Spillback

Spillback is a term used in disease ecology and epidemiology to describe the transmission of a pathogen from humans or domestic animals back into wildlife, with the possibility that the pathogen then re-enters human or livestock populations. It represents a reverse or bidirectional spillover: a pathogen first crosses from a reservoir (often humans or domestic animals) into wildlife, becomes established in the wildlife population, and may subsequently spill back into humans or domestic animals.

Spillback is facilitated by close contact at the wildlife–livestock–human interface, including farming operations, companion animals, zoos, rehabilitation centers, and urban-adjacent wildlife habitats. Pathogens with broad host ranges, environmental stability, and efficient cross-species transmission are most capable of spillback. Wildlife reservoirs can sustain transmission even when human cases decline, creating a persistent source of reintroduction.

Notable cases include SARS-CoV-2 spillback from infected humans to farmed mink in Europe and North America, with subsequent mink-to-human transmissions reported in some settings, and the detection of SARS-CoV-2 infection and antibodies in free-ranging white-tailed deer in North America, indicating sustained circulation and potential spillback risk. While the relative frequency and impact of spillback vary by pathogen, it is a concern for surveillance and outbreak dynamics.

Spillback complicates disease control and eradication efforts by creating wildlife reservoirs that are difficult to monitor and manage. It underscores the need for a One Health approach that integrates human, animal, and environmental health surveillance, biosecurity measures at farms and at the wildlife–human interface, and ongoing research into cross-species transmission and prevention strategies. See also: reverse zoonosis, zoonotic spillover, One Health.