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Dear Sir:
We recently published theoretical models evaluating the impact of targeted larval management on the disease burden of malaria.1 If the adult productivity of various potential larval habitats is highly heterogeneous, we showed that targeted larval interventions could effectively and efficiently reduce the entomological inoculation rates and incidence of malaria in areas of low to intermediate transmission. In terms of approaches to malaria control, our results highlight the importance of gathering empirical data on the contribution of various types of larval habitats to adult vector abundance. In a letter published in this issue, Killeen and others raise several concerns regarding our publication of operational implementation of targeted interventions. Their criticisms, listed in decreasing order of importance, are 1) operational difficulties of targeted larval management are insurmountable and that exhaustive treatment is a better strategy; 2) heterogeneity in productivity of adults is probably not of the order of magnitude to warrant targeted control and is poorly understood in the field; and 3) we underestimated the impact of untargeted larval interventions. We address their concerns in this order.
First and foremost, our theoretical models dealt with strategic issues regarding targeted intervention rather than operational issues. The models were meant to emphasize the potential importance of heterogeneity in adult production and not to serve as a guide for implementation of a targeted program. Killeen and others highlight the difficulties associated with translating research findings into implementation in the field and the absence of scientific consensus on productivity of larval habitats. They suggest an alternative strategy of comprehensive habitat treatment, instead of targeted interventions, as a pragmatic approach. Moreover, the historical work of Soper (funded by the Rockefeller Foundation) was cited to show the advantages of an eradication program that eliminated Anopheles gambiae Giles from Brazil and adjacent South American countries. It should be noted, however, that the campaign of Soper was based on the fact that this mosquito was recently introduced to the continent. Thus, An. gambiae in this case was an invading species exploiting and adapting to exotic situations. Packard and Gadehla2 pointed out that, in retrospect, the dynamics of malaria transmission was much more complex and the broader context of malaria control was diminished by focusing only on eradicating An. gambiae. Although exhaustive habitat management may be easier and even cost-effective in some local situations, it is beyond the reach of most African communities. We philosophically disagree with the adage of "shoot first, ask questions later." Without addressing specific ecological questions regarding larval ecology and productivity of habitats, we believe pursuing the goal of total habitat coverage is counterproductive. More importantly, although we show the potential benefits of targeted larval control, we believe that any operational application should be placed in the perspective of integrated vector management (IVM). The principal goal of controlling locally produced mosquito populations is to reduce the threat of mosquito-borne disease transmission and to achieve a low level of mosquito annoyance.3–5 Larval interventions are but one tool and should be used with other interventions to improve efficacy and cost-effectiveness. Therefore, it is important to rationalize the use of limited resources and control efforts by identifying targets that can be attacked for the maximal benefit.6
Second, Killeen and others suggest that heterogeneity in productivity in the field is not sufficient enough and/or the perception of productivity is too variable for rational targeting. However, their own studies in Dar es Salaam, Tanzania, showed that there was a considerable heterogeneity in productivity, and habitats with absence, low, and high densities of An. gambiae larvae accounted for 38%, 42%, and 20% (N = 327), respectively.7 We think that productivity is poorly understood because too little attention has been paid to the proper estimation of adult productivity of individual larval habitats. In many cases, productivity was used interchangeably with indicators such as density or presence/absence of the immature stages. These indicators can be misleading. For example, one study showed that larval density of An. arabiensis Patton in Ethiopia increased with a decline in surface area of aquatic habitats.8 Productivity should be measured in a unit of individual habitats, and the estimation requires at least two measures (i.e., the emergence rate of adults per unit habitat area as well as size of habitats).1 Nobody really knows for sure the magnitudes of heterogeneity in productivity without further field surveys to test some of these ideas and corresponding operational activities. Targeted interventions should be evidence-based, and an understanding of productivity is a prerequisite. We believe larval interventions should be based on the best science available at the time, with recognition that extrapolation beyond the area or period of a study always requires a need for precaution.
Last, they consider that our model underestimated the effect of untargeted interventions. The basic assumption in our models is that heterogeneity in adult productivity is substantially huge among aquatic habitats and the number of productive habitats is relatively fewer than that of unproductive ones in a focal area. Although few survey studies of mosquitoes have documented distribution patterns of productivity in the field, this assumption is likely valid because the frequency distribution of abundance of organisms is often left-skewed with a flat tail such as a log-normal distribution.9–11 Under these circumstances, untargeted interventions are inefficient because aquatic habitats are randomly selected without consideration of the distribution of productivity. The statement of these authors that "In an untargeted scenario, all habitats should have an equal chance of being covered . . . Thus, effective coverage should be directly proportional to overall coverage, regardless of the distribution of productivities" is true only when habitats are uniform in productivity. In our paper, the total productivity P was represented by a numerical scale of 0 to 100, thus corresponding to 0–100%. The use of this notation was probably the reason why these authors failed to reproduce the curve with the parameter values we provided.
Our experience with larval interventions in Eritrea and Kenya convinces us that the time is right to implement an integrated approach to manage the burden of malaria in Africa. More importantly, as international initiatives have placed the emphasis on capacity building in developing countries, our colleagues in Africa certainly have the enthusiasm to pursue an IVM approach. They also are fully capable and understand how this information can be translated and used effectively in an operational program. As more entomological and ecological information is gained about the primary and secondary vectors through surveillance, the IVM approach becomes more effective, economically sound, and environmentally friendly. We strongly believe that the new perspective of habitat-based larval interventions will promote a new round of ecological surveys and result in reliable knowledge that can be translated into rational targeting.
Acknowledgements: We thank John Beier and Richard Lampman for comments.
This article has been cited by other articles:
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  W. Gu, J. Utzinger, and R. J. Novak Habitat-Based Larval Interventions: A New Perspective for Malaria Control Am J Trop Med Hyg, January 1, 2008; 78(1): 2 - 6. [Abstract] [Full Text] [PDF]
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