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A Colorado pit bull infected humans with the plague

A 17th century engraving of a “plague doctor” dressed in the heavy gown and beaked mask believed to protect people from the disease. Four people fell ill in a Colorado outbreak of the plague last year. (U.S. National Library of Medicine)

The U.S.’s largest pneumonic plague outbreak in nearly a century has been identified, and it all started with a sick dog.

That’s according to the Centers for Disease Control, which just announced the results of an investigation into a small but surprising plague outbreak in rural Colorado last year. Four cases of pneumonic plague (a form of disease that affects the lungs rather than causing the more notorious boils) were found to have originated with an infected pit bull — a first in the U.S. Typically the disease is spread via bites from fleas carried by prairie dogs and other rodents, not from contact with household pets.

The source wasn’t the only unusual thing about this outbreak. The CDC also found that one of the cases may have resulted from human-to-human transmission, something that hasn’t happened in the States since 1924.

There’s no need to break out your plague doctor mask — the disease these days is a far cry from the brutal “Black Death” of the Middle Ages. The CDC reports an average of about 8 cases per year in the U.S., and modern medicine means they’re almost never fatal. All of the patients in this outbreak (except the dog) were treated with antibiotics and have made a full recovery.

Still, pneumonic plague is scary and unpleasant, to say the least, and this is the largest outbreak to happen in the States in 90 years, according to NPR. Understanding how it came about can help health workers cope with future cases.

It started all started when a 2-year-old pit bull terrier came down with a fever last June. Its jaw was rigid, drool dripped from its mouth and it began to cough up mucus tinged with blood. After an overnight stay at a vet clinic, the dog was humanely put down, ending a sad but seemingly short-lived ordeal for its owner.

But a few days later, the owner started feeling sick. He too had a fever and was coughing up blood. A local hospital diagnosed him with pneumonia using an automated identification system, but the patient (who isn’t named in the report) didn’t respond to treatment. He was transferred to another hospital and a blood sample was sent to the Colorado Department of Public Health and Environment for study. That’s when health workers realized that this wasn’t any old case of pneumonia — they were dealing with the plague.

Researchers reached out to more than 100 people who’d come in contact with the dog and his owner and found three other people who had come down with similar symptoms. Two were employees of the vet clinic where the dog was put down. Though neither was as ill as the dog’s owner, they both sought treatment after being told that they’d been exposed to the plague. The third patient was a woman described as a “close contact” of the dog’s owner. She had touched the dog’s body after its death, but she’d also been around the dog’s owner while he was coughing up blood.

If the fourth case was caused by the man and not his pet, it would be the first time someone has contracted the plague from a fellow human since a Los Angeles outbreak in 1924, CDC researchers say.

The authors of the report pointed out several lessons from this outbreak. For one, there’s the unfortunate finding that domestic animals may harbor the plague in areas where the disease is already endemic among rodents. They also highlighted the dangers of the first patient’s delayed diagnosis. Because the automated system misidentified the cause of his illness, the plague bacteria wasn’t spotted until 10 days after he started showing symptoms. At one point the man had to be intubated to help him breathe, and he wound up spending 23 days in the hospital.

“Delayed recognition because of inaccurate laboratory tests … can lead to high numbers of potential exposures,” the researchers wrote. This case “reinforces the need for critical evaluation of results from automated systems.”

According to John Douglas, who runs the Tri-County Health Department, this outbreak is also a reminder that the plague isn’t entirely a thing of the past.

“Hopefully plague will not reemerge as it did in the Middle Ages, but it’s certainly endemic in rodent populations in western states,” he told NPR. “It’s something that those of us who live here will continue to encounter.”