Impact of indoor residual spraying of insecticides upon malaria vector populations and disease burden in rural Zambia: past, present and future in relation to resistance management with complementary active ingredients
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Date
2025-12-31
Authors
Chinula, Dingani
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University College Cork
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Abstract
With a focus on programmatic malaria vector control in Zambia that is also relevant to most of Africa. This thesis combines a district-scale epidemiological assessment of vector control tools on the malaria burden, an operational pilot assessment of micro-mosaic deployment formats for indoor residual spraying (IRS) of insecticides, and a mathematical modelling investigation into how such insecticide diversification tactics may be expected to perform for resistance management purposes.
The first chapter synthesises one of Zambia’s longest standing routine malaria surveillance datasets, to quantify the epidemiological impacts of long-lasting insecticidal nets (LLINs) and IRS over the long term. Between 2009 and 2011, the incidence of severe malaria cases in Luangwa District fell by >90% and total confirmed cases by >80%. These gains were largely attributable to scale up of durable non-pyrethroid IRS formulations and expanded access to diagnosis and treatment when a set of 8 new health facilities across the district were all opened simultaneously in mid-2016. No additional measurable effect could be attributed to LLINs, likely because coverage was already high at the outset and remained stable throughout. This chapter underscores that IRS with long-lasting formulations of non-pyrethroid insecticides and improved healthcare access are pivotal for reducing malaria burden in such highly endemic settings with predominantly anthropophilic and endophilic vectors like Anopheles funestus.
The second chapter describes entomological assessment of an operational pilot of household scale micro-mosaic applications of IRS with two different insecticide formulations, nested within a village-scale macro-mosaic of the same two formulations in the same district of southeastern Zambia over one full year that began in late 2022. The densities of An. gambiae complex and An. funestus group mosquitoes were comparable across all IRS treatment allocations and no differences in species composition across treatments were detected. Although certainty was constrained by limited replication of the village-scale observational units used, this evidence appears to confirm the feasibility of implementing IRS micro-mosaics under routine programmatic conditions without compromising immediate malaria vector control performance. Therefore, this pilot demonstrates that household level micro-mosaics may well be a practical option for national malaria control programmes to diversify IRS insecticide applications at the fine scales considered most relevant to preventing resistance emergence.
The third chapter describes the development and application of a deterministic mathematical model of mosquito life history and foraging outcomes during the first gonotrophic cycle, under various intervention scenarios in which NG-LLINs may be complemented by IRS deployed as either blanket coverage with a single insecticide or household-scale micro-mosaics of several insecticides. This model considers the cumulative outcomes of sequential mosquito feeding attempts, to calculate the fractions of mosquitoes that could harbour incipient resistance traits against one of the IRS insecticides that are expected to successfully feed, be deterred, or die while trying to feed, broken down by indoor versus outdoor location and cumulative exposure to zero, one or more IRS insecticides. The model estimates the probability of successful feeding after exposure to exactly one IRS insecticide, representing the risk of propagating an incipient resistance trait, and the selection coefficient favouring subsequent resistance spread after emergence. This modelling study indicates that NG-LLINs, when paired with simple IRS micro-mosaics, could suppress the earliest initial stages of resistance propagation in highly anthropophagic and endophagic vectors like An. funestus, but may have only negligible impact on resistance evolution trajectories in more exophagic and zoophilic species like An. arabiensis.
Overall, this thesis gives insights into how to routine malaria surveillance data platforms can guide insecticide resistance mitigation interventions and how IRS micro-mosaics may offer a practical and scalable pathway for national malaria programs to transition from reactive to pre-emptive resistance management strategies.
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Keywords
Inpatient admission , Malaria incidence , Mosaic , Micro-mosaic , Indoor residual spraying , Long lasting insecticidal nets , Next-generation long lasting insecticidal nets , Insecticide Resistance Management , Global Plan for Insecticide Resistance Management , Process Explicit Mathematical Models
Citation
Chinula, D. 2025. Impact of indoor residual spraying of insecticides upon malaria vector populations and disease burden in rural Zambia: past, present and future in relation to resistance management with complementary active ingredients. PhD Thesis, University College Cork.