The hidden threat of reverse zoonosis in sanitation gaps: A One Health perspective

Water, Sanitation, and Hygiene (WASH) are central to global public health; however, open defecation remains widespread, undermining health, economic productivity, and progress towards sustainable development goals. While its impact on human health is well documented, its contribution to human-to-animal transmission of pathogens – reverse zoonosis – remains limited and underexplored. This narrative review synthesizes current evidence on how open defecation contributes to environmental contamination that facilitates reverse zoonotic transmission, examined through a One Health perspective. Open defecation exposes animals to human faecal pathogens through both direct and indirect pathways. Molecular and case-based evidence demonstrate the presence of multispecies pathogens in shared environments and instances of human-to-animal transmission. The resulting infections, whether clinical or subclinical, impair animal health and productivity, amplify antimicrobial resistance (AMR), and promote the formation of disease reservoirs, increasing the risk of spillover events. Findings reveal limited research attention on reverse zoonosis linked to sanitation, with a lack of quantification of its burden. Addressing this neglected pathway requires integrated, cross-sectoral strategies that bridge human, environmental, and animal health. This review underscores the need to embed WASH interventions within One Health frameworks and prioritize operational research to develop holistic, evidence-based solutions that protect both human and animal populations while advancing sustainable development.