A new report from the Centers for Disease Control and Prevention reveals that a small outbreak of pneumonic plague in Colorado last year was caused by a dog infected with the disease. The organization says this is first ever incidence of a human contracting the disease from a dog in the US, and it may be the first case of human-to-human plague transmission in the country in almost a century.
The hospital to which the man had been admitted initially identified Pseudomonas luteola as the bacteria responsible for his illness using an automated diagnostic system, but as the man's symptoms worsened, further testing conducted by the Colorado Department of Public Health and Environment (CDPHE) found a bacteria called Yersinia pestis was the cause - a form of pneumonic plague.
An investigation conducted by the Tri-County Health Department (TCHD) involving interviews with the man's family revealed that on June 24th, the man's dog - a 2-year-old male American pit bull terrier - fell severely ill with fever and shortness of breath and was coughing up blood. The dog was euthanized the following day.
The dog's liver and lung tissues were tested for presence of Y. pestis after its owner became ill. The results were positive, confirming that the man had contracted pneumonic plague from the animal - the first report of such an incidence in the US.
The Centers for Disease Prevention and Control (CDC) report also reveals that two veterinary clinic employees who had close contact with the dog and a female who was a close contact of the dog's owner became ill with the disease.
Since the last patient had contact with both the dog and its owner, the CDC say it is unclear how she contracted the disease. They note, however, that this patient was exposed to the dog's owner during an incubation period of 5-6 days, while exposure to the infected dog occurred during an incubation period of 9-10 days.
"The shorter incubation period is more typical of plague and therefore supports human-to-human transmission," say the report authors. "Nevertheless, transmission from the dog cannot be excluded given the animal's role in the other three infections and because incubation periods of up to 10 days have been reported, although rarely."
All infected patients have made a full recovery following treatment with antibiotics.
Should we be worried about plague?
Y. pestis is most commonly transmitted through the bite of an infected flea that has fed on a rodent infected with the bacteria. It can also be spread through contact with contaminated fluid or tissue of an infected animal.
Human-to-human transmission of Y. pestis can occur through the inhalation of cough droplets from an infected person, but the CDC note that this has not occurred in the US since the 1924 plague outbreak in Los Angeles in which 30 people died.
Nowadays, human infection with plague is rare in the US, with an average of eight cases reported each year. The CDC note that it remains a public health concern in western US, however, because Y. pestis circulates among wild rodent populations in this area.
"The risk for plague can be minimized by avoidance of possibly infected rodents (e.g., prairie dogs) and their fleas," say the report authors. "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 authors highlight the importance of seeking immediate treatment if plague is suspected, and state that clinical specimens should be collected and tested as soon as possible. They add:
"Early recognition of plague, especially the pneumonic form, is critical to effective clinical management and a timely public health response. Veterinarians should consider plague in the differential diagnosis of ill domestic animals, including dogs, in areas where plague is endemic."
While plague is very unlikely to make a large-scale comeback comparable to the "black death" of Middle Ages - primarily due to the effective treatments we now have for the disease - the report emphasizes the fact that it remains a public health concern.
The report authors warn, however, that the delayed recognition of Y. pestis in the first patient due to reliance on automated diagnostic systems suggests health care professionals may be overlooking the possibility of plague in patients with symptoms of pneumonia.
"This delay resulted in the exposure of numerous medical personnel," they say. "Pneumonia is the only form of plague with the potential for human-to-human transmission. Delayed recognition because of inaccurate laboratory test results and atypical clinical presentations can lead to high numbers of potential exposures to health care workers, laboratory workers, and other close contacts."
In February, Medical News Today reported on a study by researchers from Weill Cornell Medical College in New York, NY, which identified traces of anthrax and Bubonic plague in the New York subway system. Reassuringly, however, the researchers found that they were not found at levels high enough to be harmful to human health.