Insecticide-treated nets: efficacy, impact and operational constraints

Insecticide-treated nets (ITNs) have long been used against mosquito bites, and have been shown to give substantial protection against malaria. This article explores some of the operational issues around the use of ITNs in emergencies. How effective are ITNs in tackling the transmission of malaria? What advantages do treated nets have over untreated ones? What operational constraints exist to the use of ITNs in emergencies? What are the key issues in implementation?

Efficacy

The efficacy of measures to prevent mosquitoes from biting humans, and so infecting people with the malaria parasite, depends on the habits of the mosquitoes and their relationship to the habits of people. In Africa, all significant vectors (mosquitoes that transmit malaria) bite at night and in the early hours of the morning, and so are susceptible to ITNs. Studies in Sub-Saharan Africa have shown that insecticide-treated nets significantly reduce the risk of morbidity and mortality in childhood.

ITNs have been thought of mainly as personal protection, but there is increasing evidence that high coverage of ITNs within a community will reduce transmission because of the mass effect on vector survival. For nets to be maximally effective coverage must be high, nets should be re-treated promptly (or long-lasting nets used) and individuals should properly deploy their nets each night. The more households within a given area that have ITNs, the greater the benefit to neighbouring households without nets, although there is still little experience of using ITNs in this way in complex emergencies.

One study has shown that using untreated bed nets in good condition is associated with a significantly lower prevalence of plasmodium falciparum infection (51% protection) and these findings suggest that an untreated net, provided that it is in relatively good condition, can protect against malaria. Even so, untreated nets do not constitute a good intervention: just a single small hole is enough to render the net useless, and even when complete it protects only the person sleeping under the net. Others in the same room or shelter are not protected, and may receive more mosquito bites. In fact, insecticide-treated nets give much more protection than untreated ones, and the safety of the insecticides used and recommended by WHO for treating mosquito nets is well established.

One of the key constraints to the large-scale, sustainable use of ITNs is the need for regular insecticide re-treatment (every six months), and the fact that they lose efficacy after three washes. An ITN that is not re-treated rapidly becomes an untreated net, and is not useful. Unless a strategy for re-treatment is built into an ITN distribution programme, the distribution will have no lasting value. Successful strategies for re-impregnation have, however, proved very difficult to identify. In Kenya, a free house-to-house strategy achieved over 95% re-impregnation, but when the system changed to free re-impregnation at sentinel sites, the success rate went down to 65%. With the introduction of cost-recovery, only 7% of nets were re-impregnated. Community-based programmes among Afghan refugees have shown higher rates of re-impregnation.

Re-treating nets is now easier than in the past: tablets or sachets are available to treat the nets, and they do not require any technical skill to use. As a treated net lasts for around six months, and in some circumstances perhaps longer, it would be useful to develop kits allowing for long-lasting efficacy, in terms of years instead of months. Other techniques to tackle the issue of re-treatment include factory pretreated nets, health education and promotional campaigns. However, factory-pretreated nets are of variable quality, and the amount of insecticide on them is unreliable. They should not be used.

This does not apply to Long-lasting Insecticide Treated Nets (LLINs), which are manufactured in a different way. Two brands have been approved by the WHO Pesticide Evaluation Scheme (WHOPES), and can be recommended for use. One brand is composed of permethrin inside a polyethylene thread. The other comprises deltamethrin stuck onto polyester fibre, however, experience with this net is limited, and only monitoring over time will show whether it lives up to its promise.

LLINs last, in general, for four to five years, and are wash-resistant up to 20 washes without the need for insecticide re-treatment. The LLINs made of polyethylene are stronger and more durable, and may be the most appropriate option for complex emergencies. It is certainly clear that, in LLINs, the insecticide is present and effective for much longer than in ITNs. Whatever type of net is being considered, quality control will always be important. WHO has recently established specifications and guidelines for net buyers and users.

Implementing ITN projects: operational issues and constraints

Many of the populations at most risk from malaria are extremely poor. Each illness episode costs a family substantial resources, and additional economic problems arise if malaria affects the men or women in charge of the household. The poverty implications of this disease are therefore enormous, making a compelling argument in favour of overall public-sector support for treated nets under normal conditions, with special emphasis on protecting pregnant women and young children. In the case of emergencies, ITNs should obviously be given free of charge.

The distribution of nets must be accompanied by health education sessions and work to raise awareness of mosquito-borne diseases, including malaria. UNICEF recommends that a good-quality education package needs to go hand-in-hand with the free distribution of ITNs. At household level, a sufficient number of ITNs must be distributed to cover the whole family. If nets are lacking, it is likely that children – who are among the most susceptible groups – will be excluded. In some emergencies, the distribution of any item of value, including ITNs, may make recipients a target for looting; in such situations indoor residual spraying (IRS) should be considered.

It could be difficult to distribute mosquito nets in camps for refugees or displaced people if the targeted beneficiaries are not accustomed to using mosquito nets. Nets must be culturally appropriate and accepted. Sometimes, nets are used not only for protection against insects, but also to provide privacy, warmth and protection against wind and dust. Keeping this in mind when setting up programmes and selecting tools would increase vector control. On the other hand, some aspects of camp life, such as the concentration of a previously dispersed population, may make it easier to distribute ITNs.

Efficacy and re-treatment under camp conditions may pose particular problems. Nets may easily be torn or damaged, and so lose their protective effect. It may also be difficult to ensure re-treatment every six months. Some types of temporary shelter may not be suited to ITNs, and may be too small to allow for hanging mosquito nets. In such circumstances, other vector control programmes like IRS should be considered. Conversely, in unstable security situations with frequent population displacement, ITNs may be a more reliable prevention method as the nets can be taken along during displacement.

Lastly, it is worth pointing out that the barriers to wide-scale bed net distribution are often not related to the particular situation, but to administrative and logistical constraints and the difficulty of transferring funds to the field; insufficient time to plan with partners; and transportation difficulties between ports and distribution sites.

Conclusion

It has been shown that, for nets to be maximally effective, coverage must be high, nets should be re-treated promptly (or long-lasting nets used) and individuals should properly deploy their nets each night. Net distribution needs to be accompanied by clear health education messages, a community participation programme and improved knowledge of at-risk groups. The size and shape of the nets have to be adapted to the size and shape of the houses, huts or dwellings concerned, and the preferences of the targeted people. Every opportunity should be taken to combine immunisation campaigns or other preventive programmes like antenatal care with the distribution and/or re-impregnation of nets. The biggest constraint, however, is the extremely slow rollout of ITN programmes, the ongoing debate about charging for nets and donor confusion about their effectiveness. Research is needed in emergencies to identify the factors and strategies that would achieve high coverage of ITNs in difficult environments.

Dr Willy Janssenhas worked extensively in complex emergencies in countries including Somalia (1991–93), Burundi (1993), Bosnia (1994–97) and the Democratic Republic of Congo (2004). He was malaria technical adviser for Belgian Technical Cooperation in national malaria control programmes in Rwanda and Mali in 1999–2003, and was malaria adviser for RBM/WHO in Eastern DRC in 2004. Currently, he works for Belgian Technical Cooperation in Rwanda.

References and further reading

S. E. Clarke et al., ‘Do Untreated Bednets Protect against Malaria?’, Trans Roy Soc Trop Med Hyg, 95, 2001: 457–62.

U. D’Allessandro et al., ‘Mortality and Morbidity from Malaria in Gambian Children after Introduction of Impregnated Bednet Programme’, The Lancet, 337, 1991: 1,499–1,502.

William Hawley et al., ‘Implications of the Western Kenya Permethrin-treated Bed Net Study for Policy, Program Implementation, and Future Research’, Am J Trop Med Hyg, 68 (sppl. 4), 2003, pp. 168–73.

Report of the Malaria Response in Emergencies Workshop, Tools, Capacity, and Coordination: Are We Reaching the Most Vulnerable Populations?, USAID, Washington DC, 21–22 April 2004.

M. Rowland et al., ‘Pyrethroid Impregnated Bednets for Personal Protection from Malaria for Afghan Refugees’, Trans Royal Soc Trop Med Hyg, 90: 357–61.

Report of a WHO meeting on best practices and lessons learned from implementing malaria control in complex emergencies in Africa 2000–2004, WHO/RBM, November 2004.