Malaria Elimination: When the Tools Are Great but Implementation Falters

Abstract

1report the outcome of a trial to control malaria in rural parts of The Gambia through larval source management (LSM). In response to the increased uptake of treated bed nets and indoor residual spraying across Africa, various integrated programs that include LSM have been, or are being implemented, in line with the World Health Organization’s (WHO) advocacy of integrated vector management (IVM). 2 Area-wide larviciding thus seemed an attractive integral component for IVM along the Gambia River where annual flooding produces numerous breeding sites in the first kilometer of landward edges of the river. Parallel to this study, several authors were also engaged in a similar trial in West Kenya, the results of which were published recently, 3 which showed unequivocally that larval control significantly enhanced the impact of bed net use only. Regretfully, in The Gambia the trial failed, with no reduction in clinical malaria, parasite prevalence, or anemia. This trial failed despite a massive 88% reduction in apparent larval densities . What went wrong? In both trials larviciding was executed by treating anopheline breeding sites at weekly intervals with the biological control agent Bacillus thuringiensis israelensis (Bti). In both sites the efficacy of the agent was demonstrated before full-scale field application. It cannot be concluded otherwise, therefore, that the tool per se cannot be the cause for the observed differences. Bti worked well against mosquitoes in Kenya, and so it did in The Gambia. The remaining causes for the observed differences in outcome could be based on differences in the ecology and behavior of local vector populations, the nature of the mosquito breeding habitat, or the way in which the larval control campaigns were executed. Perhaps all three factors interfered in some way. The authors considered the likelihood of mosquitoes dispersing over considerable areas (from outside the treated zones into the areas where larviciding took place) as unlikely, based on former mosquito dispersal studies. Nevertheless, although it was assumed that mosquitoes primarily bred along the river they were later found to occupy much larger areas of the floodplain. Second, they considered the possibility that females oviposited on damp soil, a phenomenon observed elsewhere in Africa. Flooding of these eggs during tidal movements might yield new larvae that escaped larvicidal treatment. The previous two points indicate that 1) potential breeding habitat went untreated, and 2) sites remained untreated after flooding. Although the latter sites should always have received treatment within the following week, this apparently did not happen, or at least not before such sites produced adults. The last and perhaps most critical cause of failure was described as “ … we cannot exclude the possibility that field applicators may have missed aquatic habitats.” Coverage of all potential aquatic habitats with larviciding was hindered by inaccessibility caused by deep water and other terrain features that made it impossible for field applicators carrying knapsack sprayers to reach all parts of the areas that had to be covered. It seems likely therefore that the more than originally anticipated and widespread breeding, combined with incomplete coverage of potential vector habitat, supported an overall lower but still viable vector population that sustained parasite transmission at levels that did not result in any changes in malarial indices. As such, the outcome fits the previously described non-linear relationship between transmission intensity and prevalence. 4