As in Bubonic Plague, carried by various mammals, esp. prairie dogs in the western and southwestern U.S. Associated with flea vectors. Pest: Yersinia pestis….
Geographically, you’d expect the plague to be western-bound. But significant numbers of cases are reported all over the United States since 2010.
The east coast peaks in cases demonstrate something unexpected about this diseases. Historically, we think of it as the great bubonic plague, once gone awry but now under considerable control. In the U.S., we often learn about the association of the plague with prairie dogs. Since the flea is its vector, you need only be near a prairie dog colony to potentially be exposed to this organism. (Not great news for us Western field epidemiologists out there dealing with other zoonotic diseases.)
But the matching high peaks along the west coast points to population density as well influencing where the cases are reported. This should be no surprise, since peoples’ presence is needed for human cases to be developed, and then later reported.
So initially, this map tells us that the plague could be transmitted by many other diseases, in both natural and human ecological manners, and unfortunately bearing reasons for its potential urban impacts that in some ways are very different from why and how it becomes a rural epidemiological event.
However, people travel, and in-migration routes makes their way to the heavily populated ports for international shipping.
Is latitude a feature in how and where an imported case of the plague might enter the U.S.? Perhaps, since climate does impact both vector and host activity patterns and willingness to travel or change both animal and environmental settings.
Population density is more a concern for international spread of Yersinia pestis should some new, resistant strain emerge.
Is the Pacific Rim a route of travel to consider? If you ask a west coast public health economist or epidemiologist, most certainly.
Still much of the commercial travel from the Orient follows traditional routes, and the more heavily, most heavily populated regions of the east coast are very likely to produce some of the first cases if their origins were abroad.
This leaves only a single isolated peak in the midwest to be better understood.