Indoor residual spraying is increasingly being adopted as a vector control strategy in areas of intense, year-round transmission in sub-Saharan Africa. We present the direct and indirect impact of an IRS program using lambda-cyhalothrin in an area of intense malaria transmission in Malawi. Compared with areas of the same district with similar transmission potential, children who lived in an IRS area (regardless of whether the house that they lived in was sprayed) had lower prevalence of malaria parasitemia and anemia, even after adjusting for bed net use, socioeconomic status, and house construction. These findings suggest that one round of IRS per year might reduce malaria infection burden and anemia in areas with potential for endemic malaria transmission.
However, the impact of yearly IRS in this area needs to be interpreted with caution because the survey was conducted six months after the second spray round. Indoor residual spraying in Nkhotakota District reduced An. gambiae complex and An. funestus populations as measured by window exit traps and pyrethrum spray catches (Mzilahowa T, unpublished data). In addition, residual insecticide activity was noted for at least six months after the spray round using wall cone bioassays with susceptible Kisumu strain An. gambiae s.s. Given the maximum expected residual half-life on sprayed surfaces of lambda-cyhalothrin of approximately six months, we would expect to have some effect of IRS on vector control at the time of the survey, approximately six months after the second spray round. However, given the timing of the survey, we cannot determine if a single round of IRS was sufficient to maintain adequate vector control for the whole year.
In this study, we were able to detect a community-wide effect of IRS; all children who resided in the IRS area were less likely to have malaria parasitemia or anemia regardless of whether they lived in a household that had received IRS. This finding is consistent with our understanding of how IRS works and suggests that a community-wide effect can be detected even with IRS coverage of only 77% of households in a community. Whether this degree of coverage is adequate in all settings needs to be studied further.
In our analysis, we did not find an association between ITN or untreated bed net use and either reduced malaria parasitemia or anemia prevalence in children less than five years of age. We might not detect an impact of bed net use on disease burden because of our relatively small sample size. Of note, in a larger analysis using data from eight districts in Malawi, we found a significant association between ITN use and reduced parasitemia and anemia prevalence.34
Although we explored the potential impact of the combined use of IRS and ITNs using an interaction term in our model (received IRS × slept under an ITN), we did not have a sufficiently large sample size to detect any additional benefit. This finding is an area of increased interest to the global malaria control community and deserves further study using rigorous field trials.
The U.S. President's Malaria Initiative expanded support for IRS to all of Nkhotakota District (74,772 structures) in November 2009 and to all of Nkhotakota and adjacent Salima Districts in November 2010 (approximately 163,000 structures).30
In addition, the National Malaria Control Program with the use of Government of Malawi funds has initiated an IRS program in five additional districts in the lakeside and Shire Valley, including Nkhata Bay, Nsanje, Mangochi, Karonga, and Chikwawa Districts, which will cover an additional 650,000 houses and approximately 2.7 million persons.30
Although results of this analysis suggest a substantial impact of IRS using lambda-cyhalothrin on parasitemia and anemia prevalence, recent reports of pyrethroid resistance in A. funestus
in Nkhotakota District (Mzilahowa T, unpublished data) and Likoma Island, Malawi,38
phenotypically similar to pyrethroid-resistant An. funestus
in other parts of southern Africa,39–43
are concerning and could undermine the efficacy of IRS with pyrethroids and ITNs. In response to these reports, the IRS program in Nkhotakota and Salima Districts has replaced pyrethroids with pirimiphos-methyl, an organophosphate. Close monitoring of the IRS program in Malawi will be needed to assess the impact of insecticide resistance on IRS efficacy and to guide the choice of insecticide for future spray rounds.
Given the cross-sectional nature of this household survey, we were unable to control or even measure all known confounders between the IRS and non-IRS areas of Nkhotakota District. There were some differences between northern and southern Nkhotakota in socioeconomic status, which could potentially affect the prevalence of malaria in each area. However, the northern and southern parts of the district have similar climate suitability to maintain malaria transmission and had similar levels of malaria parasitemia and anemia prevalence in 2007 before the introduction of IRS. To mitigate potential differences between the two study areas, we used an adjusted analysis to control for socioeconomic status, house construction, and the use of malaria control interventions such as ITNs. In addition, we were hampered by a relatively small sample size and thus unable to further explore the impact of bed nets either alone or in combination with IRS on parasitemia and anemia in children less than five years of age.
In this cross-sectional household survey, IRS was associated with reduced malaria parasitemia and anemia prevalence in children less than five years of age. This study supports the use of IRS in areas with intense transmission such as much of sub-Saharan Africa, but should be interpreted with caution. The impact of IRS is dependent on the use of an effective insecticide, adequate coverage, and a sufficient number of spray rounds per year to adequately reduce vector populations. The study found no impact of ITNs, regardless of whether a child lived in an area covered by IRS. However, previous work in Malawi has shown a clear protective effect from ITNs34
and a larger sample size may be required to address this specific question. As Malawi expands its IRS program and faces the threat of the spread pyrethroid-resistant An. funestus
, close monitoring is needed to ensure that IRS continues to be effective.