London School of Hygiene & Tropical Medicine

There is no need to spray DDT insecticide on walls for malaria control when people sleep under treated bed nets, according to new research published in The Lancet.

Researchers from the London School of Hygiene & Tropical Medicine working in collaboration with Durham University, the Medical Research Council (MRC) Unit in The Gambia, and the National Malaria Control Programme in The Gambia,  found that the insecticide DDT sprayed on internal walls, combined with insecticide-treated bed nets in homes does not protect children from malaria any more effectively than using insecticide-treated bed nets alone.

Malaria is spread by mosquitoes that become active and feed at night, leading to many people combining two anti-mosquito measures while they sleep. The researchers were curious to find out if there were any benefits to using multiple methods of prevention.

Their research took place in 96 villages in The Gambia, with 8,000 children checked for malaria over two years between 2010 and 2011. Insect traps were placed in houses to establish levels of exposure to malaria-carrying mosquitoes and parasites.

At the end of the study the data showed no difference between the numbers of cases of malaria where a combination of spraying DDT and nets was used, compared with the use of bed nets alone. What is more, insecticide spraying did not reduce the number of malaria-carrying mosquitoes collected from the treated homes, suggesting that spraying has no additional effect in deterring or killing mosquitoes.

The findings were not expected by the research team. Study co-author David Conway, Professor of Biology at the School, said: "We were all surprised by this result. The trial was codenamed SANTE - Spraying and Nets Towards Elimination - because there was a general idea that combining these individually effective interventions might be a really potent way to drive malaria out of some communities in Africa. The fact that this was not the case shows the importance of doing such large randomised studies in real world settings."

After analysing their results, the researchers recommended that insecticide is not used for spraying on walls in areas where there is a high use of long-lasting insecticide nets, and low to moderate numbers of malaria cases. They suggest that concentrating resources on buying additional bed nets, and encouraging their correct use could be a more cost-effective solution to help combat malaria.

Writing about the findings in a comment piece in The Lancet, School researchers Dr Jo Lines and Prof Immo Kleinschmidt discuss the mixed results of published trials. They caution: "All four experimental trials (including Pinder and colleagues' study in The Gambia) were designed to test the null hypothesis of no difference between the study groups, and because of this, those that did not find a significant difference should not be interpreted as proof of the absence of a benefit. 

"The reason for this mixture of findings is not immediately clear, and a range of possible explanations related to differences in the trial settings and methods can be suggested, including vector species, insecticides used for indoor residual spraying, effective coverage (of each intervention), and insecticide resistance to one or other of the insecticides used… In view of the uncertainties that persist, it is advisable that all national malaria control programmes investing in the combined use of the two methods should include a rigorous component of monitoring and assessment."

Prof Conway added: "The results do not mean that indoor residual spraying will never be beneficial if it is used in a situation of high bed net use in any other area. It is important to emphasise that Africa is a very large and diverse continent, with many different ecological situations, and this illustrates the general need to conduct a lot more research in different settings before making blanket recommendations about malaria control or any other public health issue."

The research was funded by the MRC and led by Durham University and the Medical Research Council's Unit in The Gambia.

Publications

• Margaret Pinder et al. Efficacy of indoor residual spraying with dichlorodiphenyltrichloroethane against malaria in Gambian communities with high usage of long-lasting insecticidal mosquito nets: a cluster-randomised controlled trial. The Lancet. DOI: 10.1016/S0140-6736(14)61007-2
• Jo Lines and Immo Kleinschmidt. Is malaria control better with both treated nets and spraying? The Lancet DOI:10.1016/S0140-6736(14)61306-4