| Summary: | Background: Mosquitoes that feed on animals can survive and mediate residual transmission of malaria even after
most humans have been protected with insecticidal bednets or indoor residual sprays. Ivermectin is a widely-used
drug for treating parasites of humans and animals that is also insecticidal, killing mosquitoes that feed on treated
subjects. Mass administration of ivermectin to livestock could be particularly useful for tackling residual malaria
transmission by zoophagic vectors that evade human-centred approaches. Ivermectin comes from a different
chemical class to active ingredients currently used to treat bednets or spray houses, so it also has potential for
mitigating against emergence of insecticide resistance. However, the duration of insecticidal activity obtained with
ivermectin is critical to its effectiveness and affordability.
Results: A slow-release formulation for ivermectin was implanted into cattle, causing 40 weeks of increased
mortality among Anopheles arabiensis that fed on them. For this zoophagic vector of residual malaria transmission
across much of Africa, the proportion surviving three days after feeding (typical mean duration of a gonotrophic
cycle in field populations) was approximately halved for 25 weeks.
Conclusions: This implantable ivermectin formulation delivers stable and sustained insecticidal activity for
approximately 6 months. Residual malaria transmission by zoophagic vectors could be suppressed by targeting
livestock with this long-lasting formulation, which would be impractical or unacceptable for mass treatment of
human populations.
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