Plague is an acute, infectious disease of humans, rodents, and ectoparasites (fleas and lice).
Throughout history, plague has destroyed entire civilizations. In the 1300s, the "Black Death," as it was called, killed approximately one-third (20 to 30 million) of Europe's population. In the mid-1800s, plague killed 12 million people in China. Thanks to better living conditions, antibiotics, and improved sanitation, plague is rare these days, occurring in a few thousand people worldwide each year.
Plague is caused by bacteria called
Yersinia pestis. These
plague bacteria are found mainly in rodents, particularly rats, and in the fleas that feed on them. Other animals and humans usually contract the bacteria from rodent or flea bites.
A
Yersinia pestis infection can cause one of three forms of plague:
Depending on the circumstances, these forms of plague may occur separately or in combination.
- Bites from infected rodent fleas
- Direct contact with infected tissue or bodily fluids
- Inhaling infected droplets.
Incubation Period for Plague
When a person becomes infected with the bacteria that cause plague, the bacteria begin to multiply within the body. For
bubonic plague, this occurs in the lymph system; for
pneumonic plague, this occurs within the lungs. (The lymph, or lymphatic system, is a major component of your body's immune system. The organs within the lymphatic system are the tonsils, adenoids, spleen, and thymus.) After 1 to 6 days, symptoms of plague can begin. The period between becoming infected and the start of plague symptoms is called the
plague incubation period.
Each type of plague has different symptoms. It is possible for a person to have symptoms of only one type; it is also possible for a person to experience symptoms from each of the three types. For example, a person may first develop
bubonic plague symptoms, followed several days later by
septicemic plague symptoms, then
pneumonic plague symptoms.
In order to make a
plague diagnosis, the doctor will ask a number of questions about a person's medical history and perform a physical exam. During the exam, the doctor will look at the skin and listen to the lungs for signs and symptoms of plague. If the doctor has a high suspicion that a person has plague, he or she will recommend certain tests.
Before diagnosing plague, the doctor will consider and rule out other possible conditions that share similar symptoms of plague. These conditions include:
- Lymphogranuloma vernereum (a sexually transmitted disease affecting the lymph system)
- Syphilis (a sexually transmitted disease)
- Appendicitis
- Hernia
- Tularemia (a serious illness usually caused by animals)
- Cat scratch fever (a disease associated with being scratched by a cat)
- Shigellosis (an infectious disease typically caused by unsanitary conditions)
- Typhoid fever (a life-threatening illness caused by Salmonella).
When plague is suspected, the person is often hospitalized and placed in isolation. Even before lab tests come back,
plague treatment will be started, typically involving antibiotics. It is also important that people who have been in close contact with a plague patient, particularly a patient with plague pneumonia, should be identified and evaluated.
Left untreated,
plague bacteria can quickly multiply in the bloodstream, causing
septicemic plague, or even progress to the lungs, causing
pneumonic plague. The plague mortality rate is 50 to 90 percent if left untreated; the rate drops to 15 percent when diagnosed and treated early.
Plague will probably continue to exist in its many localized geographic areas around the world, and plague outbreaks in wild rodent hosts will likely continue to occur. Attempts to eliminate wild rodent plague are costly and futile; therefore,
plague prevention is directed toward reducing the threat of infection in humans in high-risk areas through three techniques:
- Public health education
- Environmental management
- Preventive drug therapy.
Approximately 10 to 20 people in the United States develop plague each year from flea or rodent bites -- primarily from infected prairie dogs -- in rural areas of the southwestern United States. About one in seven of those infected die from plague. There has not been a case of person-to-person infection in the United States since 1924.
Worldwide, there have been small plague outbreaks in Asia, Africa, and South America. Globally, the World Health Organization (WHO) reports 1,000 to 3,000 cases of plague every year. Current WHO statistics show there were 2,118 cases in 2003 worldwide.
Plague occurs more frequently during spring and summer months, especially in males and people under the age of 20.
Bioterrorism is a real threat to the United States and around the world. Although the United States does not currently expect a plague attack, it is possible that
pneumonic plague could occur through an aerosol distribution. The
Yersinia pestis bacterium is widely available in microbiology banks around the world, and thousands of scientists have worked with plague, making a biological attack a serious concern.
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