Pneumonic plague is a severe bacterial lung infection caused by the gram-negative bacillus Yersinia pestis, which is found in rodents (eg, prairie dogs, squirrels, rats) and their fleas, and sometimes in cats. Yersinia pestis can also cause bubonic and septicemic plague (bubonic plague associated with sepsis and delirium), either of which may progress to the pneumonic form. The bubonic and septicemic presentations are discussed separately.
Pneumonic plague is classified as a Category A bioterrorism agent because of its ease of dissemination, contagiousness, and high mortality rate. The most likely method of dispersal would be as an aerosol, but simply having an infected individual walk around infecting others is also a likely mode of dissemination. It has an incubation period of 1-6 days (average 2-4 days) and can be transmitted person to person via droplets through direct close contact.
Pneumonic plague results in a severe fulminant illness that develops with associated high fever, chills, headache, productive cough, chest pain, hemoptysis, extreme malaise, myalgias, tachypnea, tachycardia, and pneumonia. Gastrointestinal symptoms including nausea, vomiting, abdominal pain, and diarrhea are also commonly seen. If not treated within 24 hours of onset, pneumonic plague rapidly progresses to acral cyanosis, respiratory failure, septicemia, circulatory collapse, and death. Untreated, the mortality rate is 50%-90%. With treatment, the rate is 15%.
Endemic plague is seen in the southwestern United States (Colorado, New Mexico, Arizona, California). Plague is also seen in Vietnam, India, the former Soviet Union, and parts of Africa. Naturally occurring pneumonic plague is uncommon. Hikers, campers, veterinarians, and owners of infected cats, especially those living or visiting endemic areas, are at risk for contracting plague.