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The relationship between house height and mosquito house entry: an experimental study in rural Gambia

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Most malaria infections in sub-Saharan Africa are acquired indoors, thus finding effective ways of preventing mosquito house entry should reduce transmission. Since most malaria mosquitoes fly less than 1 m from the ground, we tested whether raising buildings off the ground would prevent the entry of Anopheles gambiae , the principal African malaria vector, in rural Gambia. Nightly collections of mosquitoes were made using light traps from four inhabited experimental huts, each of which could be moved up or down. Mosquito house entry declined with increasing height, with a hut at 3 m reducing An. gambiae house entry by 84% when compared with huts on the ground. A propensity for malaria vectors to fly close to the ground and reduced levels of carbon dioxide, a major mosquito attractant, in elevated huts, may explain our findings. Raised buildings may help reduce malaria transmission in Africa.
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royalsocietypublishing.org/journal/rsif
Research
Cite this article: Carrasco-Tenezaca M et al.
2021 The relationship between house height
and mosquito house entry: an experimental
study in rural Gambia. J. R. Soc. Interface 18:
20210256.
https://doi.org/10.1098/rsif.2021.0256
Received: 25 March 2021
Accepted: 5 May 2021
Subject Category:
Life SciencesEngineering interface
Subject Areas:
environmental science, biomedical engineering
Keywords:
Anopheles gambiae, housing, malaria,
mosquitoes, sub-Saharan Africa
Author for correspondence:
Steve W. Lindsay
e-mail: s.w.lindsay@durham.ac.uk
Electronic supplementary material is available
online at https://doi.org/10.6084/m9.figshare.
c.5428748.
The relationship between house height
and mosquito house entry: an
experimental study in rural Gambia
Majo Carrasco-Tenezaca1, Musa Jawara2, Mahamed Y. Abdi3, John Bradley4,
Otis Sloan Brittain3, Sainey Ceesay2, Umberto DAlessandro2,4, David Jeffries2,
Margaret Pinder1,2, Hannah Wood3, Jakob B. Knudsen3
and Steve W. Lindsay1,4
1
Department of Biosciences, Durham University, Durham, UK
2
Medical Research Council Unit The Gambia at the London School of Hygiene & Tropical Medicine, Banjul,
The Gambia
3
Royal Danish Academy - Architecture, Design and Conservation, Copenhagen, Denmark
4
London School of Hygiene & Tropical Medicine, London, UK
JBK, 0000-0002-5348-8439; SWL, 0000-0002-3461-9050
Most malaria infections in sub-Saharan Africa are acquired indoors, thus
finding effective ways of preventing mosquito house entry should reduce
transmission. Since most malaria mosquitoes fly less than 1 m from the
ground, we tested whether raising buildings off the ground would prevent
the entry of Anopheles gambiae, the principal African malaria vector, in rural
Gambia. Nightly collections of mosquitoes were made using light traps from
four inhabited experimental huts, each of which could be moved up or
down. Mosquito house entry declined with increasing height, with a hut
at 3 m reducing An. gambiae house entry by 84% when compared with
huts on the ground. A propensity for malaria vectors to fly close to the
ground and reduced levels of carbon dioxide, a major mosquito attractant,
in elevated huts, may explain our findings. Raised buildings may help
reduce malaria transmission in Africa.
1. Introduction
The United Nations has projected that the population of sub-Saharan Africa
will more than double between 2019 and 2050, and the region will become
the worlds most populated by 2062 [1]. Coincident with the increasing
growth rate, there has been an unprecedented improvement in the housing
stock in sub-Saharan Africa, with the proportion of improved houses increasing
from 11% in 2000 to 23% in 2015 [2]. With an additional 1.05 billion people in
2050 [1], there has never been a better time to improve the quality of housing in
sub-Saharan Africa and make houses healthier for people.
In 2019, there were 384 000 deaths from malaria in sub-Saharan Africa,
representing 94% of the global total [3]. It is particularly concerning that the
decline in malaria cases has stalled recently and has reversed in some countries
despite the massive deployment of insecticide-treated nets (ITNs), indoor
residual spraying and prompt and effective treatment with antimalarials. It is
generally recognized that supplementary measures are needed to further
decrease malaria in the region. Since 79% of bites by malaria mosquitoes
occur indoors at night [4], reducing mosquito house entry could contribute to
greater malaria control.
There is growing evidence that house design can decrease the force of
malaria infection. A systematic review and meta-analysis showed that residents
of modern homes had 47% lower odds of malaria infection when compared
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with traditional homes and a 5465% lower incidence of clini-
cal malaria [5]. Similarly, a meta-analysis of 29 malaria
surveys carried out in 21 sub-Saharan African countries
between 2008 and 2015 found that modern housing was
associated with a 914% reduction in the odds of malaria
infection when compared with traditional housing, a level
of protection comparable to ITNs in the same study [6].
Modern houses are more likely to have closed eaves (the
gap between the top of the wall and the overhanging roof),
and screened windows and doors, all modifications known
to reduce the entry of malaria mosquitoes into houses [7].
Completely closing a building in the hot humid tropics, how-
ever, will reduce ventilation and increase the temperature of
the house before midnight, particularly if the roof is metal
[8,9]. Making a house hotter at night is bad for malaria con-
trol since being too hot is the main reason people give for
not sleeping under a mosquito net at night [10]. Thus, the
ideal house keeps out malaria vectors, while keeping the
occupants cool at night.
We hypothesized that raising a house above the ground
would reduce mosquito entry and keep the house cooler.
Support for this hypothesis comes from several sources.
Firstly, field studies in The Gambia showed that 80% of mos-
quitoes fly less than 1 m from the ground [11,12]. Secondly,
raised platforms can reduce biting by malaria mosquitoes
[13,14]. Thirdly, a pilot study in Tanzania showed that
screened double-storey buildings had 96% fewer malaria
mosquitoes when compared with outdoors levels, while
screened single-storey buildings raised less than 1 m from
the ground had 77% fewer [15].
We used four experimental huts (figure 1), each of which
could be adjusted to different heights, to measure house
entry of Anopheles gambiae s.l., the principal malaria mos-
quito in sub-Saharan Africa. There were two primary
objectives to (i) determine whether mosquito-hut entry
declined with increasing height and (ii) find out whether
an elevated hut would be cooler than one closer to the
ground. This experiment is, to our knowledge, the first
study to determine the impact of the height of a
house on the entry of malaria mosquitoes and on indoor
temperatures at night.
2. Material and methods
This was a proof-of-principle study in which four identical huts
were built so that each could be moved up or down. All huts had
open eaves, no windows and doors with 20 mm gaps at the top
and bottom of each to replicate badly fitting doors, common in
the region. For four nights each week, one hut was at 0 m, one
at 1 m, one at 2 m and one at 3 m (electronic supplementary
material, figure S1). The height in which each hut was positioned
was changed weekly, following a replicated Latin rectangle
design (electronic supplementary material, table S1). Four huts
were used instead of one since there is considerable variation
in mosquito numbers from night-to-night, and to complete the
study within one rainy season.
2.1. Study area
The study took place in Wellingara village (N 13°33.3650, W 14°
55.4610), Central River Region, The Gambia. This is an area of flat
Sudanese savanna located close to a large area of irrigated rice
fields. The study took place in 2019 during the rainy season,
from 5 August to 17 October, when An. gambiae s.l. are
common [9].
2.2. Experimental huts
Four experimental huts were constructed, 10 m apart, along a
straight line on the western edge of the village, closest to a
1500 ha irrigated rice field. Their design was similar to single-
room, one-storey dwellings in the area with doors on opposite
sides, but the experimental huts were smaller in size and
constructed from lightweight material so they could be easily
and safely moved up and down (doi:10.6084/m9.figshare.
14483475). The walls of the huts were constructed from 18 mm
waterproof plywood on a timber frame and had 0.30 mm tin
corrugated roofs (electronic supplementary material, figure S1).
Each hut was 3.10 m by 3.10 m in area and the walls were
2.20 m high. Each hut had two corrugated aluminiumsteel
doors, 1.80 m high and 0.70 m wide, with 20 mm narrow slits
Figure 1. Experimental huts. From left to right, at 1 m, 3 m, 0 m and 2 m. Automatic weather station is shown to the right of the huts and the village to the left
and rear.
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on the top and bottom of each door, to simulate badly fitting
doors common in villages. One door faced west, towards the
rice fields, and the other east, towards the village. The saddle
roof was constructed from aluminiumsteel sheets mounted on
a timber frame, with open eaves on the sides of the house with
the doors. Each house had two wooden beds placed parallel to
one another on opposite walls, on the sides without doors
(north and south). The huts were fixed to a steel frame, which
allowed the hut to be raised and lowered using pulley lifts
(Yale lever hoist, VSIII 2000 kg Manual Chainblock, Yale, UK).
The hoists were anchored in a steel frame that was fixed at
ground level by steel foot brackets cast into reinforced concrete
foundations (1.65 m × 1.67 m × 0.50 m high).
2.3. Study participants
A village meeting was organized to explain the study to the villagers
in Mandinka, their main language. Eight healthy men over 15 years
old living in Wellingara village provided signed-witnessed consent
and were recruited to the study. Before the first experimental session,
volunteers slept in the huts for one night to prime the huts with the
smell of humans. During the experiments, volunteers slept in the
huts for four nights every week for 10 weeks. Each night, each
pair of volunteers slept with their heads pointing westward in the
direction of the ricefields, under separate ITNs (Olyset Net, 1.30 m
wide, 1.50 m high and 1.80 m in length, Sumitomo Chemicals,
Japan) from 21.00 to 07.00 the following morning (electronic sup-
plementary material, figure S2). Each pair of volunteers was
rotated between huts each night so that at the end of each weekly
block, each pair had slept in each hut. Two field assistants were sta-
tioned on-site each night in a separate room to assist the volunteers
with ladders if they needed to leave the hut during the night.
2.4. Outcomes
The main entomological outcomes were the mean number of An.
gambiae s.l. and the mean temperature of huts. All results were
collected at different hut heights before and after midnight. Mos-
quitoes were collected using one light trap (CDCCenters for
Disease Control and Prevention, miniature light trap model
512, US John W. Hock Ltd, Gainsville, USA) in each hut, with
the light bulb 1 m above the floor and between the feet end of
the two beds. Light traps were operated from 21.00 to 07.00 the
following morning. Every night, the field assistants conducted
two supervisory visits, at 00.00 and 06.00, to make sure the
men were in the huts, assess bed net use and make sure the
light traps were working properly. After collection, mosquitoes
were placed in a 20°C freezer until dead. Mosquitoes were
identified using standard morphological identification keys
[16,17], and members of the An. gambiae complex identified to
species by PCR [1820].
Indoor temperature and relative humidity were measured in
each hut every 30 min using a data logger (TGU 4500, Tinytag,
UK) positioned in the centre of the room, 1 m above the floor.
Carbon dioxide was recorded every 30 s with data loggers (1%
CO2 + Rh/T Data Logger GasLab, Florida, USA) located
between the beds near the head of the bed, 1 m above the floor
(electronic supplementary material, figure S2). Outdoor tempera-
ture, relative humidity, wind speed, wind direction and
precipitation were recorded every 30 min by an automatic
weather station (MiniMet, Skye Instruments, Llandrindod
Wells, UK), located 10 m from the centre of the hutsline.
At the end of the collection period, all sleepers participated in
one focus group discussion. The discussion was led by M.J. and
recorded by M.C.T. The men discussed their individual experi-
ences of sleeping in the huts and their willingness to live in a
house raised from the ground. After that, they elaborated on
common ideas and perceptions of the experimental huts and
pointed out at what height they felt more comfortable and
explained why. The discussions were conducted in Mandinka,
the local language, and audio recorded. The recording was
later transcribed into English. Common experiences of the
sleepers were reported as quotes for illustration.
2.5. Statistical analysis
IBM SPSS Statistics 20 and Stata version 16 were used for the
analysis. The sample size was estimated using a computer simu-
lation based on data from a study conducted in the same area in
2017 [9], in which the mean number of An. gambiae s.l. collected
indoors over 25 nights was 6.4 mosquitoes (SD 7.1). The present
study was thus powered to detect an intervention that reduces
the number of mosquitoes that were found indoors by at least
75% at the 5% level of significance and 90% power. In the simu-
lation, the 4 × 4 Latin square was repeated three to 10 times (i.e.
12 to 40 nights). The simulation showed that eight, 4 × 4 Latin
squares would provide sufficient power to detect a 75%
reduction in mosquito house entry (i.e. 32 nights of collections).
In this study, we extended the period for two more weeks,
because of low mosquito catches in the first two weeks.
To assess the effect of hut height on mosquito house entry
and indoor climate, we used a generalized estimating equation
using a negative binomial model with a log link function for
mosquito count data and a normal distribution with identity
link for temperature and carbon dioxide, since they were con-
tinuous variables and normally distributed. In addition to hut
height, we included hut position, sleeper pair and number of
nights in the model as fixed effects. To examine the relationship
between carbon dioxide concentration and covariates, we used
linear regression. Polar plots were used to depict the direction
and strength of the wind during the day and night.
3. Results
3.1. Mosquito collections
A total of 17 432 female mosquitoes were collected in the
experimental huts over 40 nights (electronic supplementary
material, table S2). Of these, 2080 (11.9%) were An. gambiae
s.l., 13 321 (76.4%) Mansonia spp., 1823 (10.5%) Culex spp.
and the rest were other anophelines and Aedes aegypti (elec-
tronic supplementary material, table S2). Members of the An.
gambiae complex of mosquitoes were identified by PCR analy-
sis as An. coluzzii (68.0%, 157/231), An. arabiensis (29.4%, 68/
231), and An. gambiae s.s. (2.6%, 6/231). Overall, unadjusted
numbers of mosquitoes of all species entering huts decreased
with increasing height (figure 2): by 33% at 1 m, 57% at 2 m
and 69% at 3 m, compared with the hut on the ground.
The number of female An. gambiae entering the huts
declined with increasing hut height, declining by 40% (95%
CI 24 to 53%) with the floor height at 1 m, 68% (95% CI 60
to 74%) at 2 m and 84% (95% CI 77 to 88%) at 3 m when com-
pared with the hut at 0 m (table 1). Similar reductions were
seen with Mansonia spp., but not with Culex spp., for which
the number of hut-entering mosquitoes was similar in all
huts when compared with the hut at 0 m. Details of the
regression analyses for An. gambiae s.l., indoor temperature
and carbon dioxide concentrations are provided in the
electronic supplementary material, S3.
3.2. Climate measurements
Indoor temperatures declined steadily from 29.0°C at 21.00 to
25.5°C at 07.00, but were always about 2°C warmer than the
outdoor temperature (figure 3).
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There was little indication that temperature declined with
increasing hut height (table 2), although the indoor tempera-
ture in the hut at 3 m was of borderline significance when
compared with the hut at 0 m, in the period after midnight.
No differences in relative humidity were found between
huts (electronic supplementary material, table S4). The
wind was predominantly from the west, and the mean
wind speed was 0.60 m s
1
(95% CIs 54.0 to 67.1) from
20.00 to 23.59 and 0.71 m s
1
(95% CIs 65.0 to 75.1) from
00.00 to 06.59 (electronic supplementary material, figure S3).
3.3. Carbon dioxide measurements
Although there was considerable nightly variation in the pat-
tern of carbon dioxide in each hut, the mean values provide
consistent patterns and show clear trends between huts
(figure 4). Carbon dioxide levels indoors rose sharply after
the men entered the huts to a maximum value 30 min later.
Thereafter, levels gradually declined during the night before
rising sharply around 05.00 to a second, smaller peak at
07.00, when the men left the huts.
There was a decreasing trend in the concentration of
carbon dioxide with increasing height, both in the periods
before and after midnight (table 2). Before midnight, there
was a 24.8% decline in carbon dioxide at 1 m, 39.8% at 2 m
(rate ratio, RR = 0.60, 95% CI = 0.51 to 0.72) and 44.8% at
3 m (RR = 0.55, 95% CI = 0.46 to 0.66) when compared with
the hut at 0 m (electronic supplementary material, table S4).
While after midnight, there was a 19.7% decline in carbon
dioxide at 1 m, 25.4% at 2 m (RR = 0.75, 95% CI = 0.65 to
0.86) and 38.4% at 3 m (RR = 0.62, 95% CI = 0.54 to 0.71)
when compared with the hut at 0 m.
3.4. Focus group discussion
A complete transcript of the focus group discussion with
the men who slept in the experimental huts is provided in
FigShare (doi:10.6084/m9.figshare.14483475).
Briefly, sleepers preferred to sleep in the huts at 0 m and
3 m. When asked the reasons for their choice, they mentioned
that the higher hut was cooler and had fewer mosquitoes dis-
turbing them during the night. The sleepers also said they
did notice a change in temperature throughout the night,
with temperatures becoming cold after midnight.
I am very happy whenever I am going to level three [3 m hut]. Iam
very, very happy.
Initially is very, very hot inside, before twelve. By twelve it starts to get
cold.
Sleepers said that they would live in a house raised from the
ground if it was made of solid (mud block) materials like
traditional houses and had a fixed stair to access the house
if it was to be permanent.
[Huts would be more useful] if the houses are solid, like the way the
ones [built] on the ground. We would prefer that.
As I said, I prefer the stair that is fixed to the house. I prefer if all the
stairs would be in that form. It would be much more appreciated.
height (m)
0
0
10
20
30
40
123
mean no. mosquitoes (trap/night)
female An. gambiae s.l.
female Culex spp. all female and male mosquitoes
female Mansonia spp.
height (m)
0
0
50
100
150
200
123
mean no. mosquitoes (trap/night)
hei
g
ht (m)
0
0
5
10
15
20
123
mean no. mosquitoes (trap/night)
height (m)
0123
mean no. mosquitoes (trap/night)
0
50
100
150
250
200
(a)(b)
(c)(d)
Figure 2. Mean mosquito house entry in huts at different heights. Error bars are 95% confidence intervals. (a)An. gambiae s.l., (b)Mansonia spp., (c)Culex
quinquefasciatus and (d) all mosquitoes.
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3.5. Serious adverse events
There was one serious adverse event during the study, which
occurred when one study participant sleep-walked out of the
2 m high hut and received minor injuries. This accident was
most likely to have been caused by the medication the parti-
cipant was taking, and he was replaced with a healthy
volunteer. Hut doors were secured during the night to
prevent any further accidents.
mean temperature (ºC)
24.5
25.0
25.5
26.0
26.5
27.0
27.5
28.0
28.5
29.0
time
24.0
21.00
21.30
22.00
22.30
23.00
23.30
00.00
00.30
01.00
01.30
02.00
02.30
03.00
03.30
04.00
04.30
05.00
05.30
06.00
06.30
07.00
Figure 3. Mean indoor and outdoor temperatures from 21.00 to 07.00. Where purple line = hut at 0 m, red line = hut at 1 m, green line = hut at 2 m, turquoise
line = hut at 3 m and dashed black line = outside temperature.
Table 1. Female mosquitoes collected at different heights and adjusted analysis for covariates. General linearized modelling results, adjusted for house position,
sleeper pair and night. CI = condence intervals.
height of
hut (m) total
mean ratio
(95% CI)
effect estimate
(95% CI) p-value
Anopheles gambiae
0 1015 reference ——
1 601 0.60 (0.47 to 0.76) 40% (24 to 53) <0.001
2 333 0.32 (0.26 to 0.40) 68% (60 to 74) <0.001
3 131 0.16 (0.12 to 0.23) 84% (77 to 89) <0.001
Mansonia spp.
0 5475 reference ——
1 3880 0.62 (0.50 to 0.77) 38% (23 to 50) <0.001
2 2486 0.35 (0.29 to 0.43) 65% (57 to 71) <0.001
3 1471 0.24 (0.18 to 0.30) 76% (70 to 82) <0.001
Culex spp.
0 420 reference ——
1 522 1.11 (0.79 to 1.58) +11% (59 to 21) 0.546
2 444 1.13 (0.95 to 1.35) +13% (35 to 5) 0.168
3 437 1.00 (0.86 to 1.15) 0% (15 to 14) 0.974
all mosquitoes
0 6998 reference ——
1 5069 0.67 (0.43 to 1.06) 33% (6 to 57) 0.087
2 3326 0.43 (0.34 to 0.55) 57% (45 to 66) <0.001
3 2140 0.31 (0.26 to 0.37) 69% (63 to 74) <0.001
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4. Discussion
Our findings establish how house height affects host location
by the worlds most efficient vector of malaria and provides a
plausible mechanism for this behaviour. The number of
female An. gambiae mosquitoes collected in the huts declined
with increasing height, decreasing progressively with increas-
ing height of the huts floor above the ground. Huts with the
floor 3 m above the ground had 84% fewer mosquitoes than
those on the ground [21]. If this reduction correlates to a simi-
lar reduction in malaria transmission, it would be comparable
to that of an ITN that can reduce malaria transmission by
4090% [21]. Similar reductions in Mansonia spp., vectors of
lymphatic filariasis and arboviruses, were observed as the
height of a house was raised, with the huts at 3 m having
77% fewer mosquitoes than the hut at 0 m. In marked con-
trast, the number of Culex spp. entering experimental huts
was similar at any height.
Our findings are supported by a series of studies that
measured the height at which mosquitoes fly which were
conducted in the same study area [11] and two other sites
in The Gambia between 1968 and 1977 [12,22]. Mosquitoes
were collected at different heights using suction traps
mounted on scaffolding, with 80% of the total catch of mos-
quitoes collected less than 1 m from the ground [22]. The
researchers differentiated between low-flying mosquitoes,
like Anopheles spp.and Mansonia spp.,Aedes punctothoracis
and higher-flying mosquitoes that could be collected at 3.5
to 4 m, which included Culex neavei and Cx. weschei. In The
Gambia, many Culex species, like Cx. neavei, feed primarily
Table 2. Environmental measurements outdoors and indoors. General linearized modelling results, adjusted for house position, sleeper pair and night.
CI = condence intervals.
hut height
(m)
21.00 to 23.30 00.00 to 07.00
mean
(95% CI)
difference from
reference hut
(95% CI) p-value
mean
(95% CI)
difference from
reference hut
(95% CI) p-value
outdoor temperature (°C)
26.5 (26.3 to 26.7) ——24.8 (24.7 to 24.9) ——
indoor temperature (°C)
0 28.4 (28.1 to 28.6) reference 26.4 (26.2 to 26.5) reference
1 28.4 (28.1 to 28.7) 1.05 (0.99 to 1.12) 0.103 26.6 (26.2 to 26. 8) 1.11 (0.90 to 1.37) 0.328
2 28.3 (28.1 to 28.6) 0.98 (0.93 to 1.03) 0.423 26.3 (26.2 to 26.4) 0.95 (0.83 to 1.10) 0.507
3 28.3 (28.0 to 28.5) 0.97 (0.91 to 1.02) 0.211 26.2 (26.1 to 26.4) 0.88 (0.76 to 1.01) 0.067
indoor carbon dioxide levels ( ppm)
0 760 (710 to 800) reference 710 (680 to 750) reference
1 710 (680 to 740) 75.2 (60.6 to 93.4) 0.010 680 (650 to 710) 80.3 (70.1 to 92.0) 0.002
2 710 (680 to 740) 60.2 (50.7 to 71.5) <0.001 670 (640 to 700) 74.6 (64.9 to 85.9) <0.001
3 690 (660 to 730) 55.2 (45.9 to 66.4) <0.001 660 (630 to 700) 61.6 (53.9 to 70.5) <0.001
carbon dioxide mean (ppm)
time
850
750
800
700
650
600
550
21.30
21.00
22.30
22.00
23.30
23.00
00.30
00.00
01.30
01.00
02.30
02.00
03.30
03.00
04.30
04.00
05.30
05.00
06.30
06.00
07.00
Figure 4. Mean carbon dioxide concentration from 21.00 to 07.00. Where purple line = hut at 0 m, red line = hut at 1 m, green line = hut at 2 m and turquoise
line = hut at 3 m.
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on birds [23] and, presumably, the ability of these mosquitoes
to fly at higher altitudes, when compared with most mosqui-
toes, allows them to feed more readily on birds roosting at
night in trees. Although An. gambiae s.l. and Mansonia spp.
normally fly close to the ground, when they reach a house
they fly upwards and typically enter through open eaves,
while mosquitoes like Aedes spp., An. pharoensis,Cx. poicilipes
and Cx. thalassius do not [12,24]. In a study conducted in the
present study area, it was found that An. gambiae and Manso-
nia spp. moved up over a 6 m high circular netting fence to
feed on people inside the fence [11]. This study raises the
question of whether house height is protective if houses
raised on stilts are closed underneath with netting or other
materials. Reassuringly, a study in Tanzania found that mos-
quito entry decreased in two-storey houses when compared
with one-storey houses [15]. In Kenya, thatched-roofed
houses with mud walls and raised on stilts had 81% fewer
malaria vectors than houses built from the same material
but on the ground [25]. Similarly, in São Tomé, houses on
stilts had half the number of An. gambiae s.l. than those
built on the ground [26]. These observations could be due
to the position of the house, rather than the house height.
In our study, however, because we adjusted for house pos-
ition in the design, we can be confident that house entry of
An. gambiae s.l. declines with house height.
In the current experiments, temperature declined pro-
gressively through the night. There was no consistent
evidence that indoor temperature declined with the height
of the hut. This was unexpected since it was hypothesized
that air flowing underneath the raised huts would decrease
their indoor temperature when compared with the hut
located on the ground, and at higher altitudes, where wind
speed increases [27]. There are two likely explanations for
our findings. (i) The huts in our study were built with
materials of low thermal mass that lose heat rapidly. If the
buildings were made of mud, concrete or brick, high thermal
mass materials, we may have found a difference in cooling
with increased height. (ii) The hut at 0 m had a gap of
500 mm between the ground and wooden floor. The air
beneath the hut acts as an insulating layer and is likely to
make the hut cooler than if it had been built directly on the
ground. By contrast, in Tanzania, two-storey houses were
2.3°C (95% CI 2.2 to 2.4) cooler than single-storey houses [15].
Carbon dioxide levels rose rapidly after the study partici-
pants entered the huts in the evening and then slowly
declined through the night until rising around 05.00 to a
second peak at 07.00, when the men left the huts. Both
peaks were partly associated with increased physical activity,
the first, chatting and preparing to go to bed, and the second,
praying and getting ready to leave the huts in the morning.
The second peak is also likely to be under circadian control,
representing an increase in metabolic activity prior to awa-
kening [28]. Similar upticks of activity are seen with an
increase in core temperature before wakening [28,29]. The
prolonged decline in carbon dioxide during the night is
associated with a decline in physical activity and sleep,
with an associated reduction in respiratory rate and increased
carbon dioxide concentrations in the body [30]. Carbon diox-
ide levels declined as the height of the hut increased, most
probably due to stronger winds at higher elevations [27].
Since carbon dioxide and other volatiles produced by
humans are strong attractants for blood-questing mosquitoes
[31], the lower concentrations experienced in elevated huts
are likely to contribute partly to the decline in mosquitoes
seen with increasing hut height. Carbon dioxide concen-
trations and human-produced volatiles emanating from a
house are key to understanding our findings and probably
explains why houses with more people have more
mosquitoes than those with few or none [32].
During the focus group discussion, participants stated a
preference to sleep in huts at 0 m and 3 m. It is likely that
their preference for the hut at ground level was associated
with the familiar style of housing in this community, all of
which are single-storey ground-level buildings. This con-
clusion was supported by a preference for solidmaterials.
Hence, there is a perceived normfor local houses to be
built on the ground and of solid materials. Nonetheless, par-
ticipants did appreciate the hut at 3 m because of the absence
of mosquitoes. The huts were designed to answer a particular
hypothesis, and they were not intended to represent what
actual houses would look like in the future. These would
be robustly constructed with lightweight materials, well
ventilated and with safe stairs for children and adults [15].
With the population of sub-Saharan Africa expected to
increase sharply over the next decades [1], there is a need
to produce millions of healthy, comfortable, safe and cheap
houses. While most rural houses in the region are single-
storey buildings, constructed on the ground [33], there are
numerous examples of indigenous structures built off the
ground (figure 5). Raised structures are built for four reasons.
Firstly, as grain stores, elevated to prevent infestations with
rodents, such as the Kongo Granaries in Angola [34] and
multi-storey Dogon granaries in Mali [35]. Secondly, houses
are built on stilts to avoid damp or flooded ground, such as
those constructed by the Lafofa people in southern Sudan
[35], the lake village of Ganvie, in Benin, a UNESCO World
Heritage site [36,37], and Makoko, one of Lagoslargest infor-
mal settlements [38], with a third of its area built over a lake.
Such structures are typically built on the coast or riverside as
protection against flooding [35]. Clearly, installing sturdy
two-storey buildings in areas prone to flooding is highly
desirable, particularly as flooding becomes more common
due to climate change [39]. Thirdly, they may be used as a
defence and in the past were used as protection from slave-
raiding parties [40]. Fourthly, today two-storey houses or
higher are common because of a shortage of land, in order
to increase living space and have a more efficient use of
land [41,42]. These structures are part of the urban landscape
of many African cities and towns, and alongside busy roads
where shops are frequently two-storey structures. As land
becomes in increasingly short supply, the building of
multi-storey infrastructure will undoubtedly increase.
There are several limitations to our study. Firstly, the exper-
imental huts were smaller than traditional single-room houses
and made with marine plywood walls instead of mud or
cement blocks. This made them hotter than traditional
houses during the day, and the movement of carbon dioxide
in these buildings will differ from typical houses. Secondly,
in the study area, adults usually go to bed 23 h later [43]
than in our study. Thirdly, we do not know whether closing
the space under an elevated house will result in more mosqui-
toes flying up the sides of the building and entering the
dwelling room in higher numbers than a similar building
that is open underneath. Studies in Tanzania, however, indi-
cate that this will not reduce protection from mosquitoes
[15]. Fourthly, with our current design, we were unable to
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determine exit rates of mosquitoes leaving the huts or measure
the number of mosquitoes that died during the night. Finally,
our study was conducted in experimental huts, which means
that the performance of the house and perceptions from poss-
ible users will differ when compared with implementation or
scaling up a prototype.
Our findings show that the number of malaria mosquitoes
entering a hut declines with the increasing height of the hut.
This results from the habit of most mosquitoes flying less
than 1 m above the ground and lower production of carbon
dioxide and other attractants in the cooler elevated houses.
Essentially, mosquitoes are less likely to feed on a person
sleeping in an elevated house than one on the ground. At
3 m, this reduction in an indoor entry is equivalent to the pro-
tections afforded by an insecticide-treated bed net [21]. Raising
houses off the ground, like any intervention, is not evolution-
ary proof, and over time, mosquitoes may adapt and feed
higher off the ground than before. Nonetheless, we rec-
ommend elevating houses off the ground since they are
likely to reduce mosquito biting and keep the occupants
cooler at night [7] and therefore more likely to use an ITN.
This research is likely to be relevant to many hot and humid
parts of sub-Saharan Africa where An. gambiae s.l. is the
major vector of malaria and places where high temperatures
reduce the use of bed nets. Raising houses off the ground is
likely to reduce mosquito house entry.
Ethics. Study subjects provided signed-witnessed consent. The study
was approved by The Gambia Government and Medical Research
Councils joint ethics committee (SCC 1607v1.3, October 17, 2018)
and the Department of Biosciences ethics committee, Durham Uni-
versity, UK (January 11, 2019).
Data accessibility. Data files and video are available on FigShare (doi:10.
6084/m9.figshare.14483475) [44].
Authorscontributions. S.W.L. conceived the study; J.B.K., H.W. and
O.S.B. designed the experimental huts; M.J.A., M.J. and M.C.T. over-
saw construction; S.W.L., M.C.T., M.J., J.B. and D.J. designed the
study; M.C.T. collected field data; M.J., M.P. and U.D. contributed
in field supervision; M.C.T., S.W.L and J.B. contributed in data
(d)(c)
(f)(e)
(b)(a)
grain storesraised houses
two-storey buildings
Figure 5. Raised and two-storey constructions in sub-Saharan Africa. (a) Dogon granaries in Mali, 2016 (Hamaji Magazine); (b) granaries for crop storage in Chad,
2017 (UN Environment Programme); (c) Makoko in Nigeria, 2016 (The Guardian); (d) Ganvie in Benin, 2018 (Scribol Magazine); (e) double-storey bamboo prototype
house in Tanzania [15]; ( f) two-storey house with store in the ground floor in Uganda (S.W.L.).
royalsocietypublishing.org/journal/rsif J. R. Soc. Interface 18: 20210256
8
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analysis; M.C.T. and S.W.L. wrote the manuscript. All authors read
the manuscript and commented on it.
Competing interests. We declare we have no competing interests.
Funding. This work was supported by pump-prime funding from the
BOVA Network (Building Out Vector-borne diseases in sub-Saharan
Africa). S.W.L. and M.C.T. are supported by the Global Challenges
Research Fund for networks in vector-borne disease research which
is co-funded by BBSRC, MRC and NERC (BB/R00532X/1), and
M.C.T. is supported by the Durham Global Challenges Centre for
Doctoral Training. The funders had no role in study design, data
collection, data analysis, data interpretation or writing of the
report. All authors had full access to all data in the studies and
had final responsibility for the decision to submit for publication.
Acknowledgements. We would like to acknowledge the support of
Dr Mamadou Ousmane Ndiath, Malaria Laboratory Manager at
the MRCG and the hard work and enthusiasm of Amfaal Fofana
and Bakary Fadera who supported the fieldwork data collection.
We are grateful for the cooperation of the villagers of Wellingara
and Walikunda, study participants and the MRCG field staff for
their support.
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... Thus, using materials to construct walls that increased ventilation and had a low thermal mass resulted in few mosquitoes indoors and cooler indoor temperatures. In addition, elevating a house also reduces mosquito entry, as shown by an experimental hut study in The Gambia, where individual huts were raised or lowered to different heights [5]. ...
... Recent research shows that the number of An. gambiae s.l. enter an inhabited building declines with increasing height, with 84% fewer mosquitoes when houses are elevated 3 m from the ground [5]. ...
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... Recently, it has been demonstrated that house entry by malaria mosquitoes can be reduced substantially by elevating the house off the ground [7] and by increasing ventilation to reduce indoor levels of carbon dioxide [8], making it difficult for mosquitoes to locate a blood meal and transmit malaria. Indoor air pollution mostly due to smoke from open fires but also due to dust and allergens from earth floors, muddy walls, and thatched roofs increases the risk of respiratory tract infections [9,10]. ...
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... Housing improvements tend to be implemented as a package and, in line with this, our study found that metal-roof sleeping spaces were more likely to have floors and walls made of finished materials than thatch-roof sleeping spaces. Improving house construction should be a focus for malaria reduction, with increasing evidence in support of screened, selfclosing doors, closed eaves, raising buildings off the ground, screened windows on either side of building for ventilation and solid roofs [16,38,39]. As well as contributing to the development agenda, there is also evidence that improved housing can reduce risk of other major causes of death in children including diarrhoea, growth failure and anaemia [40]. ...
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Access to adequate housing is a fundamental human right, essential to human security, nutrition and health, and a core objective of the United Nations Sustainable Development Goals1,2. Globally, the housing need is most acute in Africa, where the population will more than double by 2050. However, existing data on housing quality across Africa are limited primarily to urban areas and are mostly recorded at the national level. Here we quantify changes in housing in sub-Saharan Africa from 2000 to 2015 by combining national survey data within a geostatistical framework. We show a marked transformation of housing in urban and rural sub-Saharan Africa between 2000 and 2015, with the prevalence of improved housing (with improved water and sanitation, sufficient living area and durable construction) doubling from 11% (95% confidence interval, 10–12%) to 23% (21–25%). However, 53 (50–57) million urban Africans (47% (44–50%) of the urban population analysed) were living in unimproved housing in 2015. We provide high-resolution, standardized estimates of housing conditions across sub-Saharan Africa. Our maps provide a baseline for measuring change and a mechanism to guide interventions during the era of the Sustainable Development Goals. The prevalence of improved housing (with improved drinking water and sanitation, sufficient living area and durable construction) in urban and rural sub-Saharan Africa doubled between 2000 and 2015.
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Introduction: Unprecedented improvements in housing are occurring across much of rural sub-Saharan Africa, but the consequences of these changes on malaria transmission remain poorly explored. We examined how different typologies of rural housing affect mosquito house entry and indoor climate. Methods: Five typologies of mud-block houses were constructed in rural Gambia: four were traditional designs with poorly fitted doors and one was a novel design with gable windows to improve ventilation. In each house, one male volunteer slept under a bednet and mosquitoes were collected indoors with a light trap. Typologies were rotated between houses weekly. Indoor conditions were monitored with data loggers and the perceived comfort of sleepers recorded with questionnaires. We used pyschrometric modelling to quantify the comfort of the indoor climate using the logger data. Primary measurements were mean number of Anopheles gambiae and mean temperature for each house typology. Findings: In thatched-roofed houses, closing the eaves reduced A gambiae house entry by 94% (95% CI 89-97) but increased the temperature compared with thatched-roofed houses with open eaves. In houses with closed eaves, those with metal roofs had more A gambiae, were hotter (1·5°C hotter [95% CI 1·3-1·7]) between 2100h and 2300 h, and had 25% higher concentrations of carbon dioxide (211·1 ppm higher [117·8-304·6]) than those with thatched roofs. In metal-roofed houses with closed eaves, mosquito house entry was reduced by 96% (91-98) by well fitted screened doors. Improved ventilation of metal-roofed houses made them as cool as thatched houses with open eaves. Metal-roofed houses with closed eaves were considered more uncomfortable than thatched ones with closed eaves. In metal-roofed houses, ventilated houses were more comfortable than unventilated houses before midnight, when people retired to bed. Interpretation: Closing the eaves reduced vector entry in thatched houses but increased entry in metal-roofed houses. Metal-roofed houses with closed eaves were, however, protected against malaria vectors by well fitted screened doors and were made comfortable by increasing ventilation. House designs that exclude mosquitoes and are comfortable to live in should be a priority in sub-Saharan Africa. Funding: Sir Halley Stewart Trust, Global Clinical Trials, and Global Challenges Research Fund.
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Despite compelling evidence that modern housing protects against malaria, houses in endemic areas are still commonly porous to mosquitoes. The protective efficacy of four prototype screened doors and two windows designs against mosquito house entry, their impact on indoor climate, as well as their use, durability and acceptability was assessed in a Gambian village. A baseline survey collected data on all the houses and discrete household units, each consisting of a front and back room, were selected and randomly allocated to the study arms. Each prototype self-closing screened door and window was installed in six and 12 units, respectively, with six unaltered units serving as controls. All prototype doors reduced the number of house-entering mosquitoes by 59-77% in comparison with the control houses. The indoor climate of houses with screened doors was similar to control houses. Seventy-nine percentage of door openings at night occurred from dusk to midnight, when malaria vectors begin entering houses. Ten weeks after installation the doors and windows were in good condition, although 38% of doors did not fully self-close and latch (snap shut). The new doors and windows were popular with residents. The prototype door with perforated concertinaed screening was the best performing door because it reduced mosquito entry, remained fully functional, and was preferred by the villagers. Screened doors and windows may be useful tools for reducing vector exposure and keeping areas malaria-free after elimination, when investment in routine vector control becomes difficult to maintain.