Mosquito larval source management: evaluating evidence in the context of practice and policy

  • By: Robert Newman, Abraham Mnzava & Zsofia Szilagyi
  • On: August 29, 2013, 10:04
thumbnail image: Mosquito larval source management: evaluating evidence in the context of practice and policy

Authored by Tusting and colleagues, a new Cochrane review published in the August 2013 issue of the Cochrane Database of Systematic Reviews, looks at the effectiveness of larval source management for malaria control.[1]  The publication of this review is timely, but it is disappointing to learn that there continues to be a lack of robust evidence on the impact of larval source management on parasite prevalence and malaria incidence. Only thirteen studies were deemed appropriate for inclusion, and the authors conclude that the quality of evidence in the review was only moderate.

In addition, it is concerning that the reviewers ultimately decided that the standard Cochrane meta-analysis approach was appropriate in the evaluation of larval source management, given that both the interventions used and the local settings in which they were applied were highly variable. The success of larval source management depends on the local environment as well as the particular approach used to kill the larvae. Therefore, great caution should be exercised in drawing generalizable conclusions from this current analysis.

Despite these caveats, the authors’ findings are largely in line with conclusions of other major reviews on larval source management as well as the World Health Organization’s (WHO) current position statement on larviciding.[2]

Malaria transmission occurs in 99 countries, with Africa being the most affected continent. In 2010, an estimated 219 million cases occurred (range: 154 – 289 million) while the disease caused about 660,000 deaths (range: 490 000 – 836 000), mainly in children under five years of age.[3]  In recent years, the expansion of proven and cost-effective vector control interventions – i.e. the use of long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) – and a scale-up of diagnostic testing and treatment has led to a 26% reduction in malaria mortality rates around the world and an estimated 33% drop in Africa alone.

To date, larval source management has been a marginal intervention in Africa, applied mainly in urban, low-burden and elimination settings. It is not considered to have made a tangible contribution to the recent successes. Despite this, larval control interventions have attracted a significant amount of political attention in recent years. Most headlines have been generated by larviciding, which involves the regular application of a biological or chemical insecticide to water bodies to reduce the number of mosquito larvae and pupae. Following a 2012 review by its Malaria Policy Advisory Committee, WHO now recommends that larviciding should only have a limited role in malaria control; it is deemed appropriate only in areas where mosquito breeding sites are few, fixed and findable.

The growing interest in larviciding has been fueled by a desire to find new solutions for malaria control in Africa and to reduce dependence on foreign aid and on foreign-manufactured commodities. Larviciding later received priority status under a tripartite agreement, signed in 2009 between Venezuela, Cuba and the Economic Community Of West African States (ECOWAS), a regional group of fifteen countries. Under this agreement, the two Latin American countries are actively helping to expand larviciding activities in Africa, including through the provision of financial support and technology transfer for the establishment of Africa-based factories for larviciding products.

However, some of the larviciding products that have entered the African market have not been reviewed by the WHO Pesticide Evaluation Scheme (WHOPES), which coordinates the assessment of larval control compounds and formulations for safety and efficacy. The use of non-recommended products may result in poor efficacy of the intervention, and may also have a serious adverse impact on human health and the environment. Of note, the use of recommended products is not itself a guarantee of success, as an incorrect targeting and application of larviciding can also lead to intervention failure.

WHO is also concerned that the rise of interest in larviciding could result in a diversion of financial resources away from scientifically-proven vector control tools to methods whose effectiveness remains far less certain. Given that future projections on global malaria funding are not encouraging, and that there is an annual funding gap of 2.8 billion USD, these trends could prove devastating for communities in localities and regions where governments decide to scale down LLINs or IRS to finance larviciding programmes instead.

To respond to the growing interest in larviciding, WHO released an operational manual in July 2013 to provide step-by-step guidance to malaria endemic countries on the planning, implementation, management and evaluation of larval source management programmes.[4]  WHO stressed that larval control programmes need to be fully tailored to local environmental conditions and should be based on comprehensive feasibility and cost-effectiveness studies. The organization also pointed out that larval source management is a major financial and technical undertaking, requiring both community support and long-term political commitment. It also requires a cadre of trained field personnel, entomologists and public health professionals with detailed knowledge of local malaria transmission and vector control, ability to gather, analyse, and act upon detailed monitoring data, and capacity to manage complex personnel and logistics systems.

It is hoped that the Cochrane review by Tusting et al will contribute to the scientific debate over the applicability of larval control interventions, and also lead to more research regarding the public health impact of larval control programmes in different settings and using different approaches. The WHO Vector Control Technical Expert Group and the WHO Malaria Policy Advisory Committee to which it reports will review additional evidence as it becomes available. Until there is more compelling evidence, larval control should continue to be viewed as a supplementary measure for malaria control in carefully selected settings. Promoting the widespread use of larval source management in rural areas of sub-Saharan Africa would be premature.

Robert D. Newman, Abraham Mnzava, and Zsofia Szilagyi

WHO Global Malaria Programme (infogmp@who.int)

How to cite: Newman RD, Mnzava A, Szilagyi Z. Mosquito larval source management: Evaluating evidence in the context of practice and policy [editorial]. Cochrane Database of Systematic Reviews 2013;8:ED000066. dx.doi.org/10.1002/14651858.ED000066


References:

[1] Tusting LS, Thwing J, Sinclair D, Fillinger U, Gimnig J, Bonner KE, Bottomley C, Lindsay SW. Mosquito larval source management for controlling malaria. Cochrane Database of Systematic Reviews 2013;8:CD008923. DOI: 10.1002/14651858.CD008923.pub2. 
[2] World Health Organization. Interim position statement: The role of larviciding for malaria control in sub-Saharan Africa. Geneva: WHO, 2012. Available at: www.who.int/entity/malaria/publications/atoz/larviciding_position_statement/en/
[3] World Health Organization. World Malaria Report. Geneva: WHO, 2012.
[4] World Health Organization. Larval source management: a supplementary measure for malaria vector control: an operational manual. Geneva: WHO, 2013. Available at: http://www.who.int/malaria/publications/atoz/9789241505604/en/index.html

Competing interests: The authors have completed the Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (available upon request) and declare (1) no receipt of payment or support in kind for any aspect of the article; (2) no financial relationships with any entities that have an interest related to the submitted work; (3) no other relationships or activities that could be perceived as having influenced, or giving the appearance of potentially influencing, what was written in the submitted work.

Image credit: Sinclair Stammers/Science Photo Library

Contact the Editor in Chief, Dr David Tovey (dtovey@cochrane.org): Feedback on this editorial and proposals for future editorials are welcome.

Keywords:

The Cochrane Library - Independent high-quality evidence for health care decision making

Copyright © 2014 John Wiley & Sons, Ltd. All rights reserved