In most African countries bednets have become common and are contributing to saving countless lives of children. Scaling up of this intervention continues in the second decade of this millennium. Indoor residual spraying is widely practiced though a less common sight in many parts of Africa where spray teams do not reach far-off communities in rural settings.
Meanwhile, scientists are looking for additional tools that, they claim, are urgently needed to augment the limited arsenal of interventions currently available. The choice for larval control is obvious – after all, there are numerous examples of historical interventions that were largely based on larval control and very successful indeed.
Although these early interventions used environmentally unsound methods like diesel oil or the highly toxic Paris green, we now live in an era where green alternatives are widely available. Notably the biocontrol agent Bacillus thuringiensis israelensis, highly specific for mosquito larvae, has proven highly effective in many parts of the world. Moreover, resistance to it seems much less of a problem than that experienced with chemical larvicides. A perfect tool at the dawn of the second malaria elimination era, so it seems.
That larval control can really contribute to a reduction in malaria has recently been reported from western Kenya. The study by Ulrike Fillinger and colleagues unequivocally demonstrated that integrated control using bednets and larval control has a significant add-on effect over the use of nets only. The article, published in the WHO Bulletin, is all thumbs-up for larval control.
Since then, two worrying reports have been published, one from the Gambia (American Journal of Tropical Medicine and Hygiene, forthcoming) by Silas Majambere and colleagues, the other from Tanzania (Malaria Journal). In the Gambia, a trial to control malaria vector larvae along the Gambia river floodplain through ground-based application (men with knapsack sprayers filled with Bti) did not result in any significant change in malarial indices. The study from urban Dar es Salaam, by Prosper Chaki and colleagues, assessed the effectiveness of operational, community-based larval habitat surveillance systems. Not only was it difficult for community-based resource persons to have access to breeding sites in the urban environment, they were also not very successful in detecting larvae in sites (they detected larvae in only 12.6% of the sites that actually contained larvae). The tools are perfect, their application falters.
In The Gambia, it was the difficulty for Bti spraymen to access all breeding sites that probably led to a sustained (albeit lower) vector population that, however, managed to maintain transmission at such level that changes in disease indicators did not become visible. This is spite of the fact that apparent densities were reduced by more than 80%. In Dar es Salaam the problems related to surveillance teams not finding access to private property (gardens etc.) that held breeding sites. In both cases, larval control may fail to yield success. In The Gambia, the authors claim, aerial application could have been more effective, as all sites would have been reached. In Dar es Salaam it was concluded that a new surveillance system, that should lead to better access of breeding sites on private property, is needed. What that should look like was not mentioned.
The lessons from the past clearly indicate that area-wide integrated pest management tactics will only bear fruit if coverage of breeding sites with the intervention is (near-)perfect. Thus, if accessibility of sites (The Gambia) due to terrain features or private property (Dar es Salaam) becomes an issue, then one may wonder what the value of larval control will be in future.
Larval control is a great strategy with huge potential, and is very likely to play a major role in the malaria elimination efforts in the coming years. But if we don’t find the ways and means to effectively apply it, we’re heading for trouble. In The Gambia resources are needed to fund aerial application of Bti, in Dar es Salaam authority is needed to ensure that teams have access to every square inch of the city. But will this be possible?