Introduction - Biological and Environmental Control of Disease Vectors - page 3

underground drainage channels, and con-
struction of tidal gates to control the salinity in
coastal swamps (Konradsen et al ., 2004; Jobin,
2012). Keiser et al . (2005) conducted a meta-
analysis of the impact of environmental
management on the prevention of malaria-
attributable morbidity and mortality, and
concluded from a total of 24 studies, most of
which had been conducted pre-1955, that
environmental modifi cation, manipulation and
changes in human habitation provided a high
protective ei cacy on clinical malaria parameters
(79.5-88% reduction in risk). Although these
conclusions are promising, and warrant the
renewed interest in alternative methods of
disease control, the studies included in the meta-
analysis were few and at least 50 years old; in
addition, heterogeneity between studies was
large and publication bias was detected, i.e.
studies with positive results were more likely to
be published.
However, despite the promises of environ-
mental management, once DDT and, in the
1980s, synthetic pyrethroids became widely
available, the world adopted insecticides for IRS
and to impregnate bed nets, leaving the more
localized, environmentally sensitive methods to
fade. Unarguably, DDT was highly successful in
eliminating malaria from North America,
Europe, the former Soviet Union and most
Caribbean islands (Bruce-Chwatt, 1980) as well
as Taiwan (Chen and Chen, 2009). None the
less, concerns about the impacts of DDT on the
environment and human health, initiated by
Rachel Carson's publication of Silent Spring
(Carson, 1962), led to the ban of DDT by the
Environmental Protection Agency (EPA) in the
USA in 1972. To date, the long-term impacts on
human health are disputed and the available
epidemiological studies on associations with
cancer and other diseases are inconclusive (e.g.
van den Berg, 2009, 2010; Tren and Roberts,
2010). What is known is that the replacement
of DDT with pyrethroids in South Africa was
partly to blame for a sharp increase in malaria
incidence by fi vefold to around 60,000 cases per
year as mosquitoes developed pyrethroid resist-
ance (Hargreaves et al ., 2000) and the malaria
parasite evolved resistance to sulfadoxine-
pyrimethamine (SP) (Blumberg and Frean,
2007). In fact, malaria incidence only decreased
again after the reintroduction of DDT in IRS

programmes, and after the drug artemether-
lumefantrine was made available (Barnes et al .,
2005). The World Health Organization (WHO)
now recommends the use of DDT against
malaria vectors in Africa and other areas where
mosquitoes are still susceptible to DDT, as
reasoned in a recent WHO position statement:
'The Convention has given an exemption for the
production and public health use of DDT for
indoor application to vector-borne diseases,
mainly because of the absence of equally
ef ective and ei cient alternatives. ... It is
expected that there will be a continued role for
DDT in malaria control until equally cost-
ef ective alternatives are developed' (WHO,
2011).

1.1.2 Current insecticide-based control
methods and their limitations

The insecticides currently recommended by the
WHO Pesticide Evaluation Scheme (WHOPES)
to be used in IRS belong to four insecticidal
compound classes: pyrethroids, organochlorines
(DDT), organophosphates and carbamates; only
pyrethroids are licensed to be used on long-
lasting insecticidal nets (LLINs). Both methods
are the mainstay in modern day malaria control,
and have led to incredible reductions in malaria
incidence: LLINs and IRS are estimated to avert
approximately 220,000 deaths of children
under the age of 5 every year (WHO, 2012b).
Insecticide-based control methods are predicted
to remain an essential component of disease
control strategies, particularly in high trans-
mission settings and in areas where most vectors
are still exhibiting indoor biting and indoor
resting behaviours and are still susceptible to
insecticides. However, the reliance on the small
arsenal of insecticidal compounds that is
currently available (Nauen, 2007) and the rapid
evolution of insecticide resistance in mosquitoes
(Ranson et al ., 2011; Asidi et al ., 2012) are
putting existing global eradication ef orts at risk,
recently prompting the WHO to publish a
document on the management of insecticide
resistance for malaria control (WHO, 2012b). In
fact, particularly now that more and more
attention is being paid to malaria eradication,
and reductions in entomological inoculation
rates (EIRs) occur even in historically high