Low-coverage vaccination strategies for the conservation of endangered species

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Abstract
The Ethiopian wolf Canis simensis is a specialist carnivore found only in seven isolated mountain pockets in Ethiopia, where they prey on rodent communities. They live in family packs with an intricate social organization. Fewer than 500 individuals now survive following rabies outbreaks in 1992 and 2003 that severely depleted the population in the Bale Mountains region. Work carried out there as part of the Ethiopian Wolf Conservation Programme ( http://www.ethiopianwolf.org ) has included the development of a vaccination strategy that could be an important model for other conservation projects. The aim is to control the spread of disease through habitat corridors between subpopulations using only low vaccination coverage. This reduces the extent of rabies outbreaks, and should significantly enhance the population's long-term survival chances. Cover photograph by Martin Harvey ( http://www.wildimagesonline.com ). A vaccination strategy against rabies in the endangered Ethiopian wolf aims to control the spread of disease through habitat corridors between subpopulations using only low vaccination coverage. This approach reduces the extent of rabies outbreaks, and should significantly enhance the long-term persistence of the population. The conventional objective of vaccination programmes is to eliminate infection by reducing the reproduction number of an infectious agent to less than one1, which generally requires vaccination of the majority of individuals. In populations of endangered wildlife, the intervention required to deliver such coverage can be undesirable and impractical2; however, endangered populations are increasingly threatened by outbreaks of infectious disease for which effective vaccines exist3,4. As an alternative, wildlife epidemiologists could adopt a vaccination strategy that protects a population from the consequences of only the largest outbreaks of disease. Here we provide a successful example of this strategy in the Ethiopian wolf, the world's rarest canid5, which persists in small subpopulations threatened by repeated outbreaks of rabies introduced by domestic dogs6. On the basis of data from past outbreaks, we propose an approach that controls the spread of disease through habitat corridors between subpopulations and that requires only low vaccination coverage. This approach reduces the extent of rabies outbreaks and should significantly enhance the long-term persistence of the population. Our study shows that vaccination used to enhance metapopulation persistence through elimination of the largest outbreaks of disease requires lower coverage than the conventional objective of reducing the reproduction number of an infectious agent to less than one1.