The current vector control tools used against Aedes mosquitoes have several limitations, necessitating the development and testing of additional tools for proactive dengue prevention. This study demonstrated that while the push-pull system reduced human-vector contact of Ae. aegypti mosquitoes, the majority of protection was provided by the FTPE. The likely reason for this is the poor response of mosquitoes to the BGS. Numerous previous studies on push-pull technologies have demonstrated the higher efficacy of the push than the pull [14, 22, 28–30]. While the push-pull system may need further development the success of the FTPE was encouraging and indicated their potential for the control of arboviral diseases.
The FTPE remained protective for three months, therefore, it is possible to meet high levels of coverage in an urban setting with just one application, reducing the difficulties of re-application and user noncompliance. These promising results indicate that FTPE could potentially be used to protect individuals in the peridomestic space longer than current personal protection methods [5]. This may be particularly useful during arboviral disease outbreaks that tend to coincide with the 3–5 month rainy season [31].
Our finding of FTPE efficacy as a spatial repellent is consistent with previous studies evaluating transfluthrin against Anopheles arabiensis and Ae. aegypti mosquitoes [32–34]. However, we may not generalize that these transfluthrin treated passive emanators provide protection in all geographical locations. In the field, it is important to consider environmental factors before implementing this control strategy. For example, in a windy environment, the active ingredient can be blown away therefore, reducing the concentrations needed in the air to repel mosquitoes. Temperature can affect the vaporization of transfluthrin and thereby its concentration and protective efficacy. It has been reported that the optimal temperature for a transfluthrin treated emanator to provide maximum protection ranges between 21 °C to 30 °C, with a reduction in protection specifically in lower temperatures [11]. This suggests that in geographical locations where the daytime ambient temperature is below 21 °C the efficacy of these emanators for prevention of Ae. aegypti bites may be impaired. In this study the experiment were conducted within the temperature ranging between 20.9–25.5 °C, which is optimal transfluthrin evaporation.
In this study, we have demonstrated that the BGS positioned 10 meters away did not significantly protect a person from mosquito bites. However, previous experiments have shown that the BGS used alone, is an effective trap for sampling Ae. aegypti [17, 35, 36]. In this experiment, the BGS was placed near a human volunteer and they were the only “hosts” available. This demonstrated that the human cues were significantly more attractive to Aedes than the cues from the BGS. Because, preliminary work in the semi field system indicated that the BGS caught many Aedes in the absence of the human volunteers revealing that the efficacy of BGS is relative to the proximity and density of humans. This has also been observed in other studies with humans outcompeting traps at short range [12] and that whole human odor is optimally attractive to anthropophagic mosquitoes [37]. While the BGS did not provide personal protection by reducing human-vector contract as the removal trap, it could still provide some level of community protection if used on a larger scale, although other traps such as the autocidal gravid trap may be more feasible for removal trapping [38] as they don’t require carbon dioxide.
The number of mosquitoes successfully caught by the BGS during the push-pull or the pull only configuration was the same. While this showed that transfluthrin did not actively push mosquitoes into the trap it also indicated that transfluthrin exposure outdoors does not inhibit mosquitoes entering the BGS. This is contrary to Salazar who reported exposing mosquitoes to transfluthrin significantly lowered trap catches [39]. Trap catches were not affected if the mosquitoes allowed to recover for 12 h before BGS trap evaluation [39]. This suggests that the mode of action of transfluthrin is dose and distance dependent. Use of a higher dose could be further optimized to prevent diversion of repelled mosquitoes from repellent users to non-users in a community [40].
We have shown that FTPE remain protective for three months following impregnation. This is a relatively short duration compared to the previous studies which demonstrated transfluthrin treated strips protective efficacy for up to six months [11, 15]. A possible explanation for these differences could be due to the variation in transfluthrin dosage and the distance from the emanator where HLC was performed. In the current study, 10.5 g of transfluthrin (5.25 g on each of two FTPE) was used and HLC conducted 3 meters from the emanators, whereas in the study by Ogoma et al., the volunteer sat at 1 meter from a strip enclosing them on all four sides at an application of 15.1 g [11, 15]. In general, the efficacy of the emanators in both studies decreases over time as the result of the loss of transfluthrin due to evaporation. To ensure long term efficacy of the FTPE, a double layer of the hessian strips could be used or transfluthrin doses increased, provided they remain within the margin of safety for chronic inhalation exposure [41]. The FTPE is a simple proof of concept prototype and further work is required to develop a product for use as a public health intervention, including standardizing the release rate of the transfluthrin through standardization of the material upon which the transfluthrin is applied and improvement of the delivery unit to ensure it is cost effective and tamper proof. This may also include the application of UV protection to the transfluthrin treated material to prolong its efficacy outdoors [42].
The use of FTPE improves user compliance, as the replacement rate is every three months. This potentially avoids the problems associated with personal topical repellents that require daily application but tend to be applied only when people notice mosquito bites [43], resulting in lack of public health benefit [44]. As the device provides protection at the household level, it is likely to provide a more cost-effective approach to bite prevention outside of sleeping hours and to be more acceptable among community members for protection of the whole family [45]. Topical repellents are logistical prohibitive to use [46] and more suitable for targeted distribution among high risk populations such as forest goers in pre-elimination areas [47].
Dengue tends to be focal in transmission with Aedes commonly having a short flight range although there are exceptions[48]. Therefore, transmission is primarily mediated between locations by movement of infected individuals [49]. These devices are portable and may be deployed anywhere; they could be very useful if provided to those with confirmed dengue, deployed in entrance points (port and airport) where travelers are coming in from other countries or in places where new cases are suspected/outbreak is reported, such as markets [1]. The high mosquito toxicity of transfluthrin is an important feature of this tool, as it has the potential to kill a substantial proportion of mosquitoes that encounter the insecticide and reduce vector densities and vectoral capacity. Further work into the impact of such devices on the mortality of free-flying mosquitoes is recommended.
The limitations of this study are that the data for longevity experiment was collected at 0, 3, and 6 months only. Whereby at three months after impregnation the FTPE still offered 44% protection, but with this experimental designed we missed the exact time point (between 3 and 6 months) when the FTPE stopped providing significant protection. We recommend that future studies conducting the same kind of experiment need to conduct weekly or monthly evaluations in order to provide a more precise estimate of efficacy over time, especially when testing the label claims of long lasting spatial repellent products. Moreover, we only evaluated one distance setup for use of the “push-pull” in the SFS. In the field, the positioning of this system will vary due to the local building layout resulting in varying protection levels. We recommend that further studies on push-pull should 1) focus on improving the attraction of pull components, since they need to outcompete humans and 2) explore optimal positioning of components to determine the effective distance at which the push and pull could work synergistically.