Eave tubes for malaria control in Africa: a modelling assessment of potential impact on transmission
© The Author(s) 2016
Received: 23 June 2016
Accepted: 26 August 2016
Published: 2 September 2016
Novel interventions for malaria control are necessary in the face of problems such as increasing insecticide resistance and residual malaria transmission. One way to assess performance prior to deployment in the field is through mathematical modelling. Modelled here are a range of potential outcomes for eave tubes, a novel mosquito control tool combining house screening and targeted use of insecticides to provide both physical protection and turn the house into a lethal mosquito killing device.
The effect of eave tubes was modelled by estimating the reduction of infectious mosquito bites relative to no intervention (a transmission metric defined as relative transmission potential, RTP). The model was used to assess how RTP varied with coverage when eave tubes were used as a stand-alone intervention, or in combination with either bed nets (LLINs) or indoor residual spraying (IRS).
The model indicated the impact of eave tubes on transmission increases non-linearly as coverage increases, suggesting a community level benefit. For example, based on realistic assumptions, just 30 % coverage resulted in around 70 % reduction in overall RTP (i.e. there was a benefit for those houses without eave tubes). Increasing coverage to around 70 % reduced overall RTP by >90 %. Eave tubes exhibited some redundancy with existing interventions, such that combining interventions within properties did not give reductions in RTP equal to the sum of those provided by deploying each intervention singly. However, combining eave tubes and either LLINs or IRS could be extremely effective if the technologies were deployed in a non-overlapping way.
Using predictive models to assess the benefit of new technologies has great value, and is especially pertinent prior to conducting expensive, large scale, randomized controlled trials. The current modelling study indicates eave tubes have considerable potential to impact malaria transmission if deployed at scale and can be used effectively with existing tools, especially if they are combined strategically with, for example, IRS and eave tubes targeting different houses.
KeywordsEave tube Novel intervention Vector control Malaria Housing Eaves Anopheles Population model
Wide scale use of mosquito control interventions, such as indoor residual spraying (IRS) and long-lasting insecticide-treated bed nets (LLINs), have made a major contribution to the substantial decline in malaria burden observed over the last decade . However, new mosquito control tools are now required to address problems of insecticide resistance and residual transmission (i.e. the malaria transmission persisting following universal coverage of existing effective interventions such as IRS and/or LLINs) [2, 3].
Numerous studies show that house screening can reduce entry of mosquitoes [4–8] and can impact transmission . Other studies find better housing correlates with reductions in malaria, particularly if eaves are closed or screened to prevent Anopheles mosquitoes from entering [10–14].
Eave tubes (see  for an introduction to the technology) represent a novel twist on the house modification approach. When referring to “eave tubes”, this is actually shorthand for a package of house modification wherein windows are screened, open eaves are closed and tubes (pieces of PVC piping) are installed into the eaves at 1–2 m intervals. These open eave tubes are fitted with electrostatic netting  treated with an insecticidal active, and so when mosquitoes are drawn toward the odours emanating from the house and attempt to enter through the eaves, they are killed. The electrostatic coating on the netting provides the additional advantage of increasing the bioavailability of powdered insecticides, delivering a lethal dose of insecticide even following transient contact [16, 17]. The netting can be used with diverse classes of insecticidal powders ranging from chemicals currently approved for IRS through to novel actives such as entomopathogenic fungi . Fitting a house with eave tubes in effect turns the house into a mosquito-killing device.
To date, studies with eave tubes have centered around laboratory and semi-field investigations providing insights into potential effects at small scale [15, 18, Snetselaar et al. pers. comm.]. Where the technology has been deployed at larger scale the focus of studies has been on operational questions of feasibility of implementation and user acceptance . Thus far, there is little understanding of how eave tubes are likely to affect entomological or epidemiological outcomes when deployed at scale and/or in combination with existing control tools such as IRS or LLINs. The aim of the current study is to use a population model to help bridge this knowledge gap.
The model makes a number of simplifying assumptions. Mosquitoes that commence host-seeking are assumed to feed or die during one night. Non-human feeding and multiple feeds during one gonotrophic cycle are ignored. Vector mortality is assumed to be unaffected by vector age or infection status. Average bite rate and length of parasite extrinsic incubation period are assumed to be constant between vectors and over time. Mosquitoes locate properties and hosts within properties randomly. There is no difference in the average number of people per property in properties with and without interventions, so an intervention applied to a given proportion of properties is also applied to that proportion of the human population.
Table of baseline parameter values used by model unless otherwise indicated
Assumed cycle length
Average search time to locate a property
Search time to locate a human host when searching indoors
Average time spent resting indoors post-feed
Average time spent finding ovipositing site
Average time spent from ovipositing to host searching
Base mortality rate while searching for property or laying site
Instantaneous daily rate
Base mortality rate while searching for host inside property
Instantaneous daily rate
Base mortality rate while resting inside property (non IRS)
Instantaneous daily rate
Base mortality rate while outdoors and not searching
Instantaneous daily rate
Base mortality when attempting to feed—pre bite
Probability of death
Base mortality when attempting to feed—post bite
Probability of death
Base mortality when attempting to oviposit—pre lay
Probability of death
Base mortality when attempting to oviposit—post lay
Probability of death
Probability vector deflected away from eave tube property
Probability vector killed when attempting to enter eave tube property
70.00 % (An. gambiae)e 52.00 % (An. arabiensis)e
Probability vector killed by eave tube when exiting eave tube property
Probability vector deflected away from human under LLIN
Probability vector killed by LLIN when attacking protected human
Probability vector killed by LLIN after biting protected human
Probability vector exits non-eave tube property if deflected away from human under LLIN
Probability vector exits eave tube property if deflected away from human under LLIN
Probability deflected away from IRS protected property before entering
Probability killed by IRS whilst resting in IRS treated property
The key metric generated by the model is the relative transmission potential (RTP). This is calculated as the number of infectious bites per (adult) vector lifetime as a proportion of that with no intervention. When the following two assumptions can be considered valid, RTP also represents the relative number of infectious bites per person per unit of time. The first assumption is that the juvenile population is at the carrying capacity of the available breeding sites and density dependence effects mean that any reduction in the populations’ rate of egg production arising from the interventions explored does not materially affect the recruitment rate of new adults to the vector population. When this assumption holds true, then the population age composition matches lifetime survival probabilities and the relative change in number of infectious bites per vector lifetime is equal to the relative change in bites from the vector population as a whole, per unit of time. The second assumption is that the human population size remains constant for different interventions. If this is true, then RTP is also equal to the proportion of infectious bites per person per unit of time under a given intervention compared to that with no intervention. Thus, for a vector population in which density dependence can be assumed to result in maintenance of a constant adult recruitment rate even when adult mortality is increased by interventions, with human population size unaffected by the intervention, the RTP should map directly to a proportionate change in the entomological inoculation rate (EIR). To illustrate, a 90 % RTP means a 10 % reduction in infectious bites per vector per lifetime and, subject to the assumptions above, represents a 10 % reduction in infectious bites from the vector population per unit of time and a 10 % reduction in infectious bites received per person per unit of time. Equivalently, 10 % RTP means the infectious bites per person per unit of time have been reduced by 90 %. This metric is calculated as an average across the human population and broken down into results for sub-groups with different interventions in place.
In Fig. 5a–c the allocation of interventions between properties varies, considering the extreme scenarios of completely overlapping (Fig. 5b, i.e. interventions are always deployed together) and complementary (Fig. 5c, LLINs are specifically targeted to houses without eave tubes) deployment strategies, as well as a random allocation (Fig. 5a). These figures show that benefits of adding LLINs exclusively to properties which are already protected with eave-tubes are only marginal. For example, in Fig. 5b where interventions are completely overlapping (i.e. deployed together in the same property), if 60 % of properties have eave tubes then adding LLINs to even 100 % LLINs provides only about a 5 % additional reduction in RTP. However, using LLINs only in properties which have no eave tube protection, as in Fig. 5c, gives benefits comparable to those achieved when all properties have LLINs, and better reduction in RTP for all properties overall.
The modelling study reveals that eave tubes could reduce the number of infectious bites a malaria mosquito will transmit in a population, and from Fig. 2, it appears that the impact of eave tubes could be substantial even with low proportions of properties outfitted with this intervention. With only 50 % eave tube coverage, the average infectious bites per vector lifetime, per person, are reduced by more than 80–90 % for the whole human population. The benefit is greater for those who are in the houses to which eaves tubes have been fitted, but even those in houses without screening and eave tubes gain substantial community benefit.
As with all models, the outputs in the current study depend on the assumptions. The baseline parameters provided in Table 1 were selected as representative of the available literature. Sensitivity analysis (Additional file 2) demonstrates that although variation in different parameters can affect the quantitative results, the non-linear reduction in relative transmission potential with increasing coverage appears robust, indicating a mass action effect, similar to that observed with LLINs. This is an important finding because it suggests that there should be community benefits in locations where only a modest proportion of houses receive eave tubes (either because of poor adoption or because properties are not amenable to have tubes fitted).
Both deflection and reduced kill are predicted to degrade eave tube efficacy (Fig. 3). Greater deflection means that fewer mosquitoes encounter the active and if they aren’t killed, then the combined effects could make the eave tubes much less effective. Yet it is worth noting that basic house screening without the addition of insecticide, which would be represented in the current model as 100 % deflection without kill, has been shown to reduce malaria transmission in multiple studies [9, 30–32]. Furthermore, one study in the Gambia demonstrated explicitly that unscreened houses adjacent to screened houses did not suffer increased disease burden due to deflection of mosquitoes . These empirical data suggest that the model outputs are likely conservative with respect to overall impact since any level of killing should improve control relative to screening alone. Also any non-human host feeding, such as on livestock, is not captured in the model, which could further dilute malaria transmission . Nonetheless, the model reveals the potential importance of having an effective active ingredient within the tubes and supports the need for regular retreatment or replacement of the electrostatic netting to ensure the killing effect is maintained and any risks of deflection are minimized.
The model results for combining eave tubes with existing interventions demonstrate benefits of developing integrated strategies, although this depends crucially on how the interventions are deployed with respect to one another. Under the baseline assumptions, eave tubes perform better than either LLINs or IRS for a given level of coverage. If eave tubes are fitted to the exact same houses as receive IRS or LLINs, there is potential for marked redundancy between technologies (Figs. 5b, 6b). However, with random distribution (Figs. 5a, 6a), or better still strategic distribution wherein overlap in interventions is minimized (Figs. 5c, 6c), there is greater complementarity. This result is important in terms of optimizing interventions on a per house basis. Not all houses within a location will necessarily be amenable to installation of eave tubes (either because of the physical nature of the house or perhaps user acceptance). Targeting these houses with IRS, or ensuring the occupants have full access to LLINs, would maximize control. Likewise, compliance with LLINs or IRS can sometimes be very low (refusal rates for IRS can be as high as 70 % for example ). These households could provide primary targets for installation of eave tubes.
Overall, the results of the modelling suggest that the eave tube technology could affect malaria incidence by reducing the number of infectious bites from mosquitoes. Individual householders should gain immediate personal protection, as well as relief from nuisance mosquitoes, which should encourage adoption. As coverage increases, mass action effects should yield additional community-wide benefits. There also appears potential for integration with existing interventions. These results support the further research and development of the eave tube technology.
indoor residual spraying
long-lasting insecticide-treated bed net
randomized controlled trial
relative transmission potential
entomological inoculation rate
JLW and MBT conceived of this study. PAL designed and executed the model with contributions from JLW. JLW and MBT wrote the paper with contributions from PAL. All authors read and approved the final manuscript.
MBT is listed on a patent for eave tube design that is currently pending. The authors JLW and PAL declare that they have no competing interests.
Availability of data and material
Most data supporting our findings not within the manuscript itself can be found in Additional files.
This work was supported by European Union Seventh Framework Programme Grant 306105, FP7-HEALTH-2012-INNOVATION-1.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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