Another Month, Another Zoonosis: Plague in Colorado

This month, the Morbidity Mortality Weekly Report and the New England Journal of Medicine reported a cluster of pneumonic plague that occurred in Colorado during the summer of 2014.

Yersinia pestis on blood agar plate (left) and chocolate agar plate (right).
Source: CDC Public Health Image Library.
Common Domain.

For a review of plague and its forms (bubonic, septicemic, and pneumonic), please refer to the CDC webpage.

There are several points to be made about this outbreak.

(1) This outbreak is the largest pneumonic plague epidemic in the United States in 90 years. The index case was a dog. The dog’s owner, two veterinary employees, and a companion of the dog’s owner became ill. Pneumonic plague is rare. Of reported cases of plague in the United states, over 80% have been bubonic. It’s a lucky thing that pneumonic plague is rare because it is the most dangerous form of the disease from an epidemiologic perspective. Patients with pneumonic plague can spread the bacterial infection person-to-person via droplets (produced with coughing for example). This can lead to rapid amplification of case numbers.

(2) The automated blood culture system misidentified the bacteria cultured from the index human patient. Rather than correctly identifying it as Yersinia pestis, the machine misidentified it as Pseudomonas luteola. This resulted in a delay of diagnosis. Errors in the identification of Y. pestis by automated blood culture systems have previously been described.  This highlights the risk of relying solely on sophisticated equipment to identify pathogens without the ability to perform traditional microbiological and biochemical methods of bacterial identification.

(3) Plague is enzootic to the American Southwest and West.  (See map.)

Source: CDC
http://www.cdc.gov/plague/maps/

The diagnosis of plague needs to be considered in the differential of patients who present with lymphadenitis, undifferentiated septicemia, and community acquired pneumonia in the context of exposure to ill animals. The risk of transmission from pets is greatest with cats. Dogs are a rare source of plague in humans.

(4) Y. pestis is a potential agent of bioterrorism, listed as a category A agent by the CDC. As such, plague may need to be considered in the differential anytime there is an unusual cluster of severe illness or progressive pneumonia.

(5) This case again highlights the importance of communication across health specialties including veterinary medicine, human medicine, public health, and ecology / wildlife biology.

REFERENCES:

Foster, CL, et al. 2015. Sick as a dog. NEJM 372(19): 1845- 50.

Gage, KL, et al. 2000. Cases of cat-associated human plague in the Western US, 1977- 1998. Clin Infect Dis 30:893- 900.

Gould, LH, et al. 2008. Dog-associated risk factors for human plague.  Zoonoses Public Health 55:448- 54.

Kool, JL. 2005. Risk of person-to-person transmission of pneumonic plague. Clin Infect Dis 40:1166- 72.

Kugeler, KJ, et al. 2015. Epidemiology of human plague in the United States, 1900-2012. Emerg Infect Dis 21:16- 22.

Nichols, MC, et al. 2014. Yersinia pestis infection in dogs: 62 cases (2003–2011). J Am Vet Med Assoc 244:1176- 80.

Runfola, JK, et al. 2015. Outbreak of Human Pneumonic Plague with Dog-to-Human and Possible Human-to-Human Transmission- Colorado, June–July 2014. MMWR 64(16): 429- 34.

Wang, H, et al. 2011. A dog-associated primary pneumonic plague in Qinghai Province, China. Clin Infect Dis 52:185- 90.