Insecticide-Treated Nets

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Abstract

Insecticide-treated nets (ITNs) are the most powerful malaria control tool to be developed since the advent of indoor residual spraying (IRS) and chloroquine in the 1940s, and as such they have been an important component of global and national malaria control policies since the mid-1990s. Yet a decade later, coverage is still unacceptably low: only 3% of African children are currently sleeping under an ITN, and only about 20% are sleeping under any kind of net. This review charts the scientific, policy and programmatic progress of ITNs over the last 10 years. Available evidence for the range of programmatic delivery mechanisms used at country level is presented alongside the key policy debates that together have contributed to the evolution of ITN delivery strategies over the past decade. There is now global consensus around a strategic framework for scaling up ITN usage in Africa, which recognizes a role for both the public sector (targeting vulnerable groups to promote equity) and the private sector (sustainable supply). So, while progress with increasing coverage to date has been slow, there is now global support for the rapid scale-up of ITNs among vulnerable groups by integrating ITN delivery with maternal and child health programmes (and immunization in particular), at the same time working with the private sector in a complementary and supportive manner to ensure that coverage can be maintained for future generations of African children.

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.

  • What is the effectiveness of different approaches to increasing coverage of ITNs (with and without IPT) in programme settings, by combining

References (123)

  • L. Dapeng et al.

    The protective efficacy of bednets impregnated with pyrethroid insecticide and vaccination against Japanese encephalitis

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (1994)
  • G. Dolan et al.

    Bed nets for the prevention of malaria and anaemia in pregnancy

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (1993)
  • C.A. Goodman et al.

    Cost-effectiveness of malaria control in sub-Saharan Africa

    Lancet

    (1999)
  • H. Guyatt et al.

    Use of bednets given free to pregnant women in Kenya

    Lancet

    (2003)
  • H.L. Guyatt et al.

    The cost of not treating bednets

    Trends in Parasitology

    (2002)
  • J.L.K. Hii et al.

    Area effects of bed net use in a malaria endemic area in Papua New Guinea

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (2001)
  • S.C. Howard et al.

    Evidence for a mass community effect of insecticide-treated bednets on the incidence of malaria on the Kenyan coast

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (2000)
  • J.H. Kolaczinski et al.

    Chronic illness as a result of low-level exposure to synthetic pyrethroid insecticides: a review of the debate

    Food and Chemical Toxicology

    (2004)
  • J. Lines et al.

    Insecticide-treated nets in Tanzania. Correspondence

    Lancet

    (2001)
  • S.M. Magesa et al.

    Trial of pyrethroid impregnated bednets in an area of Tanzania holoendemic for malaria. Part 2. Effects on the malaria vector population

    Acta Tropica

    (1991)
  • B. McPake et al.

    Community financing of health care in Africa: an evaluation of the Bamako initiative

    Social Science and Medicine

    (1993)
  • O. Müller et al.

    Evaluation of a prototype long-lasting insecticide-treated mosquito net under field conditions in rural Burkina Faso

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (2002)
  • J.K. Njagi et al.

    Prevention of anaemia in pregnancy using insecticide-treated bednets and sulfadoxine-pyrimethamine in a highly malarious area of Kenya: a randomized controlled trial

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (2003)
  • M.L. Quinones et al.

    Permethrin-treated bed nets do not have a ‘mass-killing effect’ on village populations of Anopheles gambiae s.l. in The Gambia

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (1998)
  • H. Reyburn et al.

    A randomized controlled trial of insecticide treated bednets and chaddars or top-sheets, and residual spraying or interior rooms for the prevention of cutaneous leishmaniasis in Kabul, Afghanistan

    Transactions of the Royal Society of Tropical Medicine & Hygiene

    (2000)
  • M. Rowland et al.

    Pyrethroid impregnated bed nets for self protection from malaria for Afghan refugees

    Transactions of the Royal Society of Tropical Medicine & Hygiene

    (1996)
  • J.R. Schellenberg et al.

    Effect of large-scale social marketing of insecticide-treated nets on child survival in rural Tanzania

    Lancet

    (2001)
  • T. Smith et al.

    Mosquito nets for the elderly?

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (2002)
  • R.W. Snow et al.

    Severe childhood malaria in two areas of markedly different falciparum transmission in east Africa

    Acta Tropica

    (1994)
  • R.W. Snow et al.

    A trial of bed nets (mosquito nets) as a malaria control strategy in a rural area of The Gambia, West Africa

    Transactions of the Royal Society of Tropical Medicine and Hygiene

    (1988)
  • S. Abdulla et al.

    Impact on malaria morbidity of a programme supplying insecticide treated nets in children aged under 2 years in Tanzania: community cross sectional study

    British Medical Journal

    (2001)
  • J.A. Alaii et al.

    Perceptions of bed nets and malaria prevention before and after a randomized-controlled trial of permethrin treated bed nets in western Kenya

    American Journal of Tropical Medicine and Hygiene

    (2003)
  • A.N. Asidi et al.

    Experimental hut evaluation of bed nets treated with an organophosphate (chlorpyrifos methyl) or a pyrethroid (lambdacyhalothrin) alone and in combination against insecticide resistant A. gambiae and C. quinquefasciatus mosquitoes

    Malaria Journal

    (2005)
  • Barlow, S.M. and Sullivan, F.M. (2004) A Generic Risk Assessment Model for Insecticide Treatment and Subsequent Use of...
  • F.N. Binka et al.

    Impact of spatial distribution of permethrin-impregnated bed nets on child mortality in rural northern Ghana

    American Journal of Tropical Medicine and Hygiene

    (1998)
  • F.N. Binka et al.

    Impact of permethrin impregnated bednets on child mortality in Kassena-Nankana district, Ghana: a randomized controlled trial

    Tropical Medicine and International Health

    (1996)
  • E.N. Browne et al.

    The impact of insecticide-treated bednets on malaria and anaemia in pregnancy in Kassena-Nankana district, Ghana: a randomized controlled trial

    Tropical Medicine and International Health

    (2001)
  • F. Chandre et al.

    Modifications of pyrethroid effects associated with kdr mutation in Anopheles gambiae

    Medical and Veterinary Entomology

    (2000)
  • J.D. Charlwood et al.

    The effect of permethrin-impregnated bednets on a population of Anopheles farauti in coastal Papua New Guinea

    Medical and Veterinary Entomology

    (1987)
  • Chavasse, D., Reed, C. and Attawell, K. (1999). Insecticide Treated Nets: A Handbook for Managers. Malaria...
  • H. Cheng et al.

    Large-scale spraying of bednets to control mosquito vectors and malaria in Sichuan, China

    Bulletin of the World Health Organisation

    (1995)
  • P.G. Coleman et al.

    Rebound mortality and the cost-effectiveness of malaria control: potential impact of increased mortality in late childhood following the introduction of insecticide treated nets

    Tropical Medicine and International Health

    (1999)
  • V. Corbel et al.

    Dosage-dependent effects of permethrin-treated nets on the behaviour of Anopheles gambiae and the selection of pyrethroid resistance

    Malaria Journal

    (2004)
  • F. Darriet et al.

    Impact of resistance of Anopheles gambiae s.s. to permethrin and deltamethrin on the efficacy of impregnated mosquito nets

    Med Trop (Mars)

    (1998)
  • Darriet, F., Robert, V., Tho Vien, N. and Carnevale, P., eds (1984). Evaluation of the Efficacy of Permethrin...
  • A. Diabate et al.

    The role of agricultural use of insecticides in resistance to pyrethroids in Anopholes gambiae s.l. in Burkina Faso

    American Journal of Tropical Medicine and Hygiene

    (2002)
  • D.A. Diallo et al.

    Child mortality in a West African population protected with insecticide-treated curtains for a period of up to 6 years

    Bulletin of the World Health Organization

    (2004)
  • T.P. Eisele et al.

    Effect of sustained insecticide-treated bed net use on all-cause child mortality in an area of intense perennial malaria transmission in western Kenya

    American Journal of Tropical Medicine and Hygiene

    (2005)
  • J. Etang et al.

    Reduced bio-efficacy of permethrin EC impregnated bednets against an Anopheles gambiae strain with oxidase-based pyrethroid tolerance

    Malaria Journal

    (2004)
  • Ettling, M.B. (2002). The Control of Malaria in Viet Nam from 1980 to 2000: What Went Right? Manila: World Health...
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