Less is more: repellent-treated fabric strips as a substitute for full screening of open eave gaps for indoor and outdoor protection from malaria mosquito bites

Abstract

Background Providing protection from malaria vector bites, both indoors and outdoors, is crucial to curbing malaria parasite transmission. Screening of house entry points, especially with incorporated insecticides, confers significant protection but remains a costly and labour-intensive application. Use of spatial repellents has shown promise in creating areas of protection in peri-domestic areas. Methods This study aimed at comparing the protection provided by transfluthrin-treated and untreated complete screens over open eave gaps with incomplete transfluthrin-treated eave strips as a potential replacement for a full screen. Human landing catches were implemented independently inside and outside an experimental hut under controlled semi-field conditions, with insectary-reared Anopheles arabiensis mosquitoes. Results The odds of a female mosquito finding a human volunteer indoors and attempting to bite were similar whether the eaves were completely open or there was an untreated fabric strip fixed around the eaves. However, when the eave gap was completely screened without insecticide, the odds of receiving a bite indoors were reduced by 70% (OR 0.30, 95% CI 0.20–0.47). Adding transfluthrin to the full screen, further increased the protection indoors, with the odds of receiving a bite reduced by 92% (0.08, 95% CI 0.04–0.16) compared to the untreated screen. Importantly, the same protection was conferred when only a narrow transfluthrin-treated fabric strip was loosely fixed around the eave gap (OR 0.07, 95% CI 0.04–0.13). The impact of the transfluthrin treatment on outdoor biting was correlated with evening temperatures during the experiments. At lower evening temperatures, a transfluthrin-treated, complete screen provided moderate and variable protection from bites (OR 0.62, 95% CI 0.37–1.03), whilst at higher evening temperatures the odds of receiving a bite outdoors was over four times lower in the presence of transfluthrin, on either a full screen (OR 0.22 95% 0.12–0.38) or a fabric strip (OR 0.25, 95% 0.15–0.42), than when no treatment was present. Conclusion The findings suggest that transfluthrin-treated fabric strips can provide a substitute for complete eave screens. They are a simple, easy-to-handle tool for protecting people from malaria mosquito bites indoors and potentially around the house in climatic areas where evening and night-time temperatures are relatively high.

A randomized, double-blind placebo-control study assessing the protective efficacy of an odour-based ‘push–pull’ malaria vector control strategy in reducing human-vector contact

“…This observation emphasizes the importance of field evaluations of novel interventions that have been developed under more standardized conditions, prior to advocating for larger scale use. The preceding semi-field results, however, were also highly variable 8 with low efficacy (less than 20% protection) observed at low evening temperatures 10 . Our field trial was implemented during the rainy season, when malaria transmission tends to peak in western Kenya, but average night-time temperatures are low, ranging between 16 and 20 °C 36 .…”

“…Additionally, increasing levels of physiological and behavioural resistance to insecticidal tools limit the degree to which we can rely on these methods to achieve elimination and maintain current gains 7 . Vector control tools using alternative modes of action, for example by manipulating the odour-orientation of mosquito vectors 8 – 10 , are urgently sought to expand the toolbox for malaria vector control and provide options for tailoring integrated interventions to local eco-epidemiological settings 11 , 12 .…”

“…Recent studies under semi-field conditions in Kenya and Tanzania have shown that the use of passively emanated transfluthrin spatial repellents from fabric strips suspended around open eave gaps of houses can reduce human landing rates of Anopheles gambiae s.s. and Anopheles arabiensis 8 , 10 , 15 , 23 . ‘Pushing’ mosquitoes away from the protected locations may, however, put others at elevated risk since mosquitoes might be diverted to seek out people without protection 24 .…”

Evaluation of the solar-powered Silver Bullet 2.1 (Lumin 8) light trap for sampling malaria vectors in western Kenya

“…Even though traps were baited with light and CO 2 , a single trap in a system only recaptured a quarter of the mosquitoes released during any given trap night. This contrasts with human landing collections implemented in the same systems, which resulted consistently in 80% recovery of the released mosquitoes [ 77 , 78 ]. This supports the conclusion that in open outdoor spaces, where concentrations of odour cues are quickly diluted due to prevailing climatic conditions [ 79 , 80 ], light traps have severe limitations and surveillance data needs to be interpreted with caution [ 81 ].…”

The behaviour of adult Anopheles gambiae, sub-Saharan Africa’s principal malaria vector, and its relevance to malaria control: a review

“…Novel repellents, to replace DEET, are in development at a large scale, with emphasis on human safety, residual activity and ease of application. Repellents can be used in different ways with promising results as tools for malaria prevention [ 181 – 183 ]. Long-lasting spatial repellents offer promise in the future as they can provide an area protection against mosquitoes.…”