INDOOR RESIDUAL SPRAYING FOR PREVENTION OF MALARIA

Tuesday, 28th of August 2012

• INDOOR RESIDUAL SPRAYING FOR PREVENTION OF MALARIA

The Lancet Infectious Diseases, Volume 12, Issue 8, Pages 581 - 582, August 2012

Published Online: 07 June 2012

Indoor residual spraying for prevention of malaria

Original Text

Raphael N'Guessan a b, Mark Rowland a

Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) are the two primary vector control interventions for malaria prevention. Use of LLINs in malaria endemic sub-Saharan Africa has, with support from the Global Fund and other international agencies, become much more common in the past decade. LLINs are deemed a simpler intervention to implement than IRS, whereas IRS has the potential to reduce transmission more rapidly, if campaigns are implemented rigorously and with high levels of household coverage.

IRS has some advantages over LLINs for outbreak control but is usually perceived as a time-limited intervention and its application is difficult to continue indefinitely because of high cost implicated. Nevertheless, in 2010 alone, the President's Malaria Initiative attempted to protect more than 27 million residents of 15 African countries, including Benin, by means of IRS with DDT or pyrethroid insecticides.1 In countries where pyrethroid resistance is already prevalent, such as Benin, the government has embarked on a strategy of campaigns promoting high LLIN coverage plus IRS with an insecticide of choice, bendiocarb, a carbamate with activity of 2—6 months.2

In The Lancet Infectious Diseases, Vincent Corbel and colleagues3 present a cluster randomised trial of LLIN and IRS in 28 villages in southern Benin, from 2007 to 2010, the first of several studies ongoing in Africa. This unique randomised trial is soundly done, despite some limitations.

The primary aim of the study3 was to examine whether the IRS every 8 months, as practised by a President's Malaria Initiative programme, provided additional benefit over existing LLINs. The outcome variables were clinical malaria, mosquito population densities, and selection of vector resistance to pyrethroids and carbamates. The surprising conclusion was that in an endemic malaria setting with perennial transmission and high frequency of pyrethroid resistance, IRS with a short-lived carbamate insecticide once a year provides no additional benefit to LLINs in the reduction of malaria incidence or prevalence: the reasons are not fully clear. The investigators found no indication of carbamate resistance having been selected. Perhaps, spraying annually was just not sufficient given the short-lived residual activity of bendiocarb.2 Whether IRS did enhance LLIN efficacy during the active period of bendiocarb still needs to be studied.

Corbel and colleagues3 also trialled a new vector control intervention of durable wall lining treated with a bendiocarb (carbamate-treated plastic sheeting), which can last longer than spray residue.4 This hope was confirmed by means of mosquito bioassay. The wall liners were retreated rather frequently but, more importantly, the coverage of durable lining on interior surfaces was far from complete. We fear that some mosquito surviving the exposure to LLINs could have encountered untreated refugia, leading to ineffectiveness of the combination and spurious conclusion about the strategy as a whole. This possibility was recognised by the authors as a major limitation.

The investigators did a secondary analysis to identify whether pyrethoid resistance negatively affected LLIN efficacy. Results showed that the capacity of LLINs to provide protection was affected in an environment in which resistance was prevalent. However, this finding is far from conclusive because no suitable control group was available. The coverage of LLIN was quite similar across the study groups and the frequency of pyrethroid resistance (kdr) rose to similarly high rates—ie, to 86% (95% CI 80—92) in the group with LLIN coverage targeted to pregnant women and children younger than 6 years and to 91% (84—98) in the group with universal coverage of LLINs.

Corbel and colleague's study should not discourage initiatives to deploy IRS for malaria transmission control in endemic countries. The study draws attention to the need to improve the timing and duration of spray cycles in endemic settings. Elsewhere, for example in Bioko, Equatorial Guinea, application of bendiocarb has been used to good effect to control pyrethroid-resistant Anopheles gambiae.5 Further trials are needed in other settings to reach stronger conclusions. The findings do indicate that the policy of adding IRS with short-lived insecticide to LLINs without the resources to repeat the spray cycles will provide only short-term gains.

We hope that the study will help encourage the development of alternative long-lasting formulations of active ingredients for IRS to combine with LLINs within homes for more cost effective control of malaria and management of resistance in sub-Saharan Africa.

The threat posed by pyrethroid resistance must be assessed and tackled vigorously.6, 7 Because of its easy detection, kdr is always monitored as proxy of the resistance issue, whereas more complex packages of resistance are common.8—10 Efficient methods now exist to diagnose associate members of the cytochrome P450 genes involved in pyrethroid detoxification in anophelines.11 Direct assessments of their combined expression on phenotypic resistance are needed.

We declare that we have no conflicts of interest.

1 The President's Malaria Initiative. Fifth annual report to Congress. Washington DC: USAID, 2011. http://www.pmi.gov/resources/reports/index.html. (accessed May 27, 2012)

2 WHO. Guidelines for testing mosquito adulticides intended for indoor residual spraying (IRS) and insecticide treated nets (ITNs). WHO/CDS/NTD/WHOPES/GCDDP/2006.3.

3 Corbel V, Akogbeto M, Damien GB, et al. Combination of malaria vector control interventions in pyrethroid resistance area in Benin: a cluster randomised controlled trial. Lancet Infect Dis; published online June 7, 2012. DOI:10.1016/S1473-3099(12)70081-6.

4 Djènontin A, Chandre F, Dabiré KR, et al. Indoor use of plastic sheeting impregnated with carbamate combined with long-lasting insecticidal mosquito nets for the control of pyrethroid-resistant malaria vectors. Am J Trop Med Hyg 2010; 83: 266-270. CrossRef | PubMed

5 Sharp BL, Ridl FC, Govender D, Kuklinski J, Kleinschmidt I. Malaria vector control by indoor residual insecticide spraying on the tropical island of Bioko, Equatorial Guinea. Malar J 2007; 6: 52. CrossRef | PubMed

6 Trape JF, Tall A, Diagne N, Ndiath O, et al. Malaria morbidity and pyrethroid resistance after the introduction of insecticide-treated bednets and artemisinin-based combination therapies: a longitudinal study. Lancet Infect Dis 2011; 11: 925-932. Summary | Full Text | PDF(276KB) | PubMed

7 Rehman AM, Coleman M, Schwabe C, et al. How much does malaria vector control quality matter: the epidemiological impact of holed nets and inadequate indoor residual spraying. PLoS One 2011; 6: e19205. CrossRef | PubMed

8 Corbel V, N'Guessan R, Brengues C, et al. Multiple insecticide resistance mechanisms in Anopheles gambiae and Culex quinquefasciatus from Benin, West Africa. Acta Trop 2007; 101: 207-216. CrossRef | PubMed

9 N'Guessan R, Corbel V, Akogbeto M, Rowland M. Reduced efficacy of insecticide-treated nets and indoor residual spraying for malaria control in pyrethroid resistance area, Benin. Emerg Infect Dis 2007; 13: 199-206. PubMed

10 Koffi AA, Ahoua Alou LP, Adja MA, Kone M, Chandre F, N'Guessan R. Update on resistance status of Anopheles gambiae s.s. to conventional insecticides at a previous WHOPES field site, “Yaokoffikro”, 6 years after the political crisis in Cote d'Ivoire. Parasit Vectors 2012; 5: 68. PubMed

11 Müller P, Donnelly MJ, Ranson H. Transcription profiling of a recently colonised pyrethroid resistant Anopheles gambiae strain from Ghana. BMC Genomic 2007; 8: 36. PubMed

a London School of Hygiene and Tropical Medicine, Keppel Street, London, UK

b Centre de Recherche Entomologique de Cotonou, Cotonou, Benin