Despite decades of mosquito control in Liberia using LLINs and IRS, malaria prevalence remains high. The current study demonstrates partial efficacy of DWL, an approach to mosquito control that is tailored to a typical Liberian house design and is a hybrid of LLINs and IRS. By covering ceilings and windows, the insecticidal material reduced mosquito access to indoor sleeping areas. Installing DWL as a whole house improvement is a promising approach that may provide an effective and appropriate mosquito control measure where LLIN usage is low. Measuring the effect of vector control interventions against malaria is challenging, due to the complexity of the movement of vectors and humans. Modification of housing is especially challenging as uptake is incomplete, and insecticidal materials can be repurposed or modified in unexpected ways. In the current study, an outbreak of Ebola virus disease occurred, which further complicated the installation of DWL and the measurement of its effect.
Mosquito control interventions that improve the whole house have been studied in a variety of contexts, but inconsistencies in experimental design and materials make it challenging to define a best practice for deployment of whole-house insecticidal materials. One theme that does emerge from the limited available literature is that covering eaves and other entry points is an important consideration. Disaster relief agencies have tested ITPS in Sierra Leone [8] finding that delivery of insecticides on the ceiling and walls of tents resulted in reduced malaria transmission. Hut trials in Tanzania [21] found that both pyrethroid and non-pyrethroid DWL performed poorly against resistant malaria vectors. In these studies, the lack of coverage of eaves was cited as a reason for the persistence of indoor biting. Variation in product formulation may further explain some of the differences in results between these studies and the current results.
Feasibility of DWL as an intervention has been analysed, with various approaches aimed at reducing cost and maximizing acceptability. Recent research into the utility of eave tubes is very encouraging [15, 16]. Eave tubes dramatically reduce the surface area of the insecticidal material, but require significant modifications to the structure of the house. In many rural Liberian homes, the large open gables (Fig. 3) likely makes the installation of eave tubes very challenging. Ceiling and eave coverage with DWL is expected to be associated with higher acceptability. Adaptation of a range of vector control interventions to various local contexts is needed, as are comparisons of cost and acceptability of DWL to eave tubes and other housing improvement.
Study findings indicate a significant effect on malaria prevalence at 12 months post baseline, which was not observed in trials of the most comparable DWL prototypes. DWL was observed to be an effective malaria control product in the Upland (inland) region of Bomi County, but less so in the Coastal region. Differences in effect may be due to more stable malaria transmission characteristics inland, compared to the coastal region. Human behaviour characteristics may also have played a role in reducing malaria control effect in the Coastal region, where families may be engaged in outdoor activities as night, related to fishing. This requires further investigation.
Similar prototypes made by the same supplier have been tested in other trials [4, 5, 21], but variation between final finishing and production settings may differ from the one reported here.
Mosquito genotyping revealed pyrethroid resistance at similar levels to that reported in 2012 [9]. Entomological efficacy of the DWL was near 100% at 12 months post installation, although mortality was delayed. The significant drop in bioavailability of the two active ingredients in DWL after 12 months usage, is responsible for the drop in malaria control effect, not resistance to these novel compounds. It is unlikely that either cross resistance is an issue or that a de novo mechanism evolved in this timeframe. Sublethal effects in Anopheles with the avermectin class of pesticides, that includes abamectin, have been described previously [22], which may have reduced mosquito lifespans sufficiently to influence transmission. The implications of sublethal effects require further investigation.
Coastal villages compared to Upland (inland) villages showed a dramatic drop in overall malaria prevalence (Table 1, Fig. 5). While DWL may have been responsible for this effect, additional factors, including ivermectin mass drug administration, and a national LLIN distribution in 2015 may have reached coastal villages more readily than inland villages. It is also notable that Ebola screening procedures excluded participants in surveys after 12 months. Severe cases were not included in the study, and their treatment-seeking behaviour is unknown.
Studies in Mozambique with comparable whole house coverage showed marked decrease in indoor biting [14]. Indoor biting was reduced and malaria infections were reduced in migrant worker camps in India where the entire shelter was made from ITPS [23]. Reductions in malaria infection were also observed in temporary shelters in Sierra Leone where walls and ceilings were insecticide treated [8]. Other house improvement approaches currently being tested include eave barrier tubes [15] and screening [24], with the latter showing a measurable effect on anaemia, even with non-insecticidal screening. The results of the current study appear to confirm the need to cover eaves and gables when deploying insecticidal materials similar to DWL. There is a compelling need for further research into the role insecticidal materials can play as housing improvements to achieve scalable means of malaria control in regions where pyrethroid resistance threatens the progress that has been made.
Combining insecticide classes by layering a non-pyrethroid wall liner on top of existing LLINs has been studied previously [6], but in contrast, the current results suggest an additive effect of the two control methods. The displacement of LLIN usage by DWL usage is problematic, however, and has been described elsewhere [2]. Usage of LLINs was not controlled in this study and many observed to forgo LLIN hanging after DWL installation. Although the efficacy of pyrethroid LLINs may be reduced in Liberia [17], the observed decrease in malaria—despite lack of control over LLIN usage between study arms—suggests that DWL as a single intervention is partially effective at decreasing transmission.
It is important to highlight that all children that were followed up had been previously cleared of infection by ASAQ treatment [25]. A drop in prevalence over the 24-month study period occured, and the majority of infections measured after baseline are assumed to be the result of re-infection following treatment, or new infections post weaning.
As a hybrid approach that benefits from aspects of successful use of LLINs and IRS, whole-house deployment of DWL is shown here to be partially effective at decreasing malaria in Liberia, despite an unexpected decline in bioefficacy. The decision to cover gables and eaves resulted from community input, an essential component when developing vector control tools [26, 27], and this likely contributed to a reduction in malaria in DWL protected houses, even after bioavailability declined. Further research into the mechanisms of decreased house entry and insecticide-mediated mortality of indoor biting mosquitoes is needed.