Inference for entomological semi-field experiments: Fitting a mathematical model assessing personal and community protection of vector-control interventions

Abstract

The effectiveness of vector-control tools is often assessed by experiments as a reduction in mosquito landings using human landing catches (HLCs). However, HLCs alone only quantify a single characteristic and therefore do not provide information on the overall impacts of the intervention product. Using data from a recent semi-field study which used time-stratified HLCs, aspiration of non-landing mosquitoes, and blood feeding, we suggest a Bayesian inference approach for fitting such data to a stochastic model. This model considers both personal protection, through a reduction in biting, and community protection, from mosquito mortality and disarming (prolonged inhibition of blood feeding). Parameter estimates are then used to predict the reduction of vectorial capacity induced by etofenpox-treated clothing, picaridin topical repellents, transfluthrin spatial repellents and metofluthrin spatial repellents, as well as combined interventions forPlasmodium falciparummalaria inAnopleles minimus. Overall, all interventions had both personal and community effects, preventing biting and killing or disarming mosquitoes. This led to large estimated reductions in the vectorial capacity, with substantial impact even at low coverage. As the interventions aged, fewer mosquitoes were killed; however the impact of some interventions changed from killing to disarming mosquitoes. Overall, this inference method allows for additional modes of action, rather than just reduction in biting, to be parameterised and highlights the tools assessed as promising malaria interventions.

Optimizing malaria vector control: A systematic review and mathematical modelling study to identify desirable characteristics of novel tools in different settings

“…Our intervention model, based on [16,39,96], offered a framework to assess the communitylevel impact of different characteristics of vector control tools. The global sensitivity analysis implies that, under a high HBI/parity proportion setting, vector tool targeting mosquitoes before they bite is the most effective.…”

“…The preprandial killing effect was modeled with dummy hosts, which kill the mosquitoes after encountering them (1 dummy host per protected host). Similarly, the disarming effect was modeled with another dummy host, assuming the mosquito needs to go through the resting period after feeding this host [39].…”

Field evaluation of a volatile pyrethroid spatial repellent and etofenprox-treated clothing for outdoor protection against forest malaria vectors in Cambodia

“…The relative reduction in vectoral capacity was predicted using the method described in Denz et al (2021) [44] and in Fairbanks et al (preprint, 2023) [42]. Firstly, the vectoral capacity is calculated for a "baseline scenario" (without the intervention).…”

“…Fairbanks et al (preprint, 2023) [42] used data from semi-field studies combined with a model to quantify the modes of action of selected vector control tools. The influence of the interventions on the mosquito feeding cycle were described relative to an unprotected human host using four characteristics; the relative reduction in the rate of landing, the relative increase in the rate of preprandial killing and disarming, the proportion of this increase in rate which is due to disarming, and the change in the probability of postprandial mortality.…”

“…As part of a phased approach to vector control product evaluation [40], a series of preceding semi-field system (SFS) experiments were conducted in Thailand to measure the primary effects (landing prevention as a proxy for bite prevention) and the secondary effects (sublethal incapacitation and mortality) of new transfluthrin- and metofluthrin-based VPSRs, and etofenprox-treated clothing, against pyrethroid-susceptible Anopheles minimus (an important vector of malaria in the GMS [41]). In addition to providing insights and estimates of intervention impact on landing inhibition, sublethal incapacitation, and mortality, the data were used to parameterize a mathematical model to predict the potential of the tools to provide community protection [42]. Vectoral capacity is a measure of the ability of the vector population to transmit a disease, defined as the total number of potentially infectious bites that would eventually arise from all the mosquitoes biting a single infectious human on a single day [43].…”