Indoor DDT spraying failed to add additional protection against malaria
The combined use of insecticidal bed nets and spraying insecticide inside homes was no more effective at protecting children against malaria than the use of bed nets alone, according to the results of a cluster randomized trial published in The Lancet.
“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,” researcher Steve W. Lindsay, PhD, disease ecologist at Durham University, United Kingdom, said in a press release. “High bed net use is sufficient to protect people against malaria in areas that have low or moderate levels of malaria like The Gambia.”
Lindsay and colleagues assessed the efficacy of indoor residual spraying vs. LLINs in a trial that included more than 8,000 children aged 6 to 14 years in 70 clusters of villages in the Upper River region of The Gambia. Village clusters were randomly assigned to receive LLINs alone (n=35) or LLINs plus indoor residual spraying with dichlorodiphenyltrichloroethane (DDT; n=35). The children were followed at the start of the 2010 transmission season through 2011, and clinical malaria was identified through passive case detection. The researchers also collected vector mosquitoes and parasites using standard light and exit traps inside homes to measure the extent of exposure to malaria transmission.
Coverage of indoor spraying and LLIN was high, reaching more than 80% from 2010 to 2011.
Results suggested the incidence of clinical malaria was similar between groups. In 2010, the incidence was 0.047 per child-month at risk in the LLIN group vs. 0.044 per child-month at risk in the indoor spraying plus LLIN group. In 2011, the incidence was 0.032 per child-month at risk in the LLIN group and 0.034 per child-month at risk in the indoor spraying plus LLIN group. The adjusted incidence rate ratio for all attacks during the 2-year study period was 1.08 (95% CI, 0.8-1.46).
Moreover, the researchers found no significant difference in the density of mosquitoes caught in light traps between the groups, suggesting indoor DDT spraying had no added benefit of killing or deterring vectors like Plasmodium falciparum and Anopheles gambiae sensu lato.
“Taken together with the results of previous studies, our findings do not support any universal recommendation for indoor residual spraying as an addition to LLINs across sub-Saharan Africa,” Lindsay and colleagues wrote. “Our advice is that high LLIN coverage is sufficient to protect people against malaria in areas of low or moderate transmission, but where LLIN coverage is low the cost-effectiveness of additional control with indoor residual spraying should be taken into account.”
In a related editorial, Jo Lines, PhD, and Immo Kleinschmidt, PhD, from the London School of Hygiene and Tropical Medicine, cautioned against forgoing indoor residual spraying based on the mixed results of four randomized controlled trials, including the one conducted in The Gambia.
All four trials, Lines and Kleinschmidt said, 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 programs investing in the combined use of the two methods should include a rigorous component of monitoring and assessment.”
Disclosure: The researchers report no relevant financial disclosures.