A malaria control method that targets mosquito larvae and pupae as they mature in standing water could be an important supplementary measure in the fight against the disease, according to a new report.
The Cochrane review—led by the London School of Hygiene & Tropical Medicine in collaboration with Durham University and other researchers in the UK and US—is the first systematic review looking at using larval source management (LSM) to control malaria, which causes an estimated 660,000 deaths worldwide every year. It found evidence that the method may significantly reduce both the number of cases of malaria by up to 75%, and the proportion of people infected with the malaria parasite by up to 90% when used in appropriate settings.
LSM is a method that targets immature mosquitoes, found in standing water, before the females develop into flying adults that are capable of transmitting malaria. This is done by permanently removing standing water, for example by draining or filling land; making temporary changes to mosquito habitats to disrupt breeding, for example by clearing drains to make the water flow; or larviciding by adding chemicals or biological larvicides to standing water to kill larvae.
Currently the use of long-lasting insecticide treated bed nets and indoor residual spraying of homes are widely used for malaria transmission control and are having a major impact on the burden of the disease. However, this success is now threatened by factors such as a growing resistance to insecticides among mosquitoes. Complementary methods of mosquito control such as LSM may become increasingly necessary in helping tackle the disease, and some malaria-endemic countries in Sub-Saharan Africa are already implementing LSM programmes, but there is a lack of consensus on how effective the method can be and in which settings it is appropriate. Few studies have so far been conducted to rigorously evaluate the intervention.
In this review, researchers identified 13 eligible studies from countries in Sub-Saharan Africa and Asia, plus one from Europe, that trialled various combinations of LSM methods in a wide variety of habitats and countries.
The authors' analysis of the data from Eritrea, The Gambia, Kenya, Mali, Tanzania, India, the Philippines, Sri Lanka and Greece suggested that where larval habitats are not too extensive and a sufficient proportion of these habitats can be targeted, LSM may reduce the number of cases of malaria and the proportion of people infected with the malaria parasite at any one time. The findings also suggest LSM could contribute to a reduction in the prevalence of splenomegaly in children (an enlargement of the spleen caused by repeated malaria infections).
LSM could therefore be particularly effective in urban areas, rural areas with high population densities or obvious breeding sites like small streams or swamps, highland regions and desert fringes. Interventions included adding larvicide to abandoned mine pits, streams, irrigation ditches and rice paddies where mosquitoes breed, and building dams, flushing streams and removing water containers from around people's homes.
Lead author, Lucy Tusting from the Department of Disease Control at the London School of Hygiene & Tropical Medicine, said: "This is the first time the evidence on larval source management for malaria control has been systematically reviewed, and our research shows that the method can be an effective supplementary measure against malaria in both urban and rural areas of Africa and Asia—wherever it is possible to target a sufficient proportion of mosquito breeding sites.
"These findings have important implications for malaria control policy. The tremendous progress made in malaria control in the last decade is now threatened by mosquito resistance to the insecticides available for long-lasting insecticide treated nets and indoor residual spraying. Thus additional methods are needed to target malaria-transmitting mosquitoes. Our research shows that larval source management could be an effective supplementary intervention in some places."
The authors note that further research is needed to assess how effective LSM is in rural Africa where breeding sites are harder to target, such as large flood plains or rice paddies. One study included in the review was from The Gambia, where mosquitoes were breeding in large swamps and rice paddies; in this case spraying swamps with larvicide using ground teams did not show any benefit.
Current World Health Organization (WHO) guidelines on the use of LSM distinguish between urban and rural areas, and WHO does not generally recommend larviciding in rural Sub-Saharan Africa unless particular circumstances limit larval habitats. But the review authors say LSM could be an important strategy in the control of malaria and should be applied in appropriate urban and rural locations with proper management and funding.
Study co-author Steve Lindsay, Professor in the School of Biological and Biomedical Sciences at Durham University, said: "This paper is a landmark publication demonstrating that in many places, larval source management should be used as a supplementary weapon against malaria. It will therefore be combined with insecticide-treated bed nets or indoor residual spraying to hammer down malaria across the tropics."
The authors note that there are limitations in the available data for analysis with a small number of eligible studies and a lack of data in many settings. The trials included were also likely to have been conducted in environments where experts considered success likely—so may not reflect the likely impact of LSM in every habitat.
The review was carried out by researchers at the London School of Hygiene & Tropical Medicine, the US Centers for Disease Control and Prevention, the Liverpool School of Tropical Medicine, Princeton University and Durham University.
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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. DOI: 10.1002/14651858.CD008923.pub2