The Ecology of Pathogen Spillover and Disease Emergence at the Human-Wildlife-Environment Interface

摘要

In this classification scheme, zoonotic emergence occurs between Stages II and IV. In the most constrained case (Stage II), spillover only ever leads to primary infection in humans with no onward transmission within human populations. Whereas Stage III is characterized by short chains of transmission (“stuttering chains”), Stage IV features human-to-human sustained epidemic transmission. The line between the two is obviously not only subjective but may create difficulties when characterizing expected pathogen behavior. For example, outbreaks of Ebola virus beginning in 1976 were typically small and situated in rural areas where outbreaks could have been classified as Stage III. However, since 2014, Ebola has been considered a Stage IV zoonosis, and modeling work undertaken during the 2014–2015 Ebola epidemic in West Africa showed that “the same epidemiological conditions that were present in 1976 could have generated a large outbreak purely by chance” (Camacho et al. ). The line, therefore, between short-chain and epidemic transmission for a pathogen is sometimes unclear.