were calculated from Generalized Linear Models.</p><p>*N.S refers to, ‘not significantly different’.</p

Geographical positioning of the Ifakara experimental huts.

<p>A map of the study area showing two sites at the edge of the village where Ifakara experimental huts are currently located. Site A has 9 huts while site B has 4.</p

Netting baffles used in the Ifakara experimental huts.

<p>Panel A shows the design and dimensions of the different baffles used on front, back and gable sides, panel B and C are pictures showing two baffles fitted inside the huts and panel D shows the general layout of the baffles as interspaced with exit traps. Note that even though this diagram shows no <i>mikeka</i> ceiling under the roofs, the ceiling is an essential feature of all completed Ifakara experimental huts as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030967#pone-0030967-g003" target="_blank">Figure 3</a>.</p

Mean and standard deviations (SD) of daily temperatures and relative humidity (%) inside Ifakara experimental huts, as compared to local huts that have either grass thatched roofing or iron-sheet roofing. Data collected for 20 consecutive days in February 20011.

<p>Mean and standard deviations (SD) of daily temperatures and relative humidity (%) inside Ifakara experimental huts, as compared to local huts that have either grass thatched roofing or iron-sheet roofing. Data collected for 20 consecutive days in February 20011.</p

Spraying inside the experimental huts.

<p>Picture of a fully suited spray person applying PMD onto inside walls of the Ifakara experimental huts using standard Expert Hudson™ sprayers.</p