Insecticide-treated Nets and the Diversion of Anopheles Biting to Livestock: A Critical Narrative Review of Zooprophylaxis in Malaria Vector Control

Abstract

Insecticide-treated nets (ITNs) remain the dominant tool for malaria prevention across endemic Africa and Asia, yet residual transmission persists where dominant vectors feed outdoors, early in the evening, or on non-human hosts. Zooprophylaxis, the practice of exploiting livestock as alternative blood-meal sources to divert Anopheles mosquitoes from humans, has been proposed for almost a century as a low-cost complement to net-based control, and a specific hypothesis has gained renewed attention: that ITN scale-up itself, by excluding mosquitoes from human sleeping spaces, increases the relative attractiveness of livestock and thereby amplifies passive zooprophylactic diversion. This critical narrative review synthesises entomological, epidemiological, theoretical and intervention literature bearing on this hypothesis, with particular attention to Anopheles arabiensis, the vector for which host diversion is most plausible. The evidence indicates that host diversion is real but conditional: it depends on vector species composition, the spatial placement of livestock relative to human dwellings, and the density ratio of animals to people, and it can reverse into zoopotentiation when livestock are housed close to or within sleeping areas. Direct tests of the ITN-livestock interaction remain scarce, and the clearest entomological demonstration reports a modest, species-specific reduction in human biting associated jointly with net coverage and cattle presence rather than a large, generalisable push-pull effect. Mathematical models show that whether livestock protect or endanger a population depends critically on mosquito mortality while host-seeking, a parameter rarely measured in the field. Converting passive diversion into a lethal event, through insecticide-treated livestock or systemic endectocides, has produced consistent entomological gains in trials but has not yet been validated against clinical malaria endpoints at scale. The review concludes that zooprophylaxis should not be promoted as a standalone control measure, that its interaction with ITNs is ecologically plausible but empirically under-demonstrated, and that insecticide-treated or endectocide-treated livestock represents the more defensible avenue for translating host diversion into measurable transmission reduction, contingent on further cluster-randomised evidence.