MALARIA: What do we know about larvicides?

Bill Brieger 16 Nov 2010
In SciDec.net we read that, “Cuba has announced plans to build biolarvicide factories in Brazil and several African countries in a bid to tackle malaria and dengue fever.” The move is based on apparent successes of efforts such as those in Angola where the Director-general of Labiofam says that, “Angola, for instance, has reduced malaria incidence by 50 per cent, and some areas have seen a 70 per cent fall,” with similar results in Accra, Ghana.
WHO says that larviciding is “indicated only for vectors which tend to breed in permanent or semi-permanent water bodies that can be identified and treated, and where the density of the human population to be protected is sufficiently high to justify the treatment with relatively short cycles of all breeding places.” What actual documented evidence is there from Angola and elsewhere in Africa about the use and effectiveness of larviciding?
An article on the history of malaria control in Liberia reviews early efforts to use synthetic insecticides for indoor residual spraying and larviciding. Unfortunately, “These projects encountered a spate of difficulties that foreshadowed the general retreat from malaria eradication efforts across tropical Africa by the mid-1960s.” What has changed now that we are in the days of rolling back malaria?
A newly published article on mosquito larval source management in areas experiencing flooding in The Gambia concluded that …
The intervention was associated with a reduction in habitats with late stage anopheline larvae and an 88% reduction in larval densities. The effect of the intervention on mosquito densities was not pronounced and was confounded by the distance of villages to the major breeding sites and year. There was no reduction in clinical malaria or anemia. Ground applications of non-residual larvicides with simple equipment are not effective in riverine areas with extensive flooding, where many habitats are poorly demarcated, highly mobile, and inaccessible on foot.
A key approach to the use of larvicides may be integrated vector management, where there is not reliance on one control measure alone. In the Kenyan highlands researchers found that, “Vector control with microbial larvicides enhanced the malaria control achieved with ITNs alone. Anti-larval measures are a promising complement to ITN distribution in the economically important highland areas and similar transmission settings in Africa.”
Larviciding was found to have a positive effect in reducing childhood malaria in Tanzania where “larviciding reduced malaria prevalence and complemented existing protection provided by insecticide-treated nets. Larviciding may represent a useful option for integrated vector management in Africa, particularly in its rapidly growing urban centres.”
The two promising articles from Kenya and Tanzania would be strengthened if large scale operations like those described for Angola were better documented and published because as was seen in Liberia many years ago it was the basic operational issues that limited program effectiveness.
Devine and Killeen report in discuss some of the practical issues of larviciding in Malaria Journal and note that, “The effective operational implementation of these campaigns is difficult, time consuming, and expensive,” in part because of “The myriad and cryptic nature of aquatic habitats and the difficulty in identifying and targeting the most productive of these (which) makes maximizing that impact very challenging.”
Devine and Killeen recommend a “new auto-dissemination methodology” based on a “detailed characterization of oviposition behaviour and of the effective transfer distances between feeding, resting and aquatic resources.” Again, these are good ideas, but what of evaluation of current large scale approaches underway? In addition, as RTI suggests programs must establish “baseline information on the acute, intermediate, and chronic effects of chemicals used in malaria vector control on workers and the general population.”
The basic question remains - what can we learn about the right conditions for larvicide use as a major tool in integrated vector management for malaria? All partners in rolling back malaria have a responsibility for helping this learning process by documenting and publishing their experiences. Maybe the proceedings of the recent Labiofam Conference in Havana will be published soon.
http://www.malariafreefuture.org/blog/?p=1082