The plague was never eradicated and continues to spread among mammals such as prairie dogs, rabbits, coyotes, black-footed ferrets, and even domestic animals. It is found on all continents except Oceania and Antarctica, and is especially common in Madagascar, Peru and the Democratic Republic of the Congo.
In the United States, most cases occur from late spring to early fall in the west, particularly in New Mexico, Colorado, Arizona, California, Oregon, and Nevada. In August 2021, California officials had to close parts of Lake Tahoe after a dead squirrel tested positive for bubonic plague. That same month, a New Mexico resident was diagnosed with plague, possibly from fleas brought into the home by a pet.
An outbreak of one of the deadliest diseases in history may sound like a scary thing to do, but thanks to antibiotics, the plague is rarely fatal if left untreated. Currently, there are an average of seven human cases per year in the United States, most of them of bubonic plague, which is contracted through the bite of a flea and is characterized by painful, swollen lymph nodes called buboes.
“This is an extremely rare infection,” says Dave Wagner, director of the Center for Biodefense and Disease Ecology at Northern Arizona University’s Pathogens and Microbiome Institute. “I always tell people to look both ways before crossing the street before worrying about the plague.”
What causes these rare plague outbreaks, and how serious is the risk to humans today? This is what you need to know.
Which animals can spread plague?
caused by bacteria yersinia pestisPlague is a zoonotic disease, which means that animals can transmit it to humans. Besides bubonic plague, other forms are septicemic plague, which spreads through the bloodstream, and pneumonic plague, which thrives in the lungs and can be spread from person to person.
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Black rats (typical house rats) have been responsible for one of the worst plague epidemics in history. But, according to Wagner, they are not the only ones to blame. “It’s primarily a disease of rodents and their fleas,” he says.
Fleas live on the blood of mammals, and they prefer to attach themselves to rodents, not just black rats. when infected with a rodent y pestis, quickly transmits the pathogen to its fleas. And when the rodent dies, the flea jumps onto its next host, which may be a human. A 2018 study concluded that the plague was not widely spread in Europe through rats, but through fleas and lice living on humans.
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Scientists aren’t sure what makes rodents such good hosts y pestis, Wagner says this probably has to do with their tendency to live in burrows where fleas abound. Rodents are likely to have a higher density of bacteria in their blood than at least other mammals. Wagner explains that fleas can suck very small amounts of blood, so there must be a lot of bacteria in that sip for the host to infect the flea.
The species carrying the plague varies by region. In Madagascar, for example, black rats are the most common host. However, in North America, plague is most common in prairie dogs as well as other indigenous rodents such as wood rats and squirrels.
Scientists have also found evidence of plague’s presence in wild animals other than rodents such as rabbits, deer, black-footed ferrets, cougars and coyotes, and even domestic cats and dogs. Some may contract the disease by eating an infected animal, while others may be in the wrong place at the wrong time when an infected flea is looking for a new host.
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“Imagine what a dog would do if all prairie dogs were dead [de peste] “They smell more delicious in their burrows,” says Wagner. They are going to dig in the burrow.
But scientists say it is difficult to ascertain whether these wild animals got plague from rats or whether they are also responsible for keeping the plague alive. Perhaps prairie dogs are to blame as this is especially evident when plague has infected their usually active and dense colonies.
“If prairie dogs were to disappear, the prime suspect would be plague,” says Wagner.
Where do plague outbreaks usually occur in the United States?
Black rats probably brought plague to North America in 1900, when the first case on the continent was reported in San Francisco. historical records and genetic studies of that specific strain of y pestis suggest that the stowaway rats arrived at the port on a steamer from Hong Kong, then moved into the city where they mingled with local rodents.
The bacterium spread to the western United States, where it began to circulate among small native rodents. But it never spread. Today, cases are generally reported west of the 100th meridian, the longitudinal line that runs along the edge of the Great Plains from North Dakota to Texas.
It is not clear why. Paul Mead, chief of the Bacteriological Disease Branch of the Centers for Disease Control’s (CDC) Division of Vector-Borne Diseases, noted that the first outbreaks occurred in eastern port cities. There were also outbreaks in Florida and other parts of the South, but the plague did not manage to establish itself there for long. Scientists suspect this may be related to soil differences in the west, along with the burrowing lifestyle of local rodents.
“The hypothesis is that… it provides an ecological niche in which [la peste] It can persist and continue to exist,” Mead says. It’s possible that the barren western soil is a better home for plague or for the amoeba that helps it replicate and survive. But he and other experts say Ultimately it is still a mystery why plague is only seen in the western United States.
Modern outbreaks are mostly seen in rural areas, says Mead, where rodent colonies tend to be large. Better sanitation and building codes in cities have also contributed to lower urban rates. However, outbreaks continue to occur in cities, such as in 2007 among tree squirrels in Denver’s City Park.
According to Mead, there are about a dozen animal plague reports each year, usually during the summer. But it’s not clear what drives the transition. It seems that “there is some sort of environmental signal that the conditions are right for the reservoir to germinate, whatever it may be, into these more specialized species,” Wagner says.
What is the risk of plague to humans?
It is highly unlikely that humans will get the plague, but if they do, the disease is not a death sentence for those who have the wide variety of antibiotics that treat it. Although some worry that antibiotic-resistant strains could change that calculus, scientists say the current risk is low.
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Plague also poses less of a risk to people living in Western countries, such as the United States, because they generally do not spend much time in flea-infested areas. Most flea species have a preferred species to feed on. Once they have found a host of that species, they are unlikely to jump to another animal unless that host has died.
Therefore, if you pass through a prairie dog colony with individuals that have recently died of plague, you may be at risk of being bitten by an infected flea. But “unless you’re living next to a prairie dog colony in the western US, I think the risk is essentially zero,” says Wagner.
There are some very rare circumstances in which humans can be infected by an animal intermediary. For example, dogs can carry infected fleas to humans. Domestic cats, which are particularly susceptible to the respiratory form of plague, can infect people around them through sneezing and other respiratory droplets, as colds, flu, and COVID-19 spread.
To prevent infection, Mead says it’s important for people living in rural western states to be aware of their surroundings. Report large prairie dog deaths to local health officials. Always give flea treatments to domestic animals and don’t allow them to roam areas with prairie dog colonies.
Why was no effort made to end the plague?
Although the risk to humans is low, it is more dangerous to some at-risk species, notably the endangered black-footed ferret. Plague is one of the main threats to the existence of this species, which feeds on prairie dogs. In recent years, conservationists have begun pioneering efforts to save these ferrets by vaccinating them against plague.
But Mead says the United States is unlikely to expand its vaccination efforts and try to eradicate plague. Those efforts may not be worth it anyway, he says, because eliminating a disease requires a deep understanding of how it spreads. Plague is now endemic in the western US, and scientists still don’t know how it persists when there are no outbreaks. It may be soil-dwelling, or it may be transmitted among wildlife, making outbreaks more difficult to trace. Plague can be suppressed in one area by vaccination so that it appears in another area.
“It’s like wildfire,” says Mead. “The bottom line is that in the West we will probably plague this as a low-level risk for some time.”