Insecticide-Treated Nets
Introduction
Insecticide-treated nets (ITNs) first came to the attention of public health experts over 20 years ago, when the first studies to evaluate the impact of the novel application of pyrethroid insecticides to the traditional mosquito net on reducing malaria vector exposure were undertaken in Africa and Asia (Darriet et al., 1984; Ranque et al., 1984; Charlwood and Graves, 1987; Lines et al., 1987). These studies confirmed the safety of pyrethroid insecticides used to treat the nets and demonstrated substantial effects on various entomological measures such as human biting rates, feeding behaviour and survival. The success of these initial studies led quickly to Phase II and III clinical field trials, which provided the crucial evidence that ITNs were successful in reducing both malaria morbidity and, importantly, all-cause child mortality.
The mortality trials showed that ITNs are the most powerful malaria control tool to be developed since the advent of indoor residual spraying (IRS) and chloroquine, more than four decades earlier. As a result, they have been an important component of global and many national malaria control policies since the mid-1990s. Yet a decade later, coverage is still unacceptably low. Why is this?
This review charts the progress of ITNs over the last ten years, from efficacy and effectiveness studies to more recent technological advances, and from incorporation of ITNs into global and national malaria control policies to their deployment using a range of programmatic delivery mechanisms.
Section snippets
Children and Adults
There is a large body of evidence from over 81 trials conducted in a range of malaria transmission settings worldwide showing that use of ITNs substantially reduces the frequency and severity of malaria. A recent systematic review of 22 of these trials (those which constituted randomized controlled trials) conducted in children and adults in sub-Saharan Africa (13), Latin America (5), Thailand (2), Pakistan (1) and Iran (1) showed that ITNs reduced clinical episodes by around 50% in both stable
ITNs Alone
Studies to determine whether the high efficacy of ITNs found in highly controlled clinical trials can be replicated when ITNs are deployed under programme conditions (Phase IV trials) have been limited. The few studies that have been conducted have however documented good impact in both small- and large-scale programme settings (see Table 3). There are many inherent difficulties in measuring impact under programme conditions, the major one being that programmes deliver interventions to specific
Risk Assessment
Synthetic pyrethroids were first developed in the early 1970s, and rapidly entered widespread use in agriculture. Since then, they have gained and kept a reputation of being one of the safest classes of insecticide, in terms of hazards to both human health and the environment.
Pyrethroids are formally classified as ‘moderately toxic’, which is lower than most other commonly used classes of insecticide. They break down quickly in tissue, soil and water, and therefore do not accumulate through
Monitoring and Impact of Insecticide Resistance
In the long term, resistance is probably the most important threat to the effectiveness of ITNs. Pyrethroid resistance mechanisms of two kinds have been reported.
‘Knockdown resistance’ (kdr) is caused by one or more substitution mutations in the target molecule on the surface of the nerve. Two kdr mutations have been reported in An. gambiae. One, which was first discovered in Kenya, is confined to permethrin (Ranson et al., 2000). The other, which confers cross resistance to a variety of
Technological Advances
To remain effective, ITNs need to be re-treated with insecticide about once a year or after two or three washes. This is a major constraint to the effective use of ITNs in rural Africa since systems for providing re-treatment are either absent or inadequate. To address this problem some manufacturers have sought to develop LLINs in which insecticide remains present on the net at toxic concentrations even after several washes. This is an active and competitive area of research. There is growing
Evolution of ITN Policy
The evolution of ITN policy over the past 10 years has been driven by key milestones in the evidence provided by the research on ITNs described above in terms of target groups (from efficacy studies) and delivery strategies (from cost-effectiveness studies). The main policy debate has been on how best to achieve a balance between equity on the one hand and sustainability on the other. Central, and closely linked, themes within this debate have been ‘who should pay? (and how much can they afford
Projects
Projects to deliver ITNs began in Asia and the Pacific region in the late 1980s and in Africa slightly later, in the early 1990s. With the exception of China, Viet Nam, Solomon Islands and The Gambia, all of which have national programmes, ITN distribution was, until a few years ago, limited to discrete, relatively small-scale projects run by a combination of government agencies, NGOs and development partners, but with limited inter-agency collaboration between partners. There are now
Conclusions and Needs
While there is currently unprecedented political and financial support for increasing access to ITNs as a key strategy to controlling malaria, with consequent benefits to poverty reduction, there are still many questions concerning the effectiveness of different, as well as combined, approaches to scaling up ITN interventions, including those listed below.
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What is the effectiveness of different approaches to increasing coverage of ITNs (with and without IPT) in programme settings, by combining
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