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The plague, one of the deadliest bacterial infections in human history, claimed an estimated 50 million lives in Europe during the Middle Ages; at the time it was known as the “Black Death.”
Although the disease is extremely rare, it still exists today. In March, a man in New Mexico died of the plague after being hospitalized for the disease, and in February, a person in Oregon was diagnosed with bubonic plague after likely contracting it from his pet cat.
The disease is transmitted by fleas that feed on rodents. Symptoms usually appear within one to seven days of infection and include painful, swollen lymph nodes (called bumps) in the groin, armpits, or neck area, as well as fever, chills, and cough.
The plague affects humans and other mammals.
According to the U.S. Centers for Disease Control and Prevention, people typically contract plague after being bitten by a rodent flea carrying the Yersinia pestis bacteria that causes the disease, or through contact with an infected animal.
Cats that become sick themselves can infect humans directly, while hardier dogs can simply carry the fleas back to their owners. People can also become sick by inhaling droplets from the cough of an infected person or animal.
According to the CDC, the bacteria survive because they circulate in small amounts in populations of certain rodents. These infected animals and their fleas serve as long-term reservoirs for the bacteria.
“The reason it has not been eradicated is because there is a reservoir in animals,” Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, said in February. “The bacteria can infect animals, and since we cannot treat all animals in the wild, they persist in nature and therefore occasionally cause a limited number of cases in humans.”
Plague occurs naturally in rural areas of the Western United States, particularly in Arizona, California, Colorado, and New Mexico. There, an average of seven human cases of plague are reported to the CDC each year. However, significantly more cases occur in parts of Africa and Asia.
According to the World Health Organization, 3,248 cases were reported worldwide between 2010 and 2015, including 584 deaths. The three countries where the disease is most common are the Democratic Republic of Congo, Madagascar and Peru.
There are three types of plague: bubonic plague, septicemic plague and pneumonic plague. Pneumonic plague – which affects the lungs – carries the risk of direct human-to-human transmission. This was the case with the major outbreak in Madagascar in 2017, which resulted in 2,348 confirmed, probable and suspected cases and 202 deaths.
The last plague epidemic in a major U.S. city occurred in Los Angeles in 1924-1925, the CDC said.
Modern antibiotics – streptomycin is the usual first-line treatment – can prevent complications and deaths if given immediately after symptoms appear. The treatment is used for the two most common types of plague: bubonic plague and pneumonic plague.
“The reason it caused so much death and destruction in the Middle Ages is because there were no antibiotics then,” Barouch said of the plague in February.
“Although it can be a serious disease, it is usually easily treatable with antibiotics as long as it is caught early. So it is now a very treatable disease. It should not cause the fear that people had of the Black Death in the Middle Ages,” he said. “If someone is experiencing symptoms consistent with the plague – usually the first symptoms are fever, chills and swollen lymph nodes – they should see a doctor because in the early stages the plague is easily treatable with antibiotics.”
According to the WHO, the mortality rate of bubonic plague is 30 to 60 percent if left untreated, while pneumonic plague is always fatal if left untreated.
However, a strain of bubonic plague has been identified in Madagascar that is highly resistant to streptomycin.
More than 80% of cases in the United States are bubonic plague, the most common form of the infection. If left untreated, bubonic plague can turn into the more serious pneumonic plague, which causes rapidly developing pneumonia as bacteria spread to the lungs.
There is a vaccine against Yersinia pestis, but it is recommended only for high-risk people, such as scientists who work directly with the bacteria, Dr. Harish Moorjani, an infectious disease specialist at Phelps Hospital in New York, part of Northwell Health, said in February.
“Most people don’t need the vaccine,” Moorjani said.
A 2019 review of experimental plague vaccines suggests that researchers are exploring different approaches to develop an effective plague vaccine.
Since different vaccine designs result in different immunity mechanisms, the authors conclude that combinations of different types could overcome the limitations of individual vaccines and effectively prevent a possible plague outbreak.
How do you protect yourself and your family?
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The most important steps to prevent an infestation include eliminating rodent nesting sites around your home, sheds, garages and recreation areas by removing brush, rock piles, garbage and excess firewood.
Report sick or dead animals to the police or local health department. Do not pick them up or touch them yourself. If you absolutely must touch a sick or dead animal, wear gloves.
If you live in an endemic area, take extra precautions. Use insect repellent containing DEET to prevent flea bites, and treat dogs and cats regularly for fleas. Do not sleep with your pets, as this increases your risk of contracting the plague. Finally, your pets should not hunt or roam in rodent habitats such as prairie dog colonies.
CNN’s Jacqueline Howard and Mira Cheng contributed to this report.
