Outbreak of Pneumonic Plague, Colorado 2014

BACKGROUND:  Plague is a rare, acute zoonosis caused by the bacterium Yersinia pestis. In the western United States human infections are commonly associated with exposure to plague infected fleas or susceptible mammalian hosts such as ground squirrels or domestic cats. In the summer of 2014 pneumonic plague was identified in a human and a dog in a rural community in Colorado. 

METHODS:  The Colorado Department of Public Health and Environment and Tri-County Health Department investigated these cases and implemented disease control measures. Because pneumonic plague is highly transmissible, all medical and veterinary personnel who had contact with the cases, and all direct contacts of the human and dog were assessed to identify whether additional plague cases might have occurred.

RESULTS:  Four human cases of plague originating from a fatal illness in a pet dog were identified from this outbreak. Prior to the human index patient becoming ill, the dog died of an acute illness which was subsequently confirmed as plague by histopathology. Two people with occupational exposure to the dog developed clinical illness and a fourth case, a close contact of the index patient, also developed plague. Mis-identification of Yersinia pestis as Pseudomonas luteola by automated laboratory diagnostic equipment led to a substantial delay in recognition of this rare disease agent and complicated the public health investigation of the cases.

CONCLUSIONS:  Dogs are an uncommon source of plague in humans. Healthcare providers who see patients who live, work, or travel to plague endemic areas should be aware that exposures may occur through uncommon routes. Healthcare providers and laboratorians should be informed that the causative organism of plague, Yersinia pestis, is often mis-identified by automated laboratory diagnostic equipment. In this outbreak, the delay in diagnosis contributed to the first case of human-to-human plague transmission in the US since 1924.