Indoor residual spraying to control malaria: how effective is it?

Paul Chinnock | 06 May 2010
Indoor residual spraying of insecticides (IRS) is accepted as being an effective way of killing mosquitoes and of reducing rates of malaria transmission. IRS, using the insecticide DDT, is generally credited with much of the success achieved in eradicating malaria from several countries (mostly in the Americas and Europe) in the 1950s. It is now regarded as one of the key tools in efforts to control - and eventually eliminate - malaria in highly-endemic areas in Africa and Asia.

But exactly how effective and how cost-effective is IRS? How does it compare in terms of effectiveness with other interventions to control malaria, in particular the use of insecticide-treated bednets (ITNs)? Such information would be helpful in policy and planning. A Cochrane systematic review, therefore, sought to find data that would enable the effectiveness of IRS to be quantified. The reviewers concluded, however, that insufficient research has so far been conducted to allow estimates to be made.

The reviewers, from South Africa and from the Swiss Tropical and Public Health Institute, restricted their review to data from controlled studies. They looked for cluster randomized controlled trials (RCTs), controlled before-and-after studies (CBAs) and interrupted time series (ITS) that compared IRS to no IRS, or to ITNs. To be included in the review, studies had to contain information on pre-specified "key malariological parameters", such as case numbers, parasite prevalence, haemoglobin levels, and all-cause death rates in children under 10.

Of 134 potentially relevant papers found in an electronic search, only six met the reviewers' inclusion criteria. Four were in sub-Saharan Africa and two in the Indian subcontinent. Three were in areas where malaria transmission is regarded as stable and three where it is unstable. Four were RCTs, one a CBA study and one an ITS. In four of the studies, the use of IRS was compared with no IRS and in three of them IRS was compared with ITNs.

The reviewers note that from the studies that met their inclusion criteria there was, in particular, a lack of information such questions as the effectiveness of different types of insecticide and the impact of IRS on child mortality.

The data from the included studies support the view that IRS is effective, but the number of studies was too low to properly quantify that effect. Most of the RCT data came from unstable malaria settings; evidence on the use of IRS in stable malaria settings was very limited. Some of the data suggested that ITNs give better protection than IRS in unstable areas but more trials are needed before any firm conclusions can be drawn.

While data has been recorded from many programmes where IRS has been introduced, in the great majority of cases no control information is available for comparison. Consequently, such data was not part of this review. Interviewed by Science Daily one of the review team, Professor Christian Lengeler, said, "Clearly, this is disappointing. With such a large body of evidence it is sad that so few of the studies provide high-quality scientific evidence. This represents a great waste of resources and efforts, and we are left with a very poor evidence base".

Speaking of the research that is now needed, Professor Lengeler said, "Because we know that IRS works, it is no longer ethically possible to carry out studies with control groups that don't receive anything. Consequently, we urgently need more high-quality studies that compare IRS with the other widely implemented vector control method, ITNs".

The effectiveness of IRS programmes may vary according to the epidemiological and socio-cultural characteristics of the areas where programmes are implemented. Research should therefore be conducted in a range of different settings. Another challenge for researchers is that, increasingly, malaria control programmes are multifaceted and involve the use concurrently of several control tools, including both IRS and ITNs as well as improved diagnostic and treatment facilities. Evaluation of the effectiveness of these combined approaches will be crucial.