Integrated Vector Management - Biological and Environmental Control of Disease Vectors - page 179

• Achieving high coverage of breeding sites
raises logistical problems (Chaki et al ., 2009).
• It could be predicted, from the Ross-
Macdonald model for malaria transmission,
that malaria transmission is most sensitive to
changes in adult mosquito survivorship and
reduction of biting rates, so larvicides would
have less impact on malaria outcomes than
measures aimed against adults (Fillinger and
Lindsay, 2011).

only be used as a supplement to ITNs or IRS in
most endemic settings, and identifi ed limited
occasions, with low transmission, where it
would be appropriate to use larviciding alone
(WHO, 2012c). (Such limitations are explored
further by Lines, Chapter 11, this volume.)

10.4 IVM Strategies for Stegomyia

Mosquitoes

• One potential limitation is the operational

and research costs involved in implementing
and sustaining LSM, although one study has
shown that, in areas with moderate and focal
malaria transmission, and where mosquito
larval habitats are well defi ned and accessi-
ble, costs for LSM compare favourably with
costs for IRS and LLINs (Worrall and Fillinger,
2011).

Throughout Asia, aedine mosquitoes not only
transmit LF, they are also vectors of dengue.
Dengue is an arbovirus transmitted by the
distinctive black and white Stegomyia (formerly
called Aedes ) 'tiger' mosquitoes. With 50 million
infections each year and 2.5 billion people at
risk, dengue is considered to be the most rapidly
spreading mosquito-borne disease (WHO,
2009b). Although LF has a successful chemo-
therapy programme in place that has reduced
and in some areas eliminated LF, there are no
vaccines or drugs that can be used to prevent
dengue. Vaccine development is ongoing, but
dengue is quickly becoming a public health
problem in Asia, the Americas and Africa. At
present, the only feasible way to control dengue
is by controlling the mosquito vector (WHO,
2009b) through the implementation of ei cient
and sustainable vector control programmes
aimed against the larvae. Environmental
management and modifi cation, involving com-
munity participation, are the primary methods
to achieve control (Marzochi, 1994). As with
malaria, dengue is most ef ectively controlled
using an IVM strategy.
In addition to the transmission of dengue,
Stegomyia mosquitoes are unusually ei cient
carriers of fi laria larvae due to a phenomenon
known as 'limitation' where microfi larial
survival rates, at low levels of parasitaemia, are
higher following ingestion by this genus than for
other genera of mosquitoes (Brengues and Bain,
1972). Hence, transmission risks remain
signifi cant even when the microfi laria rate has
been suppressed in humans after MDA. The St .
scutellaris group of aedine mosquitoes is
responsible for the majority of LF transmission
within Asia. In contrast to Africa, where
Anopheles and Culex mosquito vectors are night-
biting, LF transmission in Asia is through day-
biting

Similar concerns prompted the WHO to
issue a cautionary interim position statement on
the use of microbial larvicides for malaria
control in sub-Saharan Africa (WHO, 2012c). It
should be stressed that their statement does not
necessarily apply to other malarial endemic
regions, and does not address the use of
larviciding to control vector species of other
mosquito-borne diseases (WHO, 2012c). The
main reservations include: (i) the lack of robust
evidence available on the ei cacy or ef ectiveness
of larviciding in Africa, although this concern
may be addressed following completion of a
systematic review of LSM for controlling malaria
(Thwing et al ., 2011); (ii) the fact that larviciding
must be specially adapted to each locality; and
(iii) the level of logistics required to treat vector
breeding sites thoroughly, but selectively, over a
large area (WHO, 2012c). In summary, the
statement concludes that larviciding has a
restricted use for malaria control (with or
without other interventions) to areas where the
breeding sites are 'few, fi xed and fi ndable', and
that adult mosquito control interventions (using
ITNs, including LLINs, and IRS) are the most
cost-ef ective, provided that appropriate
insecticide resistance management is followed
(WHO, 2012c). This is because interventions
against adults reduce vector longevity, in
addition to vector density, whereas larviciding
only reduces vector density (WHO, 2012c). The
WHO recommended that larviciding should Stegomyia mosquitoes. The most