Combining insecticide spraying and bed nets no more protective against malaria than nets alone

Lead author Professor Steve Lindsay, a disease ecologist at Durham University in the UK explains, "Our findings do not support any universal recommendation for indoor residual spraying as an addition to long-lasting insecticidal nets (LLINs) across sub-Saharan Africa. High bed net use is sufficient to protect people against malaria in areas that have low or moderate levels of malaria like The Gambia."

In 2012, there were an estimated 207 million cases of malaria and 627,000 deaths worldwide, mostly among African children. Low-cost solutions to prevent the disease include LLINs or indoor residual spraying, which involves spraying insecticide on walls and roofs where mosquitoes are likely to land. Both have been shown to reduce the number of deaths due to malaria. But, until now, there has been little evidence to address whether combining these interventions would give better results.

This cluster-randomised trial involved almost 8000 children aged 6 months to 14 years in 70 clusters of villages in the Upper River region of The Gambia. Village clusters were randomly assigned to receive either LLINs alone or LLINs plus indoor spraying with dichlorodiphenyltrichloroethane (DDT). Cases of clinical malaria were measured in a group of children in each cluster in 2010 and 2011 using passive case detection (identifying cases that present at reporting health facilities) and annual surveys. Mosquito traps in houses were used to establish exposure to malaria-carrying mosquitoes and parasites. Coverage of indoor spraying (˃80%) and LLINS (83-95%) were high in both years of the study.

The researchers found that clinical cases of malaria were similar in both groups. In year 1, incidence of clinical malaria was 0·047 per child-month at risk in the LLIN group and 0·044 in the spraying plus LLIN group. In year 2, incidence was 0·032 per child-month at risk in the LLIN group and 0·034 in the spraying plus LLIN group (see table 4, page 6 in the Article).

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 researchers say that in order to get a proper measure of impact, further studies are needed to assess the effectiveness of combining LLINs with indoor spraying in areas with different levels of malaria transmission. They also recommend that where net coverage is low, the cost-effectiveness of additional control with indoor insecticide spraying should be considered.

Writing in a linked Comment, Jo Lines and Immo Kleinschmidt from the London School of Hygiene & Tropical Medicine in the UK 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."