Plague is an enzootic disease transmitted by fleas from wild rodents and caused by Yersinia pestis. Humans and their pets can enter this cycle, resulting in human plague. Human plague has three main forms:

  • Bubonic: 80–85% cases
  • Septicemic: 10% cases
  • Pneumonic: <5% cases
  • Other forms include meningeal, pharyngeal, ocular, and gastrointestinal (GI) plague.


  • Worldwide: enzootic in Africa, Asia, and Americas. Since 2000, 95% of the 22,000 cases reported to the World Health Organization (WHO) have been from countries in sub-Saharan Africa.
  • In the United States, most cases occur sporadically or in small clusters in Arizona, New Mexico, California, Colorado, Oregon, and Nevada during spring and summer.
  • A median of three cases (range 1 to 17) of plague were reported each year in the United States between 2001 and 2012, with a slight male preponderance.
  • No cases of person-to-person transmission of pneumonic plague have been reported in the United States since 1924.

General Prevention

  • Reduce rodent shelter and food sources in the immediate vicinity of the home by storing grain and animal food in rodent-proof containers.
  • Flea disinfestation of cats and dogs, especially in endemic areas
  • Avoid direct contact with ill or dead animals and never feed squirrels, chipmunks, or other wild rodents.
  • Hospital isolation precautions:
    • Patients with bubonic or septicemic plague and no evidence of pneumonia: standard precautions; add droplet precautions for first 24 hours of therapy until chest radiograph persistently clear.
    • Patients with pneumonic plague: standard and droplet precautions. Continue droplet precautions until patient has completed 48 hours of appropriate antimicrobial therapy.
  • Postexposure management:
    • All persons with exposure to a known or suspected plague source in the previous 6 days should be offered prophylaxis or be told to report illness or fever >38.3° C to their physician.
    • Persons with close (<2 m) contact with a patient with pneumonic plague should receive antimicrobial prophylaxis, but isolation is not necessary.
  • Chemoprophylaxis ≥8 years:
    • Doxycycline (PO), OR
    • Ciprofloxacin (PO) at treatment doses for 7 days from last exposure
  • Chemoprophylaxis <8 years: ciprofloxacin (PO)
  • Notify state public health authorities of cases of suspected and proven Y. pestis infection.
  • A vaccine for plague is no longer available.


  • Skin portal of entry
    • Y. pestis is transmitted most commonly to humans from fleas via the regurgitation of the organism into the bite during the flea’s blood meal (Y. pestis blocks the flea foregut, causing regurgitation).
    • Ground squirrels, rats, chipmunks, prairie dogs, deer mice, marmots, rabbits, and occasionally domestic dogs and cats harbor infected fleas and may be reservoirs of infection (enzootic).
    • Direct skin inoculation of organisms from infected animal tissue or blood occurs through breaks in the skin (e.g., cat scratch or during skinning of animals).
    • Lymphatic spread of infection to the regional lymph nodes creates a localized inflammatory response (bubo, bubonic).
    • Subsequent hematogenous spread of the organism to other organs results in high levels of circulating bacterial endotoxin (septicemic plague).
    • By hematogenous spread to lungs, both bubonic and septicemic plague can cause secondary pneumonic plague.
  • Respiratory portal of entry
    • Primary pneumonic plague is acquired via inhalation of respiratory tract droplets from a human or animal (e.g., dog or cat) with pneumonic plague.
  • Incubation period
    • 2 to 8 days for bubonic or septicemic plague
    • 1 to 6 days for pneumonic plague


Plague is caused by Y. pestis, a pleomorphic, bipolar-staining, gram-negative coccobacillus.

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