In the western United States, which species is the most common harbor for the plague organism?

• A. Dogs
• B. Cats
• C. Beavers
• D. Prairie dogs

Answer: (D)

Prairie dogs are the main reservoir for Yersinia pestis in the western United States because they live in large, dense colonies that sustain a thriving flea population capable of transmitting the bacterium. The pest persists in the rodent–flea cycle within these colonies, and fleas feeding on infected prairie dogs become carriers that spread the organism to other prairie dogs and potentially to other species. When epizootics occur, many prairie dogs die, and those fleas drop off into the environment, increasing the chance of bites to humans or domestic animals. Dogs and cats can become infected if exposed, but they do not serve as the primary natural reservoirs; beavers are not the typical host in this region. Thus Prairie dogs are the most common harbor for the plague organism in the western US.