Eave tubes for malaria control in Africa: initial development and semi-field evaluations in Tanzania

Abstract

Background: Presented here are a series of preliminary experiments evaluating "eave tubes"-a technology that combines house screening with a novel method of delivering insecticides for control of malaria mosquitoes.

Methods: Eave tubes were first evaluated with overnight release and recapture of mosquitoes in a screened compartment containing a hut and human sleeper. Recapture numbers were used as a proxy for overnight survival. These trials tested physical characteristics of the eave tubes (height, diameter, angle), and different active ingredients (bendiocarb, LLIN material, fungus). Eave tubes in a hut with closed eaves were also compared to an LLIN protecting a sleeper in a hut with open eaves. Eave tubes were then evaluated in a larger compartment containing a self-replicating mosquito population, vegetation, and multiple houses and cattle sheds. In this "model village", LLINs were introduced first, followed by eave tubes and associated house modifications.

Results: Initial testing suggested that tubes placed horizontally and at eave height had the biggest impact on mosquito recapture relative to respective controls. Comparison of active ingredients suggested roughly equivalent effects from bendiocarb, LLIN material, and fungal spores (although speed of kill was slower for fungus). The impact of treated netting on recapture rates ranged from 50 to 70 % reduction relative to controls. In subsequent experiments comparing bendiocarb-treated netting in eave tubes against a standard LLIN, the effect size was smaller but the eave tubes with closed eaves performed at least as well as the LLIN with open eaves. In the model village, introducing LLINs led to an approximate 60 % reduction in larval densities and 85 % reduction in indoor catches of host-seeking mosquitoes relative to pre-intervention values. Installing eave tubes and screening further reduced larval density (93 % relative to pre intervention values) and virtually eliminated indoor host-seeking mosquitoes. When the eave tubes and screening were removed, larval and adult catches recovered to pre-eave tube levels.

Conclusions: These trials suggest that the "eave tube" package can impact overnight survival of host-seeking mosquitoes and can suppress mosquito populations, even in a complex environment. Further testing is now required to evaluate the robustness of these findings and demonstrate impact under field conditions.

Keywords: Beauveria bassiana; Eave tubes; House improvement; Semi-field system.

Figures

Fig. 1

a Experimental hut used for initial testing of the eave tube prototype. In this picture, the hut has been modified for experiment 1b (testing eave tubes at different heights). The thatch roof was later replaced with metal sheeting (not pictured). b Overview of the semi-field model village showing the six houses. c Rice paddy to mimic common breeding sites for An. arabiensis. d Breeding sites (left arrows) and clay pot resting site (right arrow). e Close-up of resting mosquitoes inside a clay pot. f Diagram of model village showing the type and location of houses, cattle sheds, central walkway, and the zones (indicated with dashed lines) used for larval sampling. Each zone contained 8–9 larval habitats (51 total)

Fig. 2

Testing of different physical characteristics of the eave tube. a Height of the tubes from the ground, b diameter of the eave tubes, and c angle of the eave tube, relative to the end inside of the hut (note that because of the use of traps, this is the only experiment where higher numbers of mosquitoes are indicative of mosquitoes contacting the eave tubes). Open black circles indicate nightly recapture and closed red circles with error bars indicate mean recaptures ± SE. Different letters indicate significant differences (p < 0.05) based on Tukey all pair comparison

Fig. 3

Testing of different bioactives in the eave tubes compared to a control of clean netting; a PermaNet (deltamethrin), b a wettable powder formulation of bendiocarb (Ficam W), c a dry powder formulation of bendiocarb (Ficam D) loaded on electrostatically charged netting, d dry fungal spores (Beauveria bassiana) loaded on electrostatically charged netting. Open black circles indicate nightly recapture and closed red circles with error bars indicate mean recaptures ± SE. Lines in d show mean cumulative survival for each day

Fig. 4

Comparing eave tubes and LLINs. a Eave tubes screened either with netting cut from an LLIN (PermaNet 2.0), electrostatic netting treated with bendiocarb powder, or untreated netting (control). b Total recapture (inside and outside the experimental house), eaves closed and eave tubes installed with bendiocarb-treated electrostatic netting or eaves open and sleeper protected by an LLIN or an untreated net (control). c Indoor only recapture for the same experiment shown in b. Open black circles indicate nightly recapture and closed red circles with error bars indicate mean recaptures ± SE. Different letters indicate significant differences (p < 0.05) based on Tukey all pair comparison

Fig. 5

Mosquito recapture numbers in the model village. a Larval numbers over time, measured using dippers to sample larval habitats. Points and error bars indicate the mean number of larvae collected in a larval habitats (±SE) for each sampling time point. b Host seeking adult female numbers over time, measured using indoor human landing catches (HLC). Points indicate the total number of mosquitoes recaptured throughout a night for each sampling time point