House Screening - Biological and Environmental Control of Disease Vectors - page 133

doors, windows and eave curtains are un-
fi nished. Recent estimates suggest that 38% of
rural houses in northern Venezuela are
incomplete, mainly because householders can-
not af ord all the materials and labour in one
payment (Briceno-Leon, 1987). One way to
improve this is to involve the householder more
proactively in the construction process. An
example for improving and completing house
walls quickly, as suggested by Briceno-Leon
(1987), is to rent a brick compressor to a
householder who would produce the blocks
himself to get a discount from the cost of the
house. In the poorer communities, the
householder, if unemployed, could also work as
a paid-labourer to support a Ministry of Health-
recognized builder during construction of his
own house. If house screening was a requirement
in such a scheme, it could lead to both improved
uptake and af ordability. The net savings on
malaria control costs should also be taken into
consideration in a government-led construction
programme. Gunawardena et al . (1998)
estimated that it would take just over 7 years to
of set the cost of constructing new mosquito-
proof housing in southern Sri Lanka. Similar
economic analyses should be an integral part of
any planned house screening or house replace-
ment programmes.

screened ceilings alone (and open eaves) showed
even smaller deviations in indoor microclimate
from the controls. Similar fi ndings were apparent
in a trial of papyrus mat ceilings in Kenya -
modifi ed houses were on average 1.3°C cooler
during the day and 0.8°C warmer at night
compared to unmodifi ed houses, though these
dif erences were not statistically signifi cant, and
there was no ef ect on indoor humidity (Atieli et
al ., 2009).
Ventilation also needs to be considered
separately from microclimate. Bed net utilization
is af ected by the attenuated airfl ow inside the
net because it contributes to the perceived
thermal comfort (von Seidlein et al ., 2012), and
screening doors, windows and eaves seems
certain to do the same. Even if temperature,
relative humidity and evaporation do not change
signifi cantly when screening is installed,
reduced ventilation can have other implications.
In a well-ventilated house, the airfl ow helps to
break up odour plumes, making it more dii cult
for vectors to follow olfactory gradients. Because
of this, the number of mosquito bites received
can be reduced by using an electric fan in a
bedroom at night (Hewitt et al ., 1996). Ideally,
new houses should be built without an eave gap
and existing houses should have eaves blocked,
provided that air movement is not decreased
signifi cantly or sui cient ventilation can be
provided by alternative means. While increasing
ventilation must never be at the expense of
increased disease transmission, signifi cant
airfl ow attenuation may result in reduced use or
complete removal of screening. In many rural
houses ventilation is provided by deliberately
missing out one or two bricks in the wall, or by
using airbricks. The Kampala house (Fig. 7.1)
has no eave gaps, yet there are airbricks all along
the top of the wall below the roof that allow easy
passage of mosquitoes. If such bricks are to be
sold to provide ventilation, they should be
marketed with built-in integral screening of a
wide mesh size. There are advocates of installing
screening at a lower level in the house for
ventilation whilst blocking eaves completely, but
such low-level screens may be more prone to
dirt, dust and animal damage.
Screening will af ect ventilation in two
ways: (i) the weave (threads per square cm/inch)
of the fabric used; and (ii) the number of
locations that can be screened rather than

7.5.3 Ventilation and indoor
microclimate

A common perception in rural Gambia is that
closing eaves leads to a reduction in ventilation
and an increase in indoor temperatures, yet the
evidence for this is not clear cut. Lindsay and
Snow (1988) found that closing the eaves with
mud increased indoor temperature by 1°C but
other studies found smaller or no dif erences
(Njie et al ., 2009; Kirby et al ., 2010). It seems
intuitive that adding door and window screens
in a house with blocked eaves must af ect the
indoor microclimate; yet increases in tem-
perature may be slight. A study using data
loggers in over 100 fully screened houses found
them to be less than 0.5°C hotter at night and
only marginally drier than unscreened control
houses, with little negative feedback about the
screening in terms of thermal comfort or air
movement (Kirby et al ., 2010). Houses with