House Screening - Biological and Environmental Control of Disease Vectors - page 123

of a construction site, the use of the building
and its design can all contribute to this risk.
House materials correlate well with malaria
prevalence and risk of infection for the
occupants, and are also closely associated with
Chagas disease. The materials are usually the
proximal cause with socio-economic status and
ease of vector house entry as the likely ultimate
explanations for the correlation. Yet even when
income and other poverty measures are
controlled for, the correlation often remains,
highlighting the importance of exposure to
vectors indoors. Grass roofs have been associated
with an increased risk of malaria infection (Yé et
al ., 2006) and an odds ratio (OR) for malaria-
associated anaemia twice that of iron roofs
(Kahigwa et al ., 2002). Metal roofs are likely to
restrict vector house entry (not least because
they are often associated with closed eaves), as
well as prohibiting vectors resting there because
of the high temperatures of the roof surface.
Additionally, the sheet metal surface probably
presents less cracks and crevices, which may
serve as arthropod resting sites, compared to
thatch roofs. Houses with clay tile roofs
increased the intra-domiciliary prevalence of
Triatoma dimidiata in Guatemala (King et al .,
2011) and Costa Rica (Starr et al ., 1991)
compared to aluminium roofed houses, although
this was not universal across all study areas.
Houses with new (<1 year old) palm thatch roofs
had a three times higher OR for infestation by
another triatomine bug, Rhodnius prolixus ,
compared to houses with other roof types in
Venezuela (Sanchez-Martin et al ., 2006).
Interestingly, there was no increased risk
associated with older palm thatch roofs,
suggesting that humans are directly responsible
for bringing these Chagas vectors into their
houses when they choose this material for
thatching the roofs. The adults and eggs, stuck
to the palm fronds, can both be introduced this
way (Gamboa, 1963). In a similar fashion, ticks
and fl eas may be carried into houses when
grasses are cut to make bedding or for animal
feed (M. Kirby, personal observation).
Cement fl oors are linked to a halving of the
risk for malaria-associated anaemia compared
with earth fl oors (Kahigwa et al ., 2002). Cement
slab fl oors are also associated with dramatically
lower OR (0.04-0.11) for house infestation by
Chagas vectors T . dimidiata compared to dirt and

earth fl oors (King et al ., 2011), though in both
cases it is likely that this is due to correlation
with improved living conditions rather than a
direct link to vector house entry.
Mud-brick-walled houses were 44% more
likely to contain An . gambiae s.l. mosquitoes than
cement-walled houses (Kirby et al ., 2008a).
Children living in mud- and wood-walled houses
may have double the risk of malaria-associated
anaemia (Kahigwa et al ., 2002) and increased
parasitaemia (Sintasath et al ., 2005; Ernst et al .,
2006) than those in brick-walled houses. The
risk with mud walls is that without cementitious
material they will crack on drying, creating
access points through which mosquitoes can
pass and crevices suitable for triatomine bugs
(Briceno-Leon, 1987; King et al ., 2011),
sandfl ies, ticks and bedbugs. These cracks
provide an ideal microclimate for vectors to rest
in, largely free from predators and close to
domestic animals and human sources of blood.
The house in fact is often a benign environment
compared to outdoor resting sites. Lower indoor
temperatures and higher humidities (especially
where water is stored indoors in porous jars) will
reduce desiccation, a signifi cant limiting factor
in the distribution of many insect species
(Williams et al ., 1998; Tammariello et al ., 1999;
Gray and Bradley, 2005).
Although brick houses generally have fewer
cracks than mud-walled houses, areas where
brick making predominates can still have high
rates of malaria transmission because brick pits
make good mosquito breeding sites. In the
process of making bricks, water is supplied to
excavated clay pits to be mixed with the soil and
other materials. These pits can also fi ll with rain
water and ground water. Both active and
abandoned pits support high densities of
mosquito larvae and can be the predominant
source of vectors during dry seasons (Carlson et
al ., 2004).
Despite their problems, these materials
have features that help explain why they
continue to be used. Earthen walls keep interiors
relatively cool, while thatched roofs protect from
the sun and rain but still allow air to permeate.
Most importantly these materials often pose no
additional cost to the builders above the work of
gathering them. Availability and af ordability of
construction materials are critical elements in
rural tropical house construction and should be