By Jonny Lupsha, News Writer

A Denver wildlife refuge reopened after a plague outbreak in local prairie dogs, according to the U.S. Fish and Wildlife Service website. The area originally closed in late July 2019 as a precaution for visitors’ health and safety. The plague has many strains and symptoms to beware.

The Rocky Mountain Arsenal Wildlife Refuge, once again, opened its doors to visitors on August 17. According to the announcement on the U.S. Fish and Wildlife Service website, responders used insecticides to kill off plague-carrying fleas that infected black-tailed prairie dogs, which in turn infected black-footed ferrets in the area. The refuge has promised that its employees will continue to monitor the situation dutifully for the health of park visitors and the wildlife itself. Whether you’re a Colorado resident or not, it may be a good time to brush up on plague knowledge.

Bubonic Plague: Dispelling the Rat Myth

Bubonic plague is the most well-known form of plague, long thought to be transmitted exclusively by rats. With the bubonic plague, “infection was signaled not only by feeling downright horrible, but was also announced as plague—and not your run-of-the-mill medieval illness—by swellings at the neck, groin, and armpits,” said Dr. Dorsey Armstrong, Associate Professor of English and Medieval Literature at Purdue University. “With this form, human-to-human transmission seems to be almost impossible, although it may have occurred in some instances when doctors or caretakers tried to affect a cure by lancing the buboes.”

But bubonic plague didn’t come to humanity from rats—at least, not directly. “Many kinds of rodents can carry plague, and the fleas that feed on rats, guinea pigs, and other similar animals, like squirrels, can become infected with plague,” Dr. Armstrong said. However, fleas infected with plague only become transmitters of the disease due to a strange feature of their digestive system.

According to Dr. Armstrong, in addition to a regular stomach or ventriculus, fleas have a regulatory valve called a proventriculus. “When a flea feeds on a plague-infected rodent, the nourishment doesn’t pass to the ventriculus as quickly or as easily as it would if a flea were feeding on a non-infected rodent; in fact, what happens is a blockage of bacteria, and blood forms in the proventriculus, so that nourishment can’t get to the flea’s stomach,” she said. “Now you’ve got a very hungry flea who keeps biting more aggressively and frequently in order to get some nourishment, but the blockage in the proventriculus just gets bigger and bigger. Finally, the flea’s system realizes what’s happening and regurgitates the blockage out of the proventriculus, directly into the system of whatever the flea was feeding on.”

Dr. Armstrong said that once an infected rat died and the flea suffered its digestive problems, its hunger would cause it to find a food source it wouldn’t normally consider—like humans.

Pneumonic Plague: Transmission and Symptoms

Dr. Armstrong also said that most scholars agree that there were two other forms of plague: pneumonic and septicemic. Both are believed to begin as bubonic plague and mutate in a patient, then spread by human contact.

“Pneumonic plague was the second most common form of plague, and [it] set up shop in the sufferer’s respiratory system, rather than in the lymphatic system, as is the case with the bubonic form,” Dr. Armstrong said. “What’s uniquely terrifying about this form of plague is that it is now easily transmissible from person to person. A doctor or friend or relative taking care of someone infected with the pneumonic form of plague is going to be coming in contact with blood, sputum, saliva, all containing the bacterium, and they will usually themselves become infected.”

Sufferers of pneumonic plague often died by drowning in their own blood, Dr. Armstrong said, as their respiratory systems broke down and blood filled their lungs.

Septicemic Plague and “The Sign of the Plague”

Septicemic plague is an infection of the blood. “When plague bacteria enter the bloodstream, they cause something known as disseminated intravascular coagulation, or DIC,” Dr. Armstrong said. “In these instances, tiny blood clots start to form throughout the body, which results in something called localized ischemic necrosis, which is just a fancy way of saying that portions of your body tissue start to die off due to lack of circulation.”

Septicemic plague is said to kill within 24 hours, as patients’ blood failed to clot and seeped into other parts of their body like the skin and internal organs. While this happened, Dr. Armstrong said, patches of red, bumpy skin became visible all over the body. This symptom is rumored to be the source of the claim that people “showed the sign of the plague.”

This particularly nasty set of diseases has ravaged humanity in several widespread epidemics. Nowadays, plague is exceptionally rare, though it still occurs occasionally around the world—as some unhappy prairie dogs in Denver have proven.

Dr. Dorsey Armstrong contributed to this article. Dr. Armstrong is Associate Professor of English and Medieval Literature at Purdue University, where she has taught since 2002. She earned an A.B. in English and Creative Writing from Stanford University and a Ph.D. in Medieval Literature from Duke University.