Pathogen Spillback describes the transmission of infectious agents from humans back into animal populations after initial spillover from animals to humans. While spillover has long been recognized as a driver of emerging infections, spillback represents a reciprocal dynamic that can establish new reservoirs, alter pathogen evolution, and complicate long-term control efforts. Understanding spillback is essential for comprehensive One Health–oriented disease prevention.

Spillback occurs when pathogens adapt to human hosts and subsequently infect wildlife or domestic animals through close contact, shared environments, or contaminated resources. Urban expansion, wildlife trade, livestock integration, and environmental contamination increase opportunities for reverse transmission. Once established in animal populations, pathogens may circulate silently, creating persistent sources of re-introduction. These risks are increasingly discussed at Infectious Diseases Conference sessions focused on cross-species transmission dynamics.

From an ecological perspective, reverse zoonotic transmission reshapes disease landscapes. Pathogens introduced into new animal hosts may undergo genetic change, affecting virulence or transmissibility. Spillback can threaten wildlife conservation by introducing novel diseases into vulnerable species. It also complicates eradication efforts by creating non-human reservoirs that are difficult to monitor and control.

Human activities strongly influence spillback risk. Inadequate waste management, wastewater discharge, and encroachment into natural habitats facilitate environmental exposure. Domestic animals can act as bridges between humans and wildlife, amplifying transmission. Addressing spillback therefore requires coordinated environmental management, animal health surveillance, and human behavioral interventions.

Surveillance systems play a critical role in detecting spillback events. Monitoring animal populations for pathogens circulating in humans provides early warning of reservoir establishment. Integrating veterinary, wildlife, and public health data enhances detection sensitivity. Genomic analysis helps confirm transmission directionality and track adaptation across hosts.

Prevention strategies emphasize reducing bidirectional transmission opportunities. Infection control in human settings, vaccination where applicable, and biosecurity measures in animal husbandry reduce risk. Protecting wildlife habitats and regulating human–animal interfaces further limit exposure. Cross-sector collaboration ensures that interventions address shared drivers rather than isolated outcomes.

Pathogen spillback highlights the interconnectedness of human, animal, and environmental health. Ignoring reverse transmission risks undermines control gains and increases the likelihood of persistent circulation. By incorporating spillback into surveillance, policy, and prevention planning, health systems strengthen resilience against complex, multi-host infectious threats.