A dog in Colorado got sick and was taken to the vet, where, after some trial and error, it was finally diagnosed with the plague. By this point, it had been in contact with well over 100 vets, exposing them to the potentially deadly infection, according to a new case report in Emerging Infectious Diseases.

In December 2017, a 3-year-old mixed-breed dog was taken to a Colorado vet as it was suffering from lethargy and fever. It turned out the dog had been spotted sniffing a dead prairie dog four days previously. Prairie dogs, like most rodents, can carry the plague, and it can be transmissible between other species, including humans, through biting or fleas.

The vets did consider plague, but then dismissed it as unlikely due to a number of factors, including the season; plague infections mostly occur between April and October in the western US, the report states. Dogs are also not very common carriers of the infection; it is much more likely to occur in cats.

They treated the dog with antibiotics, suspecting a bacterial infection. However, the dog continued getting sicker, coughing up blood, and was referred to the Colorado State University Veterinary Teaching Hospital.

Here, the vets again considered and dismissed plague, instead suspecting a lung infection, aspiration pneumonia, common in dogs and caused by inhaling a foreign body, usually from food. Scans and a lung lobectomy seemed to confirm this diagnosis.

However, two days later, a swab test of the dog’s lung tissue revealed the true diagnosis; pneumonic plague, the most deadly strain of the Yersinia pestis bacterium, and the only one that can be transmitted from human to human through breathing in airborne particles from an infected person.

The vets still thought plague so unlikely that their results must be wrong, so they performed the standard Centers for Disease Control and Prevention (CDC) plague test, which confirmed it. In the two days prior, the dog had been moved throughout the teaching hospital, and kept in aerobic incubation – an oxygenated box that vented into an open room.

Once the diagnosis had been confirmed the hospital immediately sent out emails asking anybody who had been in contact with the dog to come forward. In fact, according to the report, word of mouth about the plague being inside the hospital traveled faster than the official communication, and in the end, a total of 116 people were identified as potentially exposed.

“Many expressed frustration that suspicion and diagnosis of plague did not occur earlier,” the study authors wrote.

On top of that, 46 animals housed in the hospital were also considered exposed. Luckily, after drug treatments, no humans or animals were found to have the Y. pestis infection. Sadly, the poorly pupper, patient zero, had to be euthanized.

It's not the first time a dog in Colorado has contracted pneumonic plague and put humans at risk. In 2014, four people contracted Y. pestis from a dog, the largest outbreak in the US since 1924. Without treatment, the infection has a 100 percent mortality rate. Luckily, it's rare, there have only been 78 reported cases of pneumonic plague in the US since 1900.

The authors concluded that as the month it occurred in – December being unusually mild that year – was a leading contributor to not diagnosing the plague earlier, with increasingly unseasonal weather and climate around the world, vets should consider the potential for plague risk as a year-round threat.