Residual bioefficacy of attractive targeted sugar bait stations targeting malaria vectors during seasonal deployment in Western Province of Zambia

Abstract

Background The primary vector control interventions in Zambia are long-lasting insecticidal nets and indoor residual spraying. Challenges with these interventions include insecticide resistance and the outdoor biting and resting behaviours of many Anopheles mosquitoes. Therefore, new vector control tools targeting additional mosquito behaviours are needed to interrupt transmission. Attractive targeted sugar bait (ATSB) stations, which exploit the sugar feeding behaviours of mosquitoes, may help in this role. This study evaluated the residual laboratory bioefficacy of Westham prototype ATSB® Sarabi v.1.2.1 Bait Station (Westham Ltd., Hod-Hasharon, Israel) in killing malaria vectors in Western Province, Zambia, during the first year of a large cluster randomized phase-III trial (Clinical Trials.gov Identifier: NCT04800055). Methods This was a repeat cross-sectional study conducted within three districts, Nkeyema, Kaoma, and Luampa, in Western Province, Zambia. The study was conducted in 12 intervention clusters among the 70 trial clusters (35 interventions, 35 controls) between December 2021 and June 2022. Twelve undamaged bait stations installed on the outer walls of households were collected monthly (one per cluster per month) for bioassays utilizing adult female and male Anopheles gambiae sensu stricto (Kisumu strain) mosquitoes from a laboratory colony. Results A total of 84 field-deployed ATSB stations were collected, and 71 ultimately met the study inclusion criteria for remaining in good condition. Field-deployed stations that remained in good condition (intact, non-depleted of bait, and free of dirt as well as mold) retained high levels of bioefficacy (mean induced mortality of 95.3% in males, 71.3% in females, 83.9% combined total) over seven months in the field but did induce lower mortality rates than non-deployed ATSB stations (mean induced mortality of 96.4% in males, 87.0% in females, 91.4% combined total). There was relatively little variation in corrected mortality rates between monthly rounds for those ATSB stations that had been deployed to the field. Conclusion While field-deployed ATSB stations induced lower mortality rates than non-deployed ATSB stations, these stations nonetheless retained relatively high and stable levels of bioefficacy across the 7-month malaria transmission season. While overall mean mosquito mortality rates exceeded 80%, mean mortality rates for females were 24 percentage points lower than among males and these differences merit attention and further evaluation in future studies. The duration of deployment was not associated with lower bioefficacy. Westham prototype ATSB stations can still retain bioefficacy even after deployment in the field for 7 months, provided they do not meet predetermined criteria for replacement.

Deployment of attractive targeted sugar baits in western Zambia: installation, monitoring, removal, and disposal procedures during a Phase III cluster randomized controlled trial

“…The main threats to ATSB station physical integrity observed during deployment in western Zambia were the development of holes/tears on the bait station membrane and mold growth. Although it is known that ATSB stations in good condition remain efficacious throughout a seasonal deployment in this context [ 18 ], it is not known if threats to physical integrity, including holes/tears and presence of mold, may be associated with reduced product efficacy (e.g., due to reductions in product attractancy and vector feeding). Cage and semi-field studies using the Sarabi ATSB v1.2 in Kenya suggest that mold may not be a major deterrent to Anopheles mosquito attractancy and feeding on the bait station [ 19 ].…”

“…A subset of ATSB stations were removed in Year 1 as part of a bio-efficacy study [ 18 ] and in Year 2 as part of an ATSB durability study (Karabo et al in preparation). ATSB stations included in these studies were monitored through separate complimentary datasets.…”

Time to loss of physical integrity of Attractive Targeted Sugar Bait (ATSB) stations in Western province, Zambia: a survival analysis

“…Earlier prototypes of the Westham ATSB stations used in Mali (Westham Innovations LTD., Tel Aviv, Israel) 5 maintained their attraction and killing effect while lowering biting rates and reducing entomological inoculation rates while deployed in the eld for more than 6 months 2,5 . This suggests that subsequent prototypes such as the Sarabi v1.2 ATSB stations that were used in the Zambia trial could remain bio-e cacious and maintain physical integrity longer than the earlier ATSB models, as vector control products continue to be re ned and improved.…”

“…This approach to vector control has been called Attractive Targeted Sugar Bait (ATSB). ATSB stations are a promising innovation, and a potential addition to existing vector control tools 2,3 . ATSB stations thus differ from the existing recommended vector control methods that target indoor human blood feeding (Insecticide Treated Nets) and resting behaviors (Indoor Residual Spraying, IRS).…”

“…Due to the novelty of the Westham Sarabi v.1.2 ATSB stations, there is limited understanding of their durability, and their bio-e cacy under different conditions. In a recent study by Mwaanga et al (2024) 2 , bio-e cacy of ATSB stations was measured for ATSB tools which remained physically intact during eld deployment. The results indicated that ATSB stations which remain physically intact can also remain bio-e cacious following deployment for at least seven months in the eld.…”