Yersinia pestis: First Case of Plague Transmission from a Dog to a Human in the United States

Four human cases of pneumonic plague, detected in Colorado last summer, were traced to a dog infected with Yersinia pestis. According to a recent report* from the Centers for Disease Control and Prevention (CDC), this is the first reported instance of human infection from a dog carrying the bacteria in the United States. And it is likely the first case of human-to-human transmission of the plague in the U.S. in 90 years.  On July 8, 2014, the Colorado Public Health Laboratory detected Yersinia pestis , the bacteria responsible for the plague, in a blood sample taken from a man hospitalized for pneumonia. The hospital had initially identified Pseudomonas luteola as the cause of the illness using an automated diagnostic system. An investigation into the patient’s environment then revealed that his dog had been euthanized on June 25 after presenting with a high fever, jaw stiffness, ataxia of the forelimbs, and eventually dyspnea and hemoptysis. The dog’s owner reported the illness three days later. Two people who had come into contact with the dog and another who had come into contact with its owner developed hyperthermia and respiratory distress; two of them showed radiographic signs of pneumonia. Analysis of specimens collected from these three individuals and the dog confirmed infection with Y. pestis . Pneumonic plague is rare in the United States, with only 74 cases reported between 1900 and 2012. This outbreak is thus the largest since the 1924 plague epidemic in Los Angeles, and its origin is atypical. In fact, until now, dogs infected with Yersinia pestis—which are generally asymptomatic—were not considered a direct source of human infection. The plague is primarily carried by rats and transmitted to humans through flea bites from infected rodents. It can also spread through contact with the secretions or tissues of an infected animal. According to the recent CDC report, plague should now be included in the differential diagnosis of canine respiratory diseases in areas where the disease is endemic . Similarly, the limitations of automated diagnostic systems in identifying rare bacteria such as Y. pestis should be taken into account. Furthermore, patients suspected of having pneumonic plague must be isolated prior to laboratory confirmation and treated with appropriate antibiotics. This is, in fact, the only form of plague that can be transmitted from person to person via the respiratory route. Any delay in diagnosis—due to erroneous test results or atypical clinical presentations—can lead to a high number of potential exposures to the bacteria (healthcare, veterinary, and laboratory personnel, etc.). Although human plague is rare in North America, it poses a public health problem in the western United States, where Y. pestis circulates among wild rodent populations (particularly prairie dogs) via their fleas . Early recognition of the disease, particularly the pneumonic form, is essential for effective treatment and to prevent further transmission. If left untreated, pneumonic plague has a mortality rate exceeding 93%.   * Outbreak of human pneumonic plague with dog-to-human and possible human-to-human transmission, Colorado, June–July 2014. Weekly, 2015, 64 (16); 429–434. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6416a1.htm?s_cid=mm6416a1_w