Despite decades of mosquito control in Liberia using LLINs and IRS, malaria prevalence remains high. The current study demonstrates partial efficacy of DL, 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, we provided an insecticidal material to reduce access to indoor sleeping areas. Installing DL 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, we also experienced an outbreak of Ebola virus disease which further complicated the installation of DL 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 DL 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 DL 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 DL 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 DL to eave tubes and other housing improvement.
We report a significant effect on malaria prevalence at 12 months post baseline, which was not observed in trials of the most comparable DL prototypes. 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 we report here.
Mosquito genotyping revealed pyrethroid resistance at similar levels to that reported in 2012 [9]. Efficacy of the DL was near 100%, although mortality was delayed. We conclude that bioavailability is responsible for the drop in effect, not resistance to these novel compounds. DL was observed to be an effective malaria control product for the first 12 months, and 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 inland villages showed a dramatic drop in overall malaria prevalence (Table 1, Fig. 5). While DL 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 DL. 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 DL 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 DL 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 DL 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]. We measured a drop in prevalence over the 24-month study period, 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 DL 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 [20], and this likely contributed to a reduction in malaria in DL 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.