Insecticide-treated mosquito nets have replaced house spraying as the preferred method for preventing malaria in many endemic areas. But which is most effective? Entomologists from the London School of Hygiene and Tropical Medicine and the South African Medical Research Council reviewed trials of the two strategies in Africa, Asia and Melanesia.
Recent results with treated nets are comparable with contemporary spraying trials but older spraying projects were more successful. Longer trials of bednets might reveal a greater impact and the almost universal use of pyrethroid insecticides should be reviewed. House spraying is a public health service and, if switching to nets, programmes should not try to make the poor pay for them.
In the 1940s-60s, spraying the inside surfaces of houses with insecticide, usually DDT, was the main weapon against malaria. Governments and donors are increasingly unable or unwilling to fund large-scale spraying programmes and there has been a recent resurgence of malaria. In the last 15 years, pyrethroid-treated bednets have become more popular as a means of controlling mosquitoes.
The arguments for and against using nets include:
- Most mosquitoes bite indoors late at night; bednets intercept mosquitoes as they search for sources of blood.
- Mosquitoes may not rest long enough on walls and ceilings to pick up a lethal dose of insecticide.
- Householders often re-plaster mud walls after they have been sprayed with insecticide.
- Net treatment requires less insecticide per family than house spraying. This can offset the cost of bulk-purchased nets, which last for several years.
- Community members must be actively involved to ensure that nets are used when it is uncomfortably hot and when there are not enough nuisance insects to make net use seem worthwhile.
- For nets to be fully effective, people should go to bed when malaria mosquitoes start biting and stay there until they stop.
- Nets must be kept in good repair, hung carefully and re-treated regularly.
This study found that:
- Insecticide-treated nets were at least as effective as spraying in six recent side-by-side comparisons.
- However, these recent results were not as good as several older rigorous spraying trials.
- Most of the insecticide-treated net trials have not continued or been reported for long enough to show their full potential.
The researchers recommend that policy-makers engaged in the fight against malaria should:
- consider the use of powerful insecticides with relatively low-irritancy to mosquitoes, such as fenitrothion and carbosulfan, rather than currently-used pyrethroids
- maintain a capacity to react to malaria epidemics by house spraying
- recognise the community-wide benefit of reducing mosquito numbers by applying insecticide to nets or walls in a high percentage of homes, particularly in protecting non-immune displaced groups or preventing the spread of infection into malaria-free areas
- avoid trying to transferring the responsibility for malaria control from donors or taxpayers to poor individuals if there is a switch from house spraying to insecticide-treated nets
- ensure high net re-treatment rates through a system of annual visits by trained supervisors providing free insecticide to village health workers.
Source(s):
'Comparison of house spraying and insecticide-treated nets for malaria
control' by C. Curtis and A. Mnzava, Bulletin of the World Health Organisation
78 (2000)
'Treated nets vs house spraying' by C. Curtis and A. Mnzava, Bulletin of
the World Health Organisation 79 (2001)
id21 Research Highlight: 15 August 2001
Further Information:
Christopher Curtis
Professor of Medical Entomology
London School of Hygiene and Tropical Medicine
London WC1E 7HT
UK
Tel:
+44 (0)20 7927 2339
Fax:
+44 (0)20 7636 8739
Contact the contributor: chris.curtis@lshtm.ac.uk
London School of Hygiene and Tropical Medicine, UK
Abraham Mnzava
Entomologist for Southern Africa
WHO Regional Office for Africa
95 Park Lane
Harare
Zimbabwe
Contact the contributor: mnzavaa@who.co.zw
WHO Regional Office for Africa
Other related links:
The Malaria Foundation has information on many malaria-related issues,
including research, plus a forum for discussion.
Check the Roll Back Malaria site for the latest news and information on
this WHO initiative.
See the website of the WHO's pesticide evaluation scheme.
The Multilateral Initiative on Malaria is an international collaboration
for scientific research into malaria.
