CDC light traps underestimate the protective efficacy of an indoor spatial repellent against bites from wild Anopheles arabiensis mosquitoes in Tanzania

Abstract

Background Methods for evaluating efficacy of core malaria interventions in experimental and operational settings are well established but gaps exist for spatial repellents (SR). The objective of this study was to compare three different techniques: (1) collection of blood-fed mosquitoes (feeding), (2) human landing catch (HLC), and (3) CDC light trap (CDC-LT) collections for measuring the indoor protective efficacy (PE) of the volatile pyrethroid SR product Mosquito Shield™ Methods The PE of Mosquito Shield™ against a wild population of pyrethroid-resistant Anopheles arabiensis mosquitoes was determined via feeding, HLC, or CDC-LT using four simultaneous 3 by 3 Latin squares (LS) run using 12 experimental huts in Tanzania. On any given night each technique was assigned to two huts with control and two huts with treatment. The LS were run twice over 18 nights to give a sample size of 72 replicates for each technique. Data were analysed by negative binomial regression. Results The PE of Mosquito Shield™ measured as feeding inhibition was 84% (95% confidence interval (CI) 58–94% [Incidence Rate Ratio (IRR) 0.16 (0.06–0.42), p < 0.001]; landing inhibition 77% [64–86%, (IRR 0.23 (0.14–0.36) p < 0.001]; and reduction in numbers collected by CDC-LT 30% (0–56%) [IRR 0.70 (0.44–1.0) p = 0.160]. Analysis of the agreement of the PE measured by each technique relative to HLC indicated no statistical difference in PE measured by feeding inhibition and landing inhibition [IRR 0.73 (0.25–2.12) p = 0.568], but a significant difference in PE measured by CDC-LT and landing inhibition [IRR 3.13 (1.57–6.26) p = 0.001]. Conclusion HLC gave a similar estimate of PE of Mosquito Shield™ against An. arabiensis mosquitoes when compared to measuring blood-feeding directly, while CDC-LT underestimated PE relative to the other techniques. The results of this study indicate that CDC-LT could not effectively estimate PE of the indoor spatial repellent in this setting. It is critical to first evaluate the use of CDC-LT (and other tools) in local settings prior to their use in entomological studies when evaluating the impact of indoor SR to ensure that they reflect the true PE of the intervention.

Evaluating human landing catches as a measure of mosquito biting and the importance of considering additional modes of action

“…Recent epidemiological trials for an transfluthrin-based passive emanator have demonstrated reductions in malaria 27 . It is likely that the clinical impact of such interventions against malaria is greater than that measured via traditional mosquito endpoints such as indoor density or human landings 7 . Evaluation of volatile pyrethroids is best conducted using assays that can capture all modes of action including semi-field studies or experimental hut trials 28 .…”

“…HLC is a direct measurement of human-vector contact that has not been reproduced accurately using traps 3 – 5 , but it has disadvantages especially with wild-field mosquitoes, as pathogen transmission could occur 5 . There is some experimental evidence that HLCs provide a reasonable proxy of human biting 6 , 7 . However, HLC measures only mosquito attack rate and is not sensitive for measuring other effects of vector control tools beyond landing inhibition 8 .…”

Modelling the influence of climate and vector control interventions on arbovirus transmission

“…The data used to parameterise the stable netting control strategy was gathered using WHO cone bioassays and light-traps within stables. Light traps have been observed to not effectively estimate protective efficacy for spatial repellents [35]. Considering this, and the relatively small number of vectors caught in the stable (n=25), the estimation for the reduction in biting may be inaccurate.…”

Evaluating human landing catches as a measure of mosquito biting and the importance of considering additional modes of action

“…Recent clinical trials have demonstrated reductions in malaria [24]. It is likely that the clinical impact of such interventions against malaria is greater than that measured via traditional mosquito endpoints such as indoor density or human landings [7]. Evaluation of volatile pyrethroids is best conducted using assays that can capture all modes of action including semi-field studies or experimental hut trials [25].…”

“…HLC is a direct measurement of human-vector contact that has not been reproduced accurately using traps [3][4][5], but it has disadvantages especially with wild-field mosquitoes, as pathogen transmission could occur [5]. There is some experimental evidence that HLCs provide a reasonable proxy of human biting [6,7]. However, HLC measures only mosquito attack rate and is not sensitive for measuring other effects of vector control tools beyond landing inhibition [8].…”