Bubonic plague is a severe bacterial infection caused by the gram-negative bacillus Yersinia pestis, which can cause 3 distinct forms of illness: bubonic, septicemic, and pneumonic plague (pneumonic and septicemic are discussed separately). Bubonic plague may progress to both the septicemic and pneumonic forms if left untreated.
Fleas transmit the bacteria from animals (eg, prairie dogs, squirrels, chipmunks, rats, cats) to humans. Direct contact, animal bites, or exposures to infected carcasses may also cause disease. Bubonic plague cannot be transmitted person to person unless it develops into the pneumonic form.
In a bioterrorist event, plague would most likely be released as an aerosol, resulting primarily in the highly lethal and contagious pneumonic form of the disease. Bubonic plague would not immediately result, but it could occur by secondary transmission by infected fleas.
Whether bubonic plague is acquired naturally or as the result of a bioterrorist attack (release of infected fleas), the onset of symptoms (after an incubation period of 4-7 days) is sudden and includes malaise, myalgias, high fever, headache, tachycardia, and the development of large tender regional lymph nodes called buboes (usually in the inguinal area).
Untreated, bubonic plague can progress to septicemic and occasionally pneumonic plague in 2-6 days, and death is frequent. The mortality rate of bubonic plague is low (1%-15%) when treated early but can reach 40%-60% when untreated.
Endemic plague is seen in the southwestern United States (Colorado, New Mexico, Arizona, California). About 10 cases are reported each year in the United States. Elsewhere in the world, plague is seen in Vietnam, India, the former Soviet Union, and parts of Africa.
People at risk for bubonic plague include hunters, hikers, abattoir workers, exotic pet owners, travelers to endemic areas, and those who live in rat-infested areas.
In 1995, a strain of multidrug-resistant (MDR) Y. pestis was isolated from a bubonic plague patient in Madagascar that was resistant to at least 8 antimicrobials, including streptomycin, tetracycline, chloramphenicol, and sulfonamides. To date, this is the only documented case of MDR Y. pestis, although there is no systematic monitoring of resistance.