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Malaria mosquito nets

Insecticide-treated mosquito nets have revolutionised malaria prevention, and have the potential to save millions of lives.

Nets treated with biodegradable insecticides protect people from malaria in two ways: by physically preventing malaria-carrying mosquitoes reaching the skin and by killing the mosquito when it lands on the net. The MRC Laboratories in The Gambia first demonstrated this in 1984, and other MRC studies later showed that use of insecticide-treated mosquito nets resulted in a 63 per cent reduction in deaths in children under 5 years1.

In 1897, Ronald Ross – who was the first Briton to win the Nobel Prize – discovered malarial parasites in mosquitoes in India and suggested that nets might provide protection.

Net advantage

Treated nets were used during the Second World War and then reconsidered in the 1980s when synthetic pyrethroids, which are safe insecticides, came on the market. Treated nets are cheaper and more effective than spraying insecticide: treating a family’s nets needs only about one sixth as much insecticide as indoor residual spraying of their house2. Also, nets can be re-impregnated.

The MRC’s work triggered large-scale trials in northern Ghana, Kenya and The Gambia, which led the UN, World Bank and WHO Tropical Diseases Research Programme to fund research to improve methods for treating nets with insecticide. Since 1998, insecticide-treated nets have been used in the WHO’s Global Malaria Programme. This aims for 80 per cent of people in Africa at risk of malaria to be using treated nets by 2010. The WHO estimates that malaria causes over a million deaths a year, the vast majority children.

In Vietnam, insecticide-treated nets were introduced in 1991 and gradually took over from spraying. Consequently, deaths from malaria were virtually eradicated there by 20033.

Treated nets have both a personal protection effect to the individual user and a community-wide effect because the occupied nets act as baited traps for mosquitoes. For people sleeping in the vicinity of treated nets, even if they do not have their own nets, risk from bites decreases by 75 per cent. Personal nets reduce bites to an individual by a further 69 per cent4.

Getting them out

Researchers have shown that donation, rather than selling, is the best way of distributing nets to poor families in African villages. When net donation was combined with a measles vaccination campaign in Zambia, the use of nets increased from 17 to 80 per cent5. Now there are programmes all over Africa and Indonesia linking vaccination campaigns and free nets. Over the last two years, 38 million long-lasting insecticidal nets have been distributed free of charge linked to measles vaccination6. Over three years these nets can be expected to prevent more than 500,000 deaths (based on data from West Africa and Kenya indicating that 5.5 deaths are prevented each year for every 1,000 children using an insecticidal net, 75 per cent of nets being used, 1.7 children using each net and nets remaining effective for three years7).

The costs of providing treated nets for 1,000 people for one year, including annual replacement, cost of insecticide, labour cost and transport, is £540 - less than half of what spraying would cost2. Costs of free nets, including insecticide, labour and transport, for the 350 million people in lowland tropical Africa is £189m a year, which would be almost halved if more expensive nets that did not need replacing were used4. Even without the saving, the cost is 17 per cent of what is spent on de-fleaing cats in Europe and the USA8!

The cost of disease

Malaria is a prime factor holding back economic and social development, especially in Africa9. The direct costs of malaria include a combination of personal and public expenditures on both prevention and treatment of the disease. According to the WHO, in some countries malaria may account for 40 per cent of public health spending. The indirect costs of malaria include lost productivity or income associated with illness or death.

Malaria’s direct and indirect costs have been shown to be a major constraint to economic development. Annual economic growth between 1965 and 1990 in countries with severe malaria averaged 0.4 per cent of GDP per capita, compared with 2.3 per cent in the rest of the world. Malaria prevention is therefore of interest not just to the developing countries themselves, but also to the developed world.

References

1 Alonso et al. (1991). The effect of insecticide-treated bed nets on mortality of Gambian children. The Lancet, 337, 1499.

2 Curtis et al. (1998). A comparison of use of a pyrethroid either for house spraying or for bednet treatment against malaria vectors. Trop Med Inl Hlth, 3, 619

3 Curtis et al. (2003). Insecticide treated nets: impact on vector populations and relevance of initial intensity of transmission and pyrethroid resistance. Vect Borne Dis, 40, 1

4 Curtis et al. (2006). Insecticide-treated bednets. J Amer Mosq Contr Assoc, 2, 501

5 Grabowsky et al. (2005). Integrating insecticide-treated bednets into a measles vaccination campaign achieves high, rapid and equitable coverage with direct and voucher-based methods. Trop Med Inl Hlth, 10, 1151

6 Data from Mark Grabowsky, Measles and Malaria Partnership and Yemane Ye-ebiyo, Centre for National Health Development, Ethiopia

7 Lengeler (2004). Insecticide treated bednets and curtains for malaria control: a Cochrane review. The Cochrane Library, issue 3, Oxford, UK

8 Rust, (2005). Advances in the control of Ctenocephalides felis (cat flea) on cats and dogs. Trends in Parasitology, 21, 232

9 Sachs & Malaney, (2002). The economic and social burden of malaria. Nature, 415, 680

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