Although human plague is rare in the United State, a small outbreak of pneumonia caused by Yersinia pestis is believed to have been caused by dog-to-human, or possibly human-to-human, transmission in Colorado. A description of these cases appears in the Centers for Disease Control and Prevention (CDC) appears in the Morbidity and Mortality Weekly Report (MMWR).

The Colorado Department of Public Health and Environment laboratory confirmed Y. pestis in a blood specimen collected from a man hospitalized with pneumonia in July 2014. An automated system in the hospital laboratory had previously misidentified the organism as Pseudomonas luteola and was sent for further testing after the patient’s condition continued to deteriorate. It was then discovered that the patient’s dog had recently fallen ill with fever, jaw rigidity, drooling, and right forelimb ataxia and was humanely euthanized. Tissue samples from the dog tested positive for Y. pestis and three other individuals who had been in close contact with the ill dog also tested positive for Y. pestis and received diagnoses of the plague. One of the infected individuals had also had contact with the patient. The initial patient recovered after hospitalization for 23 days and the other three recovered as well.

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All four patients in this outbreak had laboratory-confirmed plague and three patients had clinical and radiographic evidence of pneumonia. The patients were treated with antibiotics, including levofloxacin, streptomycin, azithromycin, and doxycycline. Human-to-human transmission is supported in this outbreak due to a shorter incubation period that is typical of plague, but dog-to-human transmission cannot be excluded because of the animal’s role in three infections. Incubation periods of up to 10 days have been reported in previous cases (although rarely).

Primary pneumonic plague is rare in the United States and only 74 cases have been reported from 1900–2012. However, this represents the largest outbreak and the first instance of possible human-to-human transmission since an outbreak in Los Angeles in 1924. It also highlights the importance of early recognition, especially in the pneumonic form, as delays in diagnosis can lead to additional transmission.

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