Zambia recently adopted mosaic application patterns as an insecticide deployment strategy for managing resistance in malaria vector populations. While macro-mosaic applications of indoor residual spraying (IRS), a commonly used supplement to long-lasting insecticidal nets (LLINs), vary the active ingredients used from one village to the next, micro-mosaics vary them between households. Here, a convenient procedure for micro-mosaic deployment of two different IRS formulations at household level was piloted under routine programmatic conditions in south-eastern Zambia, nested within a village-scale macro-mosaic that allowed comparison with each of the two formulations alone, in terms of the persisting residual densities and population composition of key malaria vector mosquito taxa. A community-based entomological surveillance system was established to capture mosquitoes with Centre for Disease Control (CDC) light traps approximately once per month in each of 15 houses distributed across each of nine health facility catchment areas (HFCAs) scattered along a main road running through Luangwa district, for one full year immediately following completion of an IRS spray round. Each HFCA was purposively assigned one of three different IRS regimes in a macro-mosaic, avoiding allocation of the same regime to any two adjacent HFCAs, except in one case that was necessitated by environmental regulations: (i) Dichlorodiphenyltrichloroethane (DDT) across all houses, (ii) a mixture of clothianidin and deltamethrin (CD) across all houses or (iii) a micro-mosaic wherein either DDT or CD was applied to every second house by teams of two sprayers working in parallel. To compare the residual populations of primary and secondary malaria vectors, persisting despite these IRS regimes and high background coverage with LLINs containing deltamethrin and a synergist piperonyl butoxide (PBO), generalized linear mixed effects models were fitted to the numbers of captured mosquitoes identified to group, complex and species levels. Over 1620 trap nights, the numbers of primary and secondary malaria vectors captured were generally very low, confounding precise estimation of mean densities or comparisons with satisfactory statistical power. Nevertheless, the densities of all Anopheles species identified were approximately comparable across all IRS regimes (p ≥ 0.1420), and no differences were observed between regimes regarding species composition of the Anopheles funestus group (p ≥ 0.072) or Anopheles gambiae complex (p ≥ 0.1410). Household-level micro-mosaic applications of distinct IRS formulations may be just as immediately effective over the short term as conventional macro-mosaic deployments of each formulation separately in different villages, potentially offering a useful additional option for mitigating against resistance evolution over the long term.
Chinula et al. (Sun,) studied this question.
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