May is the beginning of flea season. Not only does that impact pets. It can also potentially impact pet owners. 

A report issued by the Centers for Disease Control and Prevention (CDC) on May 1 concluded that Yersinia pestis, the bacterium that causes plague, was present in a blood sample collected from a man hospitalized with pneumonia last summer in Colorado. The source of the illness appears to be his dog.

Specimens from the dog, along with three humans who had contact with the dog, including two veterinary technicians, revealed the presence of acute Y. pestis infection. It had been previously misidentified as Pseudomonas luteola by an automated hospital laboratory that had run tests on the man hospitalized with pneumonia, the report notes.

The chain of events began with the owner’s dog, a two-year old pit bull. The owner took his dog to the veterinarian due to the dog’s fever, a rigid jaw, and the fact that he was drooling and coughing up mucus mingled with blood, reports The Washington Post. The dog spent the night at the veterinary clinic and was euthanized the next day.

The dog’s owner began to feel sick and have some of the same symptoms as his dog (fever and coughing up blood). He was misdiagnosed as having pneumonia using an automated identification system and didn’t respond to treatment. Later lab work performed by the Colorado Department of Public Health and Environment (CDPHE) correctly diagnosed the cause, noted The Washington Post.

The CDPHE reached out to 100+ people who had contact with the dog and found similar symptoms in the dog’s owner, two veterinary technicians who had treated the dog, and one close friend of the dog’s owner. Acute Y. pestis infection was present in the samples of all three, reported The Washington Post.

All three were successfully treated with medications, reported ABC News.

Not all plagues are alike. The more commonly known bubonic plague killed 25 million people in the Medieval Ages. The plague identified in the CDC report was the pneumonic plague. People become infected via direct contact with an animal that is sick or has died from plague, according to the CDC. 

“Pneumonic plague is the worst form,” John M. Douglas, MD, of Tri-County Health Department in Colorado and one of the study authors told ABC News. “It’s the one that you least want to get. You get sick fast and the chances of getting a rocky or even fatal course [are increased].”

The CDC report also issued a caution to veterinarians to consider plague when diagnosing ill animals, and the limitations of automated diagnostic systems when identifying rare diseases.