A California man was confirmed to have contracted the plague earlier this week, the fifth case of the infamous disease in the United States this year.
The man, a South Lake Tahoe resident, was California's first case of plague in five years, according to the El Dorado County health department. In July, Colorado also saw its first case in five years when a southwestern region resident, who has since recovered, was infected, according to the Colorado Department of Public Health and Environment. Navajo County public health officials documented a case in Arizona late July. And two cases this year were reported in New Mexico, including a man who died.
Reports of plague may sound scary, but experts say the bacterial infection is not something to fret about.
“Bubonic plague in the U.S. is not the same scenario as the historical Black Death, and we do not need to be afraid of it in the same way,” Susan Jones, a professor of ecology, evolution and behavior at the University of Minnesota College of Biological Sciences, said.
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Though tragic, the cases this year aren’t unusual, she said.
The U.S. averages seven human cases of plague each year, according to the Centers for Disease Control and Prevention. In 2006, cases reached a high of 17.
Whether there are more yearly cases than average can depend on what’s going on in rodents. Plague is caused by bacteria called Yersinia pestis, which are carried by fleas that live on rodents including prairie dogs and squirrels. Humans are more likely to become infected with plague when there is an outbreak of the bacterial infection in local wild rodent populations, Jones said.
“Plague waxes and wanes in the rodents. When a lot of rodents are infected and dying, plague can more easily spill over into nearby humans,” she said. With just five human cases logged so far this year, the U.S. is still well within the normal range, though fatalities are uncommon.
“It is unusual to have more than one plague-related death in the same year, and few cases result in death,” Dave Morgan, a spokesperson for the New Mexico Department of Health, told NBC News in an email. When plague is caught early and treated with antibiotics, the chance of death falls to around 10 percent.
There had not been a plague fatality in the U.S. since 2015, which had 16 total cases, four of which resulted in deaths, according to the CDC.
Though there is limited information surrounding the death of the man in Rio Arriba County on the New Mexico-Colorado border, the New Mexico Department of Health reported the case as septicemic plague, rather than the much more common bubonic plague.
Both septicemic plague and bubonic plague are caused by the same bacteria; the name refers to what part of the body is affected, according to the Mayo Clinic. Bubonic plague occurs when the infection causes large, swollen lymph nodes, called buboes. Septicemic plague refers to an infection in the blood, and can be the first sign of infection or develop from untreated bubonic plague, according to the CDC. Neither are contagious.
Only the third form of plague — pneumonic plague — can be spread from person to person. Pneumonic plague occurs when the infection gets into the lungs, either from untreated bubonic or septicemic plague, or from inhaling infected droplets from another person. It is the most serious form of the disease, the CDC says.
"Early diagnosis is very important," Jones said, adding that in parts of the U.S. where plague is common, "physicians and veterinarians are on alert for symptoms of plague."
Many symptoms of plague, she noted, such as fever, chills and headache, can be confused with other illnesses. However, each year, epidemiologists create maps to track reports of the plague in rodents in the western U.S. If a person has spent time outdoors in these areas and has these symptoms, doctors may be more inclined to consider plague.
The majority of cases occur in the Four Corners area, where Arizona, Colorado, New Mexico and Utah meet, though plague is detected in rodents across the West. According to Jones, this is due to the squirrels and prairie dogs that live in these areas and harbor plague-carrying fleas. These fleas can make their way to humans by latching onto pets that roam in rural areas.
"When you are outdoors in areas known to harbor plague, do not touch any wild rodents or rabbits. Don't let your pets roam around. Apply flea control to pets, and repellent to yourself if you are walking or camping in the wilderness. Stay away from areas that harbor rodents, such as prairie dog towns, hay bale storage and wood piles," Jones said. "And if you are concerned that you or your pet may have been exposed to plague, contact your physician and your veterinarian."