The Plague Is Still Very Much Alive in The US — Here’s the Annoying Reason We Can’t Get Rid Of it

The plague is often relegated to the distant past, a fatal super pathogen of a bygone era that left millions dead in its centuries-long wake. In reality, the plague-causing bacteria Yersinia pestis still lurks in many parts of the world, including in the United States. Earlier this month, health officials in Central Oregon announced the first human case of the plague in the state since 2015.

The unidentified individual from Deschutes County was diagnosed with bubonic plague, the most common form of the disease that infects lymph nodes and starts with flu-like symptoms like fever, chills, and headache. Health officials stated the individual was likely infected by a pet cat, an animal highly susceptible to contracting and spreading the bacteria to humans.

“All close contacts of the resident and their pet have been contacted and provided medication to prevent illness,” Richard Fawcett, health officer for Deschutes County Health Services, said in a statement.

Why the Plague won’t die

While the news may sound like history returning to haunt us, the U.S. is no stranger to the plague, although its occurrence is quite rare. According to the Centers for Disease Control and Prevention, the U.S. averages around seven human cases of the plague per year; worldwide, this runs anywhere from 1,000 to 2,000 cases, likely even higher. The last large outbreak occurred in 2017 in Madagascar, with over 2,000 cases reported.

Although public health measures and medical treatments for the plague have evolved massively since the infection’s medieval heydays, Yersinia pestis continues to be a pernicious nuisance — and a serious cause for concern if it starts circulating among humans. (The CDC considers the plague a Category A biological agent presenting a high public health risk.)

Why the bacteria remains an enduring thorn in our side has much to do with the animals harboring the pathogen, the pathogen itself, and the inherent challenges of eradication.

Hiding in the gut

Yersinia pestis is an ancient microbe believed to have originated in or near China, evolving from another bacteria called Yersinia pseudotuberculosis around 7,000 years ago. Genetically, Yersinia pestis is a close clone of Yersinia pseudotuberculosis, which causes self-limiting gastrointestinal illness but is a bit more virulent, meaning it’s more capable of infecting and causing disease in its host.

“It’s called an enzootic disease, which means it lives in the population, in our case, at a very low level,” Emilio DeBess, the state public health veterinarian and an epidemiologist at the Oregon Health Authority, tells Inverse. “The transmission happens between rodents and fleas — they go back and forth — and it’s at a very low level, meaning when one animal dies, the fleas find another suitable host, whether it’s an animal or a human.”

Transmission is pretty straightforward. When a flea chomps down on an unwitting animal (or human) for its blood meal, the bacteria, hiding out in the gut, induces the insect to vomit it up. This is in contrast to other insect-borne diseases, like Lyme disease and malaria, where their respective pathogens tend to hide in salivary glands.

Yersinia pestis was introduced via rat-infested ships into port cities in the western U.S. around 1900 and spilled into native rodent species throughout the region from 1900 to 1950, said Rebecca Eisen, a research biologist in the CDC’s National Center for Emerging and Zoonotic Infectious Diseases.

“Human cases of plague have diminished with improved sanitation, but plague bacteria still circulate among wildlife and fleas. Many wildlife species are susceptible to plague infection, but ground squirrels, prairie dogs, chipmunks, and woodrats are some of the most common hosts,” Eisen wrote in an email to Inverse.

Repel those fleas

In the recent human case of bubonic plague in Oregon, the infection likely jumped from a pet cat. These animals tend to be more susceptible mostly because they spend considerable time outdoors, putting them at risk of flea bites or eating infected rodents.

“In some years, we see large-scale plague die-offs among rodents, and humans are at greatest risk of infection during those periods of time (know as epizootics),” wrote Eisen. “In some cases, pets (cats and dogs) might encounter plague-infected rodents or their fleas and bring the infection back to their human families.”

While there are a variety of antibiotics that easily treat the plague and reduce the risk of death associated with its other more severe clinical forms like septicemic and pneumonic plague (the latter responsible for human-to-human transmission), wholesale eradication from wildlife reservoirs is nigh impossible.

“The wide range of susceptible hosts, broad endemic area, and the lack of understanding of how the bacteria persists between outbreaks pose immense challenges to eradication,” wrote Eisen.

Some studies have found going from those stable low levels of Yersinia pestis to periodic fatal outbreaks may have to do with fleas carrying the bacteria between animals. For example, a 2022 paper found that the first time a flea feeds on its host after becoming infected — what’s called early-phase or mass transmission — isn’t effective at instigating outbreaks unless the rodents are already vulnerable to the disease for whatever reason. When the microbe reaches a stage of transmission — called blockage-dependent transmission — where it forms a biofilm in the fleas’ digestive tract, preventing the insect from feeding (essentially starving it), this appears far more successful in causing undesirable epizootics.

Insight from this and other research hasn’t yet lent to any means for eradication, such as with, say, a vaccine. But DeBess says our centuries-long history with Yersinia pestis gives us the weapons to stop the plague in its tracks before it can harm us. For one, he recommends any individuals living in the western U.S., where the plague is endemic, protect themselves when they’re outside.

“Don’t handle dead or dying animals that could have fleas looking for a blood meal [and] will jump onto humans and transmit the disease,” says DeBess.

For individuals with pets that go outdoors, even occasionally, he recommends staying up to date on their flea prevention. “That alone will prevent [the plague] from coming into your household,” says DeBess.

Eisen also recommends rodent-proofing your home and yard by keeping it free of any food or other debris that would attract a tiny critter.

The bottom line: While we might never eradicate it completely, as long as we exercise some good old-fashioned common sense, our chances of getting the plague will remain as slim as possible.