Plague is a zoonotic (shared between humans and other animals) disease caused by a bacterium, Yersinia pestis. Y. pestis is found in many parts of the world, and was introduced to the west coast of North America, near San Francisco, around the turn of the 20th century. Since its introduction, plague has spread to every state west of the Mississippi River except Arkansas, Iowa, Louisiana, Minnesota, and Missouri. About 25 NPS units in the western U.S. have reported evidence of plague in wildlife, and many continue to report cycles of epizootic (outbreaks within wildlife populations) activity. Additional parks are likely to find evidence of plague, if the organism continues its current trend of eastward expansion.
Rodents and lagomorphs make up the vast majority of infected species, but carnivores and hoofstock can also be infected. In the U.S., ground squirrels, prairie dogs, and wood rats are most commonly associated with epizootics. During an epizootic, 90 percent or more of a susceptible population can succumb to plague. In areas where susceptible species (e.g., prairie dogs) serve as keystone species, large-scale die-offs may have a significant effect on ecosystem form and function. Extirpation of black-footed ferrets from most of their historic range was very likely due, in part, to plague. Other top predators (e.g., wild felids) are susceptible to plague; the larger-scale effects of such mortality are under investigation. Management of plague is often necessary to protect native wildlife species, ecosystem function, and human health. Because of the risks to human health, safe handling practices should always be followed when working in plague-endemic areas.
To learn more about plague, click here to download the PDF fact sheet.