Installing insecticide-treated window nets (ITNs) on open eaves, windows, and wall openings in unreinforced homes is a potential malaria control measure. It can prevent mosquitoes from entering the home, providing lethal and sublethal effects on malaria vectors and potentially reducing malaria transmission. Therefore, we conducted an epidemiological study in Tanzanian households to evaluate the effectiveness of insecticide-treated window nets (ITNs) in protecting against malaria infection and vectors indoors.
In Charinze District, Tanzania, 421 households were randomly assigned to two groups. From June to July 2021, mosquito nets containing deltamethrin and synergist were installed on eaves, windows, and wall openings in one group, while the other group did not. Following installation, at the end of the long rainy season (June/July 2022, primary outcome) and the short rainy season (January/February 2022, secondary outcome), all participating household members (aged ≥6 months) underwent quantitative PCR testing for malaria infection. Secondary outcomes included total mosquito counts per trap per night (June/July 2022), adverse reactions one month after net placement (August 2021), and chemobioavailability and residues one year after net use (June/July 2022). At the end of the trial, the control group also received mosquito nets.
The study was unable to draw conclusions due to an insufficient sample size due to the refusal of some residents to participate. A large-scale cluster-randomized controlled trial, ideally involving the installation of window screens treated with a long-lasting insecticide, is needed to evaluate this intervention.
Malaria prevalence data were analyzed using a per-protocol approach, meaning that individuals who had traveled within two weeks prior to the survey or taken anti-malarial medication were excluded from the analysis.
Because the number of mosquitoes captured during the assessment was small, only an unadjusted negative binomial regression model for the number of mosquitoes captured per night by each trap was used to determine the number of mosquitoes in the room.
Of the 450 eligible households selected across all nine villages, nine were excluded because they did not have open roofs or windows prior to randomization. In May 2021, 441 households were subjected to simple randomization stratified by village: 221 households were assigned to the intelligent ventilation system (IVS) group, and the remaining 220 to the control group. Ultimately, 208 of the selected households completed the IVS installation, while 195 remained in the control group (Figure 3).
Some studies suggest that ITS may be more effective in protecting against malaria in certain age groups, housing structures, or when used with mosquito nets. Access to malaria control commodities, particularly mosquito nets, has been reported to be limited, especially among school-aged children.[46] Low availability of nets in households contributes to limited net use within households, and school-aged children are often neglected, thus becoming a source of persistent malaria transmission.[16, 47, 48] Tanzania is implementing ongoing distribution programs, including a school net program, to increase access to mosquito nets for school-aged children.[14, 49] Given the low level of net availability (50%) at the time of the survey and the fact that this group may experience more difficulty accessing nets, ITS may have provided protection for this group, thereby filling the protection gap in net use. Housing structures have previously been linked to increased malaria transmission; for example, cracks in mud walls and holes in traditional roofs facilitate mosquito entry.[8] However, there is no evidence to support this claim; analysis of study groups by wall type, roof type, and previous use of ITNs revealed no difference between the control group and the ITN group.
Although households using an indoor mosquito control system (ITS) had fewer Anopheles mosquitoes captured per trap per night, the difference was small compared to households without an ITS. The lower capture rate in households using an ITS may be due to its effectiveness against key mosquito species that feed and roost indoors (e.g., Anopheles gambiae [50]) but may be less effective against mosquito species that are more likely to be active outdoors (e.g., Anopheles africanus). Furthermore, current ITSs may not contain optimal and balanced concentrations of pyrethroids and PBO and, therefore, may not be effective enough against pyrethroid-resistant Anopheles gambiae, as shown in a semi-field study [Odufuwa, forthcoming]. This result may also be due to insufficient statistical power. To detect a 10% difference between the ITS group and the control group with 80% statistical power, 500 households were required for each group. To make matters worse, the study coincided with an unusual climate in Tanzania that year, with increased temperatures and decreased rainfall[51], which could have negatively impacted the presence and survival of Anopheles mosquitoes[52] and could have led to a decrease in the overall mosquito count during the study period. In contrast, there was little difference in the average daily density of Culex pipiens pallens in houses with ITS compared to houses without it. As mentioned previously [Odufuwa, forthcoming], this phenomenon may be due to the specific technology of adding pyrethroids and PBO to ITS, which limits their insecticidal effect on Culex pipiens. Furthermore, unlike Anopheles mosquitoes, Culex pipiens can enter buildings through doors, as found in a Kenyan study[24] and an entomological study in Tanzania[53]. Installing screen doors may be impractical and will increase the risk of occupant exposure to insecticides. Anopheles mosquitoes primarily enter through eaves[54], and large-scale interventions may have the greatest impact on mosquito density, as shown by modeling based on SFS data[Odufuwa, forthcoming].
Adverse reactions reported by technicians and participants were consistent with known reactions to pyrethroid exposure [55]. Notably, most reported adverse reactions resolved within 72 hours of exposure, as only a very small number (6%) of family members sought medical attention, and all participants received medical care free of charge. A high incidence of sneezing observed among 13 technicians (65%) was associated with failure to use the provided masks, citing discomfort and a possible link to COVID-19. Future studies may consider mandating mask wearing.
In Charinze District, no significant differences were observed in malaria incidence rates or indoor mosquito populations between households with and without insecticide-treated window screens (ITS). This is likely due to the study design, insecticide properties and residues, and high participant attrition. Despite the lack of significant differences, a decrease in household-level parasite incidence was observed during the long rainy season, particularly among school-aged children. Indoor Anopheles mosquito populations also decreased, suggesting the need for further study. Therefore, to ensure continued participant participation, a cluster-randomized controlled design, combined with active community engagement and outreach, is recommended.
Post time: Nov-21-2025