At least 116 people and 46 animals in Colorado were potentially exposed to the black plague after veterinarians struggled to diagnose a critically ill dog back in 2017.

The unusual case prompted health experts to issue an equally unusual—and perhaps startling—warning. That is, that dogs in the US may contract the deadly bacterial infection at any time of the year, and the signs may be hard to spot.

“[P]neumonic plague, although rare, should be considered in dogs that have fever and respiratory signs with potential exposure in disease-endemic areas, regardless of season and lobar [lung] distribution,” the Colorado health experts concluded. They published details of the case and their warning this week in the journal Emerging Infectious Diseases.

The plague is endemic to areas in the Western United States, meaning it circulates continually. Though it’s best known for causing the catastrophic Black Death pandemic in Europe during the fourteenth century, it arrived in the States around 1900 on rat-infested steam ships. Since then it has spread to, and quietly lurked in, rural rodent populations, including rock squirrels, wood rats, ground squirrels, prairie dogs, chipmunks, mice, voles, and rabbits. Infected populations tend to pop up in parts of Arizona, California, Colorado, Oregon, Nevada, and New Mexico. The Centers for Disease Control and Prevention reports that in recent decades there has been an average of seven human cases documented each year, with a range of one to 17 cases.

The bacterium behind the deadly disease is Yersinia pestis, which is spread by flea bites and contact with infectious people and animals. Once it finds its way into a victim, the infection can manifest in several ways. The main three ways are bubonic (infection typically starting from the skin after a flea bite and spreading to the lymphatic system, causing swollen lymph nodes, called buboes), septicemic (blood infection), and pneumonic (infection in the lungs, which can spread from person to person via airborne droplets).

In dogs, plague is rare but usually presents as bubonic or septicemic, stemming from a bite from an infected flea. And, as the authors of the report note, plague cases in the US tend to crop up when fleas are most active, typically between April and October. But, this is not always the case, as the tale of the poor pup in Colorado shows.

Peculiar plague

In December of 2017, a three-year-old, mixed-breed dog turned up at a vet’s office with lethargy and fever. Four days earlier, the dog’s human noted that the dog had been sniffing around a dead prairie dog. The vet started an antibiotic treatment, but the dog’s condition rapidly grew worse. By the next day, the dog started coughing up blood, and the vet referred the case to the Colorado State University Veterinary Teaching Hospital.

Despite the contact with the dead prairie dog, the vets there didn’t suspect plague at first because, well, it’s rare, and it was December. Moreover, clinical images of the dog’s sick lungs didn’t fit the usual pattern of a plague infection, which usually affects both lungs. Instead, only one part of a lung was affected, and it looked more like the dog had inhaled a foreign body, a more common doggy problem. To get rid of the presumed source of infection, the vets performed a lung lobectomy, removing the heavily damaged part of the dog’s lung.

With the removed lung tissue, the vets tried for several days to grow any possible bacteria that was causing the infection. But, that yielded confusing results, pointing to a bacterium related to Y. pestis that wouldn’t cause the symptoms seen in the dog. They next turned to PCR (polymerase chain reaction), a method of picking out and making copies of specific DNA segments, which can help identify organisms. But at that point, the dog’s condition had continued to worsen, and the vets had to put the dog down.

Following the CDC’s PCR protocol to look for Y. pestis, the vets found the deadly bacteria. Realizing they had plague on their hands, the vets retraced the dog’s days-long stay in the hospital to assess exposures. Based on staff surveys and the dog’s locations, they concluded that at least 116 personnel and 46 co-housed animals were potentially exposed. At-risk humans talked with their doctors to see if they should take antibiotics as a precaution. All the co-housed animals got prophylactic antibiotics.

As far as the vets could tell, no one got sick with the plague from their exposures. Still, they report that the hospital is updating its protocols to better identify plague and keep it from spreading to personnel and patients.

Emerging Infectious Diseases, 2018. DOI: 10.3201/eid2504.181195 (About DOIs).