Bubonic Plague: It’s No Joke

While looking for a dispersed camping spot in the Buffalo Gap National Grasslands, we found several of the following signs nailed to posts on some of the dirt roads we were about to explore.  Timm posted this picture of the sign on Facebook and our sister-in-law Lauren said, “I thought he posted it on Facebook as a joke.” Unfortunately, it’s real. Bubonic plague has infected the prairie dog populations in this area of South Dakota.

Bubonic plague sign in Buffalo Gap National Grasslands

Bubonic plague (often just called “the plague”) is an infectious disease of animals and humans caused by a bacterium named Yersinia pestis.  People usually contract the plague by being bitten by a flea that previously bit a rodent that had the plague.  Modern antibiotics are effective against plague, but if an infected person is not treated promptly, the disease causes severe illness or death.  In fact, without treatment, the bubonic plague kills about two thirds of infected humans within 4 days.

Bubonic plague—along with the septicemic plague and the pneumonic plague, which are the two other manifestations of Y. pestis—is generally believed to be the cause of the Black Death that swept through Europe in the 14th century and killed about 25 million people, or 30–60% of the European population.


Human Plague Cases 1970-1977  Copyright © CDC

Bubonic plague outbreaks still occur around the world, especially in areas where rats live in the home.  In the United States, the last urban plague epidemic occurred in Los Angeles in 1924.  Since then, there have been an average of 10-15 cases per year, scattered in rural areas in the western states: New Mexico, Arizona, Colorado, California, Oregon and Nevada.

At least 76 wild mammals are known to carry plague but the most common animals are all types of rodents: rats, mice, squirrels, and prairie dogs.  The most common animal to transmit plague to humans is the cat.  Cats are great hunters of rodents.  Rodents with the plague have fleas that jump from the rodent to the cat to the human. 


Two prairie dogs in South Dakota's Wind Cave National Park

On the other hand, prairie dog to human plague transmission is extremely rare.  The reason is that prairie dogs are especially vulnerable to plague.  When plague enters a prairie dog town, it rapidly kills 100% of the prairie dogs. Therefore, the prairie dog does not live long enough to transfer the disease to other animals, including humans.


Black Footed Ferret Copyright © USFWS Mountain Prairie Image Used Uner Creative Common License

Prairie dog towns infected with the plague is devastating to the entire fragile grassland ecosystem because the prairie dog is a “keystone species.”  A keystone species is an organism that has a disproportionately large effect on its environment, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community.  There are over 160 species dependent on the prairie dog for their existence. For example, the black footed ferret, once thought to be extinct, is a great example of an animal that requires the prairie dog in order to exist.  The prairie dog is the main food source for the black footed ferret. 

South Dakota prairie dogs recently contracted the plague.  Unfortunately for the prairie dog and the 160 other species that depend n them, South Dakota was the last stronghold for the prairie dog population in the United States. Four of the largest 7 remaining prairie dog colonies are found in South Dakota and 32% of all remaining prairie dogs are located there. 

Despite the sign warning humans of the existence of plague in specific areas of the grasslands, it is really the prairie dog that is in mortal danger from the disease.

A priairie dog at Devil's Tower in Wyoming stretches out on his belly.

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