House Screening - Biological and Environmental Control of Disease Vectors - page 121

change behaviour to seek suitable alternative
microclimates indoors. Anthropogenic habitat
disturbance has also been associated with
conditions conducive to increased transmission
of fl ea-borne diseases (Friggens and Beier, 2010).
In Venezuela, Rhodnius prolixus , a principle
vector of Chagas disease (also known as
American trypanosomiasis) in much of the
Americas, is associated with at least seven species
of palm trees (Sanchez-Martin et al ., 2006) and
the proximity of such palms to houses is a risk
for domestic colonization by R . prolixus
independent of whether house roofs are made
from palm thatch (Sanchez-Martin et al ., 2006).
Deforestation here may reduce the risk of Chagas
disease transmission.
Houses situated in urban areas may face
smaller challenges. In general, transmission of
diseases such as malaria (Robert et al ., 2003)
and Chagas disease is lower in urban than rural
locations, and this is strongly correlated to
vector densities and lack of breeding sites (Trape,
1987; Lindsay et al ., 1990), with other factors
such as the use of antiparasitic drugs or insect
repellents being less signifi cant. Kirby et al
(2008a) found that the risk of fi nding An .
gambiae sensu lato indoors was 89% less in town
homes than in rural homes, independent of
distance to breeding site. The quality of housing
is usually higher in urban areas and this may
of er one explanation (Knudsen and Sloof ,
1992). Nevertheless, the rapid growth of some
tropical cities (Knudsen and Sloof , 1992; Briggs
and Mwamfupe, 2000) can lead to huge
demands on building materials and methods,
resulting in high construction costs and poor
quality or unfi nished housing projects (Falconer,
1971). If urban migrants bring their parasites
and vectors with them into such environments,
urban cycles of disease can become established.
For example, Chagas disease is on the rise in
urban South America (Dias, 2007; Bayer et al .,
2009). Diseases can also adapt to peri-domestic
vectors. Many arboviruses, including yellow
fever and dengue, have well established urban
transmission cycles with dif erent vector species
to the rural cycles (Moncayo et al ., 2004).
Malaria vectors can also adapt to breed in urban
water sources such as wells (Robert et al ., 1998)
and water-fi lled domestic containers (Chinery,
1984). Even seemingly harmless anthropogenic

practices can cause problems: non-native
ornamental trees may be encouraging invasion
of mosquito species into urban centres; high
densities of tree-hole breeding Stegomyia spp.
and An . gambiae s.s. (not commonly recognized
as a tree-hole breeder) were recorded recently in
Kenyan towns (Omlin et al ., 2007).
The risk of siting a house in close proximity
to vector breeding sites can be reduced by house
improvement. For example, unlike poor quality
houses, those of good quality (plastered brick
walls and tiled or corrugated iron roofs) had no
correlation between incidence of malaria and
distance to water sources in a malaria endemic
region of Sri Lanka (Gunawardena et al ., 1998).
A similar conclusion was drawn by Van der
Hoek et al . (2003), i.e. that poor housing is an
independent risk factor for malaria separate
from house location. House improvement
therefore suggests a way that disadvantages to
health of living near irrigated land can be
overcome, leaving only the advantages of
increased agricultural productivity.

7.3 House Construction Materials:
Implications for Vector-borne
Diseases

Proper home planning and construction drive at
the root of much of the dii culty we are now
encountering in mosquito proofi ng rural homes.

(Kiker, 1941)

Although this quote refers to the quality of
housing in the southern states of the USA in the
1930s, it is equally applicable to houses in the
tropics today. Decent housing should be as basic
a human requirement as food and water; it
should provide shelter from adverse climatic
conditions and be a place of privacy in which
people can rest and sleep. Unfortunately, rest
and sleep are often disturbed if vector densities
are high, personal protection low and vectors
can enter houses easily. This of course has other
consequences beyond vector-borne diseases, e.g.
even modest sleep deprivation can reduce
natural immune responses (Irwin et al ., 1996).
Additionally, nuisance biting is important in its
own right - in fact it may be the main driving