Health officials in the Sierra Nevada region of California announced this week that a local resident tested positive for plague, an extremely rare bacterial infection usually transmitted through flea bites.
The case is still under investigation, but the officials believe the person, who is now recovering at home, may have been bitten while camping in South Lake Tahoe. Plague can be treated with antibiotics if it is caught early, but can otherwise become very serious.
Here’s what to know.
Plague still exists?
It does. Most people think of plague as the “Black Death,” a medieval pandemic that might have killed tens of millions of people in Europe. But the plague bacteria has survived through the centuries by circulating among certain rodents and fleas that serve as long-term reservoirs, making it nearly impossible to eradicate.
Experts believe plague was introduced to the United States in 1900, when rat-infested steamships pulled into port cities. The disease periodically broke out in rapidly developing urban areas with lots of rats — such as Los Angeles in the 1920s. Now, cases are rare here, occurring mostly in rural areas where people tend to interact with wild rodents.
How does plague spread and cause sickness?
Plague is caused by a bacterium called Yersinia pestis. Humans can acquire the infection when they are bitten by fleas that were living on infected chipmunks, squirrels, ferrets and other creatures. People can also become infected by handling animal tissue or body fluids, such as when skinning a carcass after hunting. Pets, particularly cats, are also susceptible to the disease when they play with or eat sick rodents.
There are three main types of plague, said Dr. Edward Jones-Lopez, an infectious disease specialist with Keck Medicine of the University of Southern California. The most common is bubonic, in which the bacteria multiply in the lymph nodes closest to a flea bite and trigger an inflammatory response. Septicemic plague occurs when the infection spreads to the bloodstream. Pneumonic plague refers to an infection in the lungs.
Plague cannot be transmitted between people except in its pneumonic form, when patients might spread infected droplets to someone close by through a cough or sneeze. Person-to-person spread has not been documented in the United States in more than a century, though there have been cases of pneumonic plague in pet owners or veterinarians exposed to sick cats.
What are the symptoms and treatment?
Within two weeks of exposure, patients typically come down with a fever, nausea, weakness and swollen lymph nodes. If there is reason to suspect a plague infection, doctors take a blood sample or a small bit of a swollen lymph gland for laboratory testing.
The Centers for Disease Control and Prevention recommends that patients be started on antibiotics immediately, even before results come back. That’s because untreated plague becomes extremely dangerous. A man in Arizona died from the disease last month.
Should I be worried?
In the United States, the plague remains extremely rare. An average of seven people become infected each year, according to the C.D.C. The vast majority of them are in Western states like Arizona, California and New Mexico.
Local officials in El Dorado County, which includes South Lake Tahoe, said they had not seen a case in the region since 2020. The California Department of Public Health, which conducts ongoing surveillance for plague among rodents, has detected the infection in only 45 ground squirrels and chipmunks in the county since 2021, all of them in the Tahoe Basin.
“The risk is incredibly low,” Dr. Jones-Lopez said. “If you’re living in this country, you’re at much higher risk of a long list of other things,” he said, adding, “things like measles.”
How can I avoid it?
Health officials recommend that people take extra precaution when hiking and camping in regions with wild rodents: Never touch or feed the rodents, and keep pets on leashes to prevent them from wandering near burrows.
Avoiding flea bites is equally important. Officials suggest wearing long pants tucked into boots and spraying DEET-containing insect repellent on socks. They recommend flea-control products for pets — to protect both your animal and yourself.