House Screening - Biological and Environmental Control of Disease Vectors - page 129

sandfl ies, is also evident for untreated curtains.
In a trial using polyester netting curtains
impregnated with carbosulfan (200 mg m −2 ),
permethrin (1 g m −2 ), or untreated, hung inside
eave gaps, doors and windows to prevent house
entry by An . gambiae in Burkina Faso, the
untreated curtains performed as well as the
permethrin-treated curtains reducing house
entry by 49% (Fanello et al ., 2003). A similar
ef ect (55% reduction) was observed for the
untreated arm in a trial of permethrin-treated
curtains (Majori et al ., 1987). While this is
further evidence for the strong mechanical
prophylaxis ef ect of house screening, it also
highlights some issues regarding the insecticide
treatment of screening materials. As daily
contact with treated screened surfaces would be
lower than that experienced by bed net users,
and because there is already extensive use of
pyrethroids on insecticidal nets and for indoor
residual spraying (IRS), using an insecticide
with a dif erent mode of action on screening is
not only desirable for slowing resistance but
may provide synergistic interaction with
pyrethroid long-lasting insecticidal nets (LLINs).
Insecticide-treated screening may reduce the
need to repair holes and would probably allow a
larger mesh size, thus improving ventilation.
Long-lasting formulations would have to be
used, because retreatment of permanently fi xed
mesh would be dii cult. An alternative strategy
would be to market removable panels that could
be replaced at the end of the life of the insecticide.
Particular care would need to be taken to ensure
netting is insecticide-treated in areas where both
mosquito and sandfl y control is required - due to
their small size sandfl ies can pass through
netting of the wide mesh sizes used against
mosquitoes - but if such netting is insecticide-
treated this penetration can be greatly reduced
(Majori et al ., 1989).

eave treatments, the success of this approach
relies on a solid understanding of local vector
fl ight height and behaviour. Some cultures,
particularly in South-east Asia, have a tradition
of building their houses raised up on stilts or
platforms. Although the rationale behind such
practice is usually related to protection against
fl ooding, avoidance of mosquito bites is also a
driving force. For example, in northern Vietnam
people build houses on stilts primarily to reduce
contact with low-fl ying An . minimus (Laderman,
1975). Raising buildings may also protect
against An . farauti , a major malaria vector in
Papua New Guinea, Indonesia and the Solomon
Islands (Lindsay et al ., 2004). It seeks a blood
meal very close to the ground, to the extent that
even raising one's feet 35 cm from the fl oor is
sui cient to signifi cantly reduce the number of
bites received (Charlwood et al ., 1984). Building
on stilts may also help to reduce the densities of
rats and other vermin. In São Tomé houses built
on stilts had half the number of An . gambiae
indoors compared to those at ground level
(Charlwood et al ., 2003); a similar association
between houses raised from the ground and low
indoor densities of Cx . quinquefasciatus has been
reported in Trinidad and Tobago (Howell and
Chadee, 2007).

7.4.6 Screening materials

Traditional materials for screening include
cotton netting or sheets for eaves and windows,
while iron mesh screens have also been used for
windows and, more rarely, doors. These
materials, while ef ective at preventing insects
from entering, reduce ventilation by 30-70%
(WHO, 1982; Rozendaal, 1997) and are easily
damaged by UV or corroded under high
humidity. Some studies (Curtis et al ., 1996;
Kampango et al ., 2012), and even local
communities under their own initiative, have
used man-made screening materials usually
associated with insecticide-treated nets, such as
polyethylene, polypropylene and polyester.
These have been either bought as rolls of netting
or more usually cut directly from old or
unwanted bed nets. Although lightweight and
easily installed, this material is also prone to
tearing and damage when exposed to direct
sunlight. Currently, a range of durable

7.4.5 Raising buildings

Although raising houses (and bedrooms in
particular) of the ground is a house design
consideration and not house screening, it is
worth discussing in this context because it has
been a recommended practice against mosquito
house entry both in the past (Celli, 1900) and
more recently (Charlwood et al ., 2003). Like