Health officials announced this week that a resident of Deschutes County — a rural part of Oregon — was diagnosed with plague, marking the state’s first human case in more than eight years. The person was likely infected by their pet cat, who had developed symptoms, according to Deschutes County Health Services.

Humans are most commonly exposed to plague from the bites of fleas carrying Yersinia pestis, the bacterium that causes the disease. Household pets can get also infected if they hunt rodents infected with plague or are similarly bitten by an infected flea.

Pets can then transfer the infection to humans via tissue or bodily fluids, such as respiratory droplets from cough or sneezes. Alternatively, they might carry home fleas that in turn bite humans.

Cats are particularly susceptible to plague because their bodies have a hard time clearing the infection and they’re more likely than some other pets to chase and capture rodents.

Plague is much rarer in dogs. However, in 2014, Colorado reported four cases of plague among people who had been in close contact with an infected pit bull terrier, including the dog’s owner and two veterinary clinic employees.

Dr. Richard Fawcett, a health officer for Deschutes County, said the cat involved in the recent case was “very sick” and had a draining abscess, which indicated “a fairly substantial” infection.

The owner’s infection likely started out in a lymph node — what’s known as bubonic plague, Fawcett said. By the time the owner was hospitalized, the infection had progressed to the bloodstream, he said. Fawcett said the patient “responded very well to antibiotic treatment.”

However, he noted that some doctors felt the patient had developed a cough while at the hospital. That could be an early sign of pneumonic plague — a version that transmits among humans — but Fawcett said it’s not clear if the disease had progressed that far.

Fawcett said doctors gave antibiotics to the patient’s close contacts out of an abundance of caution to prevent any potential infections from developing into symptoms.

“If we know a patient has the bacteria in the blood, we might decide to be on the safe side,” Fawcett said. He added that he would be “very surprised if we see any other cases.”

Prior to this week, Oregon’s last human plague case was in 2015: A teenage girl presumably got infected from a flea bite during a hunting trip, the state health department said at the time.

Where does plague occur in the U.S.?

On average, the U.S. sees around seven cases of human plague each year, mostly in the rural West. Cases are typically concentrated in northern New Mexico, northern Arizona, southern Colorado, California, southern Oregon, and western Nevada, according to the Centers for Disease Control and Prevention.

“The hotspot is really the Four Corners region” near the borders of Utah, Arizona, Colorado and New Mexico, said David Wagner, director of the Biodefense and Disease Ecology Center at Northern Arizona University’s Pathogen and Microbiome Institute.

But “we still don’t have a good handle of plague persistence in the environment in the western U.S.,” Wagner said. “It’s just so cryptic. It kind of disappears into these populations of rodents and we just don’t know what’s going on out there.”

Fawcett said he’s also not sure why cases have sporadically popped up in Central Oregon over the last dozen or so years.

“We just don’t have a lot of trouble with fleas in this part of the state,” he said, adding that rodents carrying plague don’t seem to be a particular issue either.

According to the CDC, plague tends to occur in semi-arid forests and grasslands that are home to many rodent species. Fawcett said the person who recently got sick in Deschutes County lives in a rural-suburban neighborhood, and “there is open land not far away” from the person’s home.

Deschutes County Health Services advises pet owners to keep their animals on a leash when outdoors, give them flea control products and take them to a veterinarian if they become sick after touching a rodent. The department also asks people to avoid contact with rodents and refrain from feeding squirrels or chipmunks.

When is plague deadly?

More than 80% of plague cases in the U.S. are bubonic, meaning the infection is confined to the lymph nodes. People with bubonic plague usually develop symptoms two to eight days after they’re bitten by an infected flea or come in contact with an infected animal.

The most common sign of bubonic plague is swollen, painful lymph nodes. Other symptoms include a sudden fever, nausea, weakness, chills or muscle aches. Doctors test for the infection via a blood or tissue sample, then treat it with antibiotics.

The bubonic plague that killed more than a third of Europe’s population in the fourteenth century was much a different threat from the one now, Wagner said.

“It’s the same thing that caused the Black Death, but that was in the pre-antibiotic era,” Wagner said. “Now it’s very easily treated with simple antibiotics.”

If bubonic plague isn’t treated early, however, bacteria can enter the bloodstream and cause sepsis — or septicemic plague. Typical signs of a bloodstream infection include extreme weakness, abdominal pain or bleeding from the nose, mouth or under the skin. People’s skin may also turn black, especially on their nose, fingers and toes.

An untreated bubonic or septicemic infection can lead to a lung infection, or pneumonic plague, which is often deadly. People can get also pneumonic plague directly by inhaling infectious droplets.

Fawcett said these different forms of plague can co-exist at the same time, and it’s not entirely clear when the disease transitions from one stage to another or becomes contagious among humans.

That said, he’s confident that the general risk in Deschutes County is low. As long as the health department is monitoring close contacts, he said, “I do not imagine that would be hardly any significant risk to our community.”