Semi-field evaluation of the cumulative effects of a "Lethal House Lure" on malaria mosquito mortality

Abstract

Background: There is growing interest in the potential to modify houses to target mosquitoes with insecticides or repellents as they search for human hosts. One version of this 'Lethal House Lure' approach is the In2Care® EaveTube, which consists of a section of polyvinyl chloride (PVC) pipe fitted into a closed eave, with an insert comprising electrostatic netting treated with insecticide powder placed inside the tube. Preliminary evidence suggests that when combined with screening of doors and windows, there is a reduction in entry of mosquitoes and an increase in mortality. However, the rate of overnight mortality remains unclear. The current study used a field enclosure built around experimental huts to investigate the mortality of cohorts of mosquitoes over multiple nights.

Methods: Anopheles gambiae sensu lato mosquitoes were collected from the field as larvae and reared through to adult. Three-to-five days old adult females were released inside an enclosure housing two modified West African style experimental huts at a field site in M'be, Côte d'Ivoire. Huts were either equipped with insecticide-treated tubes at eave height and had closed windows (treatment) or had open windows and open tubes (controls). The number of host-seeking mosquitoes entering the huts and cumulative mortality were monitored over 2 or 4 days.

Results: Very few (0-0.4%) mosquitoes were able to enter huts fitted with insecticide-treated tubes and closed windows. In contrast, mosquitoes continually entered the control huts, with a cumulative mean of 50-80% over 2 to 4 days. Baseline mortality with control huts was approximately 2-4% per day, but the addition of insecticide-treated tubes increased mortality to around 25% per day. Overall cumulative mortality was estimated to be up to 87% over 4 days when huts were fitted with tubes.

Conclusion: Only 20-25% of mosquitoes contacted insecticide-treated tubes or entered control huts in a given night. However, mosquitoes continue to host search over sequential nights, and this can lead to high cumulative mortality over 2 to 4 days. This mortality should contribute to community-level reduction in transmission assuming sufficient coverage of the intervention.

Keywords: Anopheles gambiae; Cumulative mortality; EaveTube; Feeding rate; Housing improvement; Lethal House Lure; Mosquito entry; Screening; Vector control.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1

Semi-field enclosure for release–recapture studies. a Metallic framework of the enclosure around 2 modified West Africa experimental huts. b Top, insert inside an EaveTube (view from outside); bottom, treated insert with visible insecticide powder. c Netting walls and door, and tarpaulin roof. d White tarpaulin floor to facilitate collection of dead mosquitoes. Six clay pots (3 visible) containing 1 l water and cotton soaked in 10% sugar solution were installed in the enclosure the morning after the release to provide resting sites and sugar sources for mosquitoes

Fig. 2

Cumulative mean (± SE) proportion of mosquitoes recovered inside huts within the enclosure over two nights. Adult Anopheles gambiae s.l. were released in the enclosure at 20:15. Mosquitoes found inside huts, or dead in the enclosure, were recovered at 05:00 each morning, over 2 nights. On the final morning all mosquitoes that could be found were recovered. Huts within the enclosure had either open windows and open tubes at eave height (control), or closed windows and tubes fitted with insecticide-treated inserts (treated). Over 2 nights mean ± SE of 0.4 ± 0.50% of mosquitoes entered treated huts and 51.9 ± 6.55% control huts. Means are based on 10 replicates of release–recapture per treatment

Fig. 3

Cumulative mean (± SE) proportion of mosquitoes recovered inside huts within the enclosure over four nights. Adult An. gambiae s.l. were released in the enclosure at 20:15. Mosquitoes found inside huts, or dead in the enclosure, were recovered at 05:00 each morning, over 4 nights. On the final morning all the mosquitoes that could be found were recovered. Huts within the enclosure had either open windows and open tubes at eave height (control), or closed windows and tubes fitted with insecticide-treated inserts (treated). Over 2 nights mean ± SE of 0.0 ± 0.00% of mosquitoes entered treated huts and 78.8 ± 3.84% control huts. Means are based on 6 replicates of release–recapture per treatment

Fig. 4

Cumulative mean (± SE) proportion of dead mosquitoes recovered inside the semi-field enclosure over two nights. Adult An. gambiae s.l. were released in the enclosure at 20:15. Mosquitoes found inside huts, or dead in the enclosure, were recovered at 05:00 each morning, over 2 nights. On the final morning all the mosquitoes that could be found were recovered. Huts within the enclosure had either open windows and open tubes at eave height (control), or closed windows and tubes fitted with insecticide-treated inserts (treated). Over 2 nights mean ± SE of 23.8 ± 2.14% of mosquitoes died when exposed to treated huts compared to 2.8 ± 1.00% with control huts. Means are based on 10 replicates of release–recapture per treatment

Fig. 5

Cumulative mean (± SE) proportion of dead mosquitoes recovered inside the semi-field enclosure over four nights. Adult An. gambiae s.l. were released in the enclosure at 20:15. Mosquitoes found inside huts, or dead in the enclosure, were recovered at 05:00 each morning, over 4 nights. On the final morning all the mosquitoes that could be found were recovered. Huts within the enclosure had either open windows and open tubes at eave height (control), or closed windows and tubes fitted with insecticide-treated inserts (treated). Over 4 nights mean ± SE of 47.1 ± 3.77% of mosquitoes died when exposed to treated huts compared to 5.2 ± 1.25% with control huts. Means are based on 6 replicates of release–recapture per treatment