Colorado Pit Bull Caused Largest U.S. Pneumonic Plague Outbreak Since 1924

A CDC investigation found a pit bull terrier caused a small outbreak of pneumonic plague in Colorado last year. This Year's Love/Flickr

A pit bull terrier was responsible for a small plague outbreak that hospitalized four people in Colorado last year, the largest pneumonic plague outbreak in the U.S. since 1924.

According to the Centers for Disease Control and Prevention (CDC), four people were infected with pneumonic plague—the type of plague that affects the lungs—which originated from a sick pit bull in rural Colorado last summer. The results of the investigation were published on Thursday.

A previously healthy middle-aged man was taken to hospital on June 28, 2014 with what doctors believed was pneumonia. Several days later he was diagnosed with plague. Investigators discovered the man's 2-year-old dog was humanely put down at a veterinary clinic several days earlier after it became ill with fever, jaw rigidity and drooling.

Plague is usually spread through bites from fleas carried by rodents such as prairie dogs, The Washington Post reports. The remains of a prairie dog colony was found at the man's property, but it was destroyed in October 2013.

Two female veterinary employees who had close contact with the dog were also diagnosed with plague; one was first diagnosed with pneumonia. A fourth patient, a woman who had close contact with the dog owner, was also first diagnosed with pneumonia before being told she had plague. She also handled the body of the dead dog and got its blood on her hands, according to the investigation.

One of the cases of plague could have been a human-to-human transmission, which would make it the first such case in the U.S. since 1924, according to the CDC. An average of eight people in the U.S. contract some form of plague every year, and only 74 cases of primary pneumonic plague have been reported in the U.S. between 1900 and 2012. More than 110 people had close contact with the dog or one or more of the humans and were monitored or medicated with prophylaxis by the CDC.

Although all four patients were treated with antibiotics and made a full recovery, plague remains a public health concern in the Western U.S., where Y. pestis, the plague-causing bacteria, is found among wild rodents.

"The risk for plague can be minimized by avoidance of possibly infected rodents (e.g., prairie dogs) and their fleas," the report says. "All suspected or confirmed plague cases and rodent die-offs in areas where plague is endemic should be reported immediately to public health officials so that exposures can be minimized to prevent additional transmission."

The report also says vets should consider the plague as a possibility when diagnosing domestic animals that fall ill.