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Human plague is caused by infection with the bacillus Yersinia pestis. It is primarily an infection of animals where transmission between animal hosts, such as rats, ground squirrels and cats, occurs via the bite of infected fleas or ingestion of infected animal tissue.

Humans may become infected when they are bitten by rodent fleas which have previously fed on infected hosts. Occasionally, infection may be acquired by direct handling of tissues or fluids of infected animals. In epidemics of the disease human infection can be acquired by the inhalation of infected respiratory secretions from individuals or animals, especially domestic cats, with plague pneumonia.

Symptoms and Signs

Human plague presents in the following ways :

  1. Bubonic Plague is the most common. After an incubation period of between 2 to 8 days, patients experience the sudden onset of fever, chills, fatigue and severe headache. This presentation is associated or closely followed by the painful swelling of regional lymph nodes. This painful swelling is called a "bubo".

    This lymphadenopathy is tender and commonly found in the groin, axilla and cervical region. The fever may continue for 3 to 4 days after which there may be a reduction in temperature and clinical improvement in the patient. This improvement is short lived, the symptoms usually worsen the next day and the patient deteriorates rapidly.

  2. Pneumonic Plague results from hematogenous spread of the infection to the lungs from the bubo. A Yersinia pneumonia develops and purulent respiratory secretions contain many bacilli. An individual infected in this way may produce infectious respiratory droplets associated with coughing. Plague pneumonia, therefore, is a serious condition because of the risk of an uninfected person acquiring primary pneumonic disease by the inhalation of infected respiratory secretions.

    Abrupt onset of fever and respiratory distress are observed and while death often occurs with 3 -6 days in untreated cases, a more rapid course where mortality can occur within 24 hours has been observed.

  3. Septicemic Plague is caused by overwhelming Yersinia septicemia. The disease progresses to coma and death without significant clinical lymphadenopathy. Hemorrhagic complications may be observed. In untreated cases mortality is usually within 24 to 72 hours.
Mode of Transmission

Usually transmitted through the bite of infected fleas (especially from the oriental rat flea). Other sources include the handling of tissues of infected animals, especially rodents and rabbits. Plague may also be transmitted by airborne droplets from patients infected with pneumonic plague. In a bioterrorist setting, plague bacilli would probably be transmitted by infectious aerosol droplets.

Incubation Period

This is from 1 to 7 days but maybe a few days longer. For primary plague pneumonia, this is usually 1 to 4 days.

Laboratory Diagnosis

The Ontario Public Health Laboratories will assist in confirmation of suspect cultures for plague, as well as consultation with physicians on suspected cases. Physicians must contact the Central Public Health Laboratory prior to the submission of cultures and/or specimens :

During work hours : 416-235-6100

After-hours Duty Officer : 416-605-3113

Specimens should be handled according to universal precautions and packaged for transport to the Central Public Health Laboratory according to the Transportation of Dangerous Good regulation.

A presumptive diagnosis can be made microscopically by identification of the gram-negative coccobacillus with "safety-pin" bipolar staining in Gram stained smears from a lymph node needle aspirate, sputum, or cerebrospinal fluid sample.

A definitive diagnosis can be made by culturing the organism from blood, sputum, and bubo aspirates.

A four-fold rise in antibody titer in patient serum is also diagnostic.


Streptomycin is the drug of choice. Tetracycline and chloramphenicol may also be used. All are highly effective if used early (within 8 to 18 hours after the onset of pneumonic plague). The patient should be under the care of an infectious disease specialist.

Post-Exposure Prophylaxis for Contacts of Cases

For face to face contacts of patients with pneumonic plague or after a confirmed or suspected bioterrorism attack, the use of doxycycline 100 mg. Orally, twice daily for 7 days or the duration of risk of exposure whichever is longer, is recommended.


Plague is a reportable disease in Ontario under the Health Protection and Promotion Act and must be reported immediately to the local medical officer of health by telephone. The disease should be reported even if it is only suspected and has not yet been confirmed.

For more information
Call the ministry INFOline at 1-800-268-1154
(Toll-free in Ontario only)
In Toronto, call 416-314-5518
TTY 1-800-387-5559
Hours of operation : 8:30am - 5:00pm
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