The Plague, Rare but Still Around, Is Both Preventable and Treatable

The bacteria that caused the Black Death in the Middle Ages persists in the U.S. and abroad, but human deaths are largely avoidable

With pandemic anxiety at a worldwide high because of COVID-19, the last thing anyone wants to see in the news is word of another plague—especially the plague, which has recently been reported in people in China and Mongolia and, closer to home, in a squirrel in Colorado.

“When you hear about the plague, you may think of the Black Death—the epidemic estimated to have killed 50 million people or more during the 1300s,” said Cummings School assistant professor Felicia Nutter, V93.

But there’s no need to panic, said Nutter, a board-certified wildlife veterinarian and epidemiologist. The plague has never left—it has been here all along, even in the United States—and it can usually be treated with antibiotics, so long as it is properly diagnosed.

Tufts Now spoke with Nutter about the ancient disease, who’s still at risk, and how to protect yourself and your pets.

Tufts Now: What is the plague, and who is at risk?

Felicia Nutter: The plague is a serious infectious disease caused by the bacteria Yersinia pestis. The plague persists in nature by infecting wild rodents—including mice, rats, squirrels, chipmunks, groundhogs, and prairie dogs—and their fleas. Fleas that have fed on infected rodents can spread the infection to humans and other animals by biting them.

Humans can also get the plague through direct contact with infected animals, both living and dead. This includes handling infected animals, coming into contact with their tissues or bodily fluids, eating infected animals, being bitten or scratched by infected animals, or inhaling infectious respiratory droplets.

People can also be exposed to plague via their pets if they’ve acquired fleas from infected wild animals or been infected themselves.

Pets can catch plague?

Yes. Cats in particular are highly susceptible to plague, developing symptoms similar to those in humans, and have been implicated in transmitting plague to humans in numerous cases.

In fact, most human cases of primary pneumonic plague in the U.S.—and there have only been a handful since 1924—have been caused by inhalation of respiratory droplets from coughing or sneezing infected domestic cats. Between 1977 and 1998, twenty-three of the 297 reported cases of human plague in the U.S. were associated with cats.

What happens if you catch the plague?

The good news is that although a very severe and potentially fatal illness, the plague can usually be treated with common antibiotics. However, if undiagnosed and untreated, people can die from the plague, just like they did in the fourteenth century.

Most human cases of infection with Yersinia pestis present as one of three different forms of disease: bubonic, septicemic, or pneumonic. All three forms can usually be treated with antibiotics—if diagnosed in time.

Bubonic plague is the most common form of plague. It is named for the large, painful, swelling that occurs in the lymph nodes—or buboes—that typically develop two to six days after initial infection. Other symptoms include fever, headache, chills, muscle aches, and fatigue. Some patients also may experience nausea, vomiting, and diarrhea. With appropriate antibiotic treatment, bubonic plague is fatal in less than five percent of cases.

If left untreated and the bacteria gets into the blood, bubonic plague can develop into septicemic plague. Septicemic plague also can develop when the bacteria enters the blood directly, for example through a cut while butchering an infected animal. Fortunately, it is similarly treatable to bubonic plague with antibiotics. Symptoms of septicemic plague are similar to those of bubonic plague, but without the swollen lymph nodes and with the additional risk of developing gangrene in extremities due to blood clots.

Pneumonic plague is the rarest, but most severe form of the disease. It is 100 percent fatal if not treated rapidly and is deadly in up to half of all cases, even with prompt antibiotic treatment.

Untreated cases of bubonic or septicemic plague can become pneumonic plague as infection spreads to the lungs. People can also become infected by inhaling respiratory droplets containing Yersinia pestis from other infected humans or animals. Symptoms, which can develop one to three days after infection, include fever, chills, and body aches and progress rapidly to pneumonia, coughing, chest pain, bloody sputum, difficulty breathing, and death.

Does the plague spread from person to person?

Yes, but human-to-human transmission of the plague requires close contact, typically being within six feet of a person who is coughing and producing lots of infected droplets in the final stages of the disease. There has not been a documented case of human-to-human transmission of pneumonic plague in the U.S. since 1924, but pneumonic plague still occurs in parts of the world where plague is more likely to occur.

Are wild animals harmed by the plague?

Plague is actually a much greater threat to both prairie dogs and black-footed ferrets than it is to humans in the U.S.

The only native ferret species and one of the most endangered mammals on the continent, the black-footed ferret preys on prairie dogs, which can account for 90 percent of their diets.

Prairie dogs are very susceptible to plague infection, and colonies can become outbreak sites with up to 100 percent mortality. This can lead to plague infection in black-footed ferrets—with 90 percent or more of infected ferrets dying from infected flea bites and other exposures to infected prairie dogs—and to the ferrets dying of starvation.

Conservationists are using multiple methods to fight plague in both species, including treating prairie dog colonies with insecticides to kill fleas and developing vaccines for both prairie dogs and black-footed ferrets.

Where does the plague occur, and how common is it?

In the U.S., the Centers for Disease Control and Prevention (CDC) reports that plague occurs in wild animals and their fleas in all states west of the 100th meridian. That includes western parts of North and South Dakota, Nebraska, Kansas, Oklahoma, and Texas, as well as the entirety of Montana, Wyoming, Colorado, New Mexico, Idaho, Utah, Arizona, Nevada, Washington, Oregon, and California. It’s most common in northern New Mexico, northern Arizona, southern Colorado, California, southern Oregon, and western Nevada—and these areas are also where most human cases occur.

Still, in the U.S., plague remains a very rare human disease. Since 2000, the CDC has reported an average of just seven human cases of plague per year.

Worldwide, plague is naturally found on all continents except for Australia and Antarctica, and there is a risk of humans contracting plague anywhere that they co-exist with a rodent reservoir and a vector such as a flea that can spread the disease between humans and animals. The World Health Organization estimates that 500 to 1,000 human cases of plague occur annually, with most cases reported from Africa (Democratic Republic of Congo and Madagascar) and South America (Peru). The last large outbreak occurred in 2017 in Madagascar, with 2,348 cases reported.

Can the plague be prevented?

There is no vaccine for Yersinia pestis available in the U.S. for people or pets. However, because public-health professionals, doctors, veterinarians, and other scientists know so much about the plague, there is a lot of good information available on how to reduce the risk of becoming infected.

If you live in, work in, or are traveling to an area where plague is known to occur, take these steps to protect yourself:

  • Reduce exposure to rodent fleas while working or enjoying the outdoors by wearing pants and long sleeves and using a repellant effective against fleas (such as one with DEET).
  • Limit exposure to fleas on your pets by using approved flea-control products. Keep pets from interacting with possibly infected wild animals by not allowing them to freely roam outdoors.
  • Wear gloves when handling animals that could be infected with plague, especially rodents. This includes skinning or butchering hunted animals from areas where plague occurs.
  • Make your home, work place, and recreation areas less attractive to rodents by removing piles of brush and leaves, keeping woodpiles well away from buildings, eliminating sources of food, and rodent-proofing buildings.
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