The evaluation of insecticide-treated screening of unimproved housing for malaria control in Tanzania: Laboratory, semi-field system, and community studies

Introduction: Unimproved housing, built with traditional materials, increases malaria transmission risk, as malaria mosquitoes readily enter through eaves, windows, and wall holes of these homes. Therefore, insecticide-treated screening (ITS) of these entry points offers a promising complementary approach. Being a new tool, this study assessed its efficacy in Tanzania following modified testing modalities in response to insecticide resistance. Methods: Three studies were conducted: 1) Laboratory cone bioassays assessed the bioavailability (mosquito mortality) of deltamethrin and piperonyl butoxide (standalone and combined) in the context of exposure time and mortality assessment timing. 2) Semi-field efficacy studies (deterrence, mortality, bloodfeeding inhibition) evaluated new (12-month stored) and 12-month-aged ITS against 12-month-aged Olyset® Plus nets and a control hut using laboratory-reared mosquitoes (4x4 Latin square design). 3) A household trial (421 households) evaluated ITS's impact on malaria infection and vector prevalence at two rainy seasons (short and long), also assessed adverse events after one month, and chemical retention and bioavailability after one year of field use. Results: Evening cone bioassays with metabolic-resistant mosquitoes showed significantly lower mortality than daytime assays. Mortality also increased from 24 to 72 hours for deltamethrin, both alone and with PBO (p<0.05). Semi-field studies showed that ITS significantly reduced blood-feeding and hut entry (p<0.05), and similar mosquito mortality to Olyset® Plus nets. Consequently, modeling suggests ITS's epidemiological impact may exceed that of PBO-net. However, the household trial showed no significant difference in malaria infection prevalence between the ITS (19.9%, 50/251) and control (28.3%, 65/230) arms (p=0.227). ITS showed no serious adverse events, but a reduced chemical retention and bioavailability after field use. Conclusion: Despite promising results from cone bioassays and SFS, the trial was inconclusive, likely due to low statistical power caused by household participation challenges. To address these limitations, a large cluster randomised trial of the intervention, ideally with a longer-lasting insecticidal effect, and community engagement are required.