The combined use of spraying insecticide inside homes and
insecticide-treated bed nets is no better at protecting children against
malaria than using bed nets alone, a study in The Gambia suggests. The
findings, published in The Lancet, should encourage donors to invest
their limited resources in additional bed nets, the more cost-effective
solution to tackling malaria.
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."
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