RESUMO
Though emerging infectious diseases ignore boundaries between species, the agencies that respond to them do not. Based on interviews with state and federal epidemiologists, veterinarians, and physicians and on case studies of disease events, this article examines how the jurisdictional and cultural divides that exist among human and animal health agencies hinder efforts to successfully contain species-jumping diseases (zoonoses). I argue that newly emergent zoonoses make these agencies' organisational cultures function as silos because the institutionalised thinking and practices developed to address the diseases that traditionally concerned each agency constrain members from building the inter-organisational bridges required to manage the latest 'hybrid' diseases. The silo effect is evident both across the human-animal health divide and within the landscape of animal health, as agencies that monitor livestock and wildlife follow distinct and sometimes competing agendas. The article also touches on moments of inter-agency cooperation in order to specify how health practitioners can begin making connections between 'organisational silos'. This article encourages sociologists of health to explore the crucial link between animal and human health; and it introduces the concept of organisational silos to capture the relational dilemmas that arise when a 'hybrid' problem systemically links agencies with disparate organisational cultures.