Using population viability criteria to assess strategies to minimize disease threats for an endangered carnivore.

Outbreaks of infectious disease represent serious threats to the viability of many vertebrate populations, but few studies have included quantitative evaluations of alternative approaches to the management of disease. The most prevalent management approach is monitoring for and rapid response to an epizootic. An alternative is vaccination of a subset of the free-living population (i.e., a "vaccinated core") such that some individuals are partially or fully immune in the event of an epizootic. We developed a simulation model describing epizootic dynamics, which we then embedded in a demographic simulation to assess these alternative approaches to managing rabies epizootics in the island fox (Urocyon littoralis), a species composed of only 6 small populations on the California Channel Islands. Although the monitor and respond approach was superior to the vaccinated-core approach for some transmission models and parameter values, this type of reactive management did not protect the population from rabies under many disease-transmission assumptions. In contrast, a logistically feasible program of prophylactic vaccination for part of the wild population yielded low extinction probabilities across all likely disease-transmission scenarios, even with recurrent disease introductions. Our use of a single metric of successful management-probability of extreme endangerment (i.e., quasi extinction)-to compare very different management approaches allowed an objective assessment of alternative strategies for controlling the threats posed by infectious disease outbreaks.