House Screening - Biological and Environmental Control of Disease Vectors - page 118

Sadly these are but a select few examples,
and not all have obvious solutions. Perhaps most
damning of all is a problem that at fi rst seems
simple to solve, that is, the very nature of our
housing. Where homes are built, the design and
the materials used can be the most important
risk factors determining the level of exposure to
vector-borne disease transmitted by many
species of domestic and peri-domestic
arthropods (Table 7.1). The current chapter will
focus on house improvements and house
screening to control disease vectors in houses
within the tropics and subtropics whilst
acknowledging that neither poorly designed
housing, nor vector-borne disease, are limited
exclusively to those regions.

(Muriuki et al ., 2005). Similar suggestions
include creating forest-free belts of 300 m to 2
km to protect against forest-dwelling mosquitoes
and sandfl ies (vectors of leishmaniasis), or siting
houses downwind of breeding sites (adult
mosquitoes that emerge from breeding sites will
be seeking a blood-meal soon afterwards, and
will follow odour plumes upwind towards the
source) (Rozendaal, 1997). However, realizing
such an aim is fraught with complications -
clearing forests may remove one vector but
create ideal habitats for another (Lindsay et al .,
2004). Enforcing restriction zones where new
houses cannot be sited may be dii cult legally,
whilst even encouraging construction on
'favourable locations' can be tricky: the
ownership of plots can be dii cult to establish,
land titles may be owned by groups and not
individuals and the land may have no inherent
monetary value (Briggs, 1991). The availability
of land within these locations may be short,
while defi nitions of 'favourable locations' are
themselves limited: an area that is suitable in
terms of limiting disease transmission may be
unsuitable in terms of climate, agricultural
productivity of the land or other economic
potential.
Clearing construction sites for house
location, particularly by deforestation, creates its
own problems. Deforestation has permitted
expansion of savannah blackfl ies Simulium
damnosum s.s. (Wilson et al ., 2002) and tsetse
fl ies (de la Rocque et al ., 2001) into areas that
were previously adverse to them, though the
extent of the ef ects of such land clearance
depend on the ecology of the local vector species.
For example, no correlation was apparent
between proximity of housing (of any type) to
the forest edge and malaria risk in southern Sri
Lanka, where An . culicifacies is the dominant
vector (Gunawardena et al ., 1998), yet the
opposite was true in an epidemic situation in the
Kenyan highlands where An . gambiae pre-
dominates (Brooker et al ., 2004). A review of the
impact of habitat clearance and the ecological
requirements of local mosquitoes in east Asia
and the Pacifi c suggests how the densities of
some species will be reduced, e.g. An . balabacensis
and An . dirus , while others may increase (Lindsay
et al ., 2004). Where habitat change reduces
vegetation, outdoor-resting mosquitoes may

7.2 House Location

The importance of house location in relation to
disease transmission has been recognized for
many years. Fullerton and Bishop (1933)
observed in Tennessee, USA, that the
construction of houses on poorly drained plots
that cannot be used for crops 'is of tremendous
importance as a malaria problem', and
associations between houses built on marshy
grounds and malaria go back centuries further
(Sallares, 2002). Where transmission is highly
localized and vector breeding sites are restricted,
locating houses at a distance from these might
provide some relief from vector-borne diseases.
Studies measuring vector density and malaria
transmission (prevalence or incidence) gradients
in Senegal (Trape et al ., 1992), Sri Lanka
(Gunawardena et al ., 1998; Van der Hoek et al .,
2003) and The Gambia (Thomas and Lindsay,
2000; Clarke et al ., 2002) have shown negative
correlations with increasing distance from
breeding sites. Similarly, distance from blackfl y
( Simulium spp.) breeding sites is negatively
correlated with several clinical indicators of
onchocerciasis (Kloos et al ., 1991; Mendoza et
al ., 1997). Gunawardena et al . (1998) estimated
that an exclusion zone of 200 m around
breeding sites (and subsequent relocation of
some houses) would reduce malaria incidence in
the whole population by 21% where Anopheles
culicifacies is the vector. Exclusion zones have
also been considered in tsetse fl y control