Plague (Yersinia Pestis)

  • What Is It?
  • Symptoms
  • Diagnosis
  • Expected Duration
  • Treatment
  • When To Call A Professional
  • Prognosis
  • Prevention
  • What Is It?

    Yersinia pestis is a species of bacteria that causes plague, an infection that leads to death quickly and that has caused several major epidemics in Europe and Asia over the last 2,000 years. One of the best known was called the Black Death because it turned the skin black. This plague epidemic in the 14th century killed more than one-third of the population of Europe within a few years. In some cities, up to 75 percent of the population died within days, with fever and ulcerated swellings on their skin.

    The last urban plague epidemic in the United States occurred in Los Angeles in 1925. Since then, an average of 13 cases of plague have been diagnosed each year, primarily in the Southwest, with about 80 percent occurring in the desert areas of New Mexico, Arizona or Colorado and about 9 percent in California. Worldwide, up to 3,000 cases of plague are reported to the World Health Organization each year.

    Plague affects many species of wild rodents, and it usually is transmitted by the bite of infected rat fleas (Xenopsylla species). Less commonly, humans become infected in other ways: 1) when the organism enters through a break in the skin after direct contact with the tissue or blood of an infected animal (for example, if a hunter is skinning an animal) or 2) by direct inhalation of infected droplets if they are in close contact with a person or animal infected with the pneumonic form of the illness (plague infection of the lungs).

    In the United States, plague usually is transmitted in one of four ways:

    • Contact with infected wild rodents, especially ground squirrels, rock squirrels, prairie dogs or chipmunks
    • Contact with carnivores (especially pet cats) that eat these wild rodents
    • The bite of a flea that lives on one of these animals
    • Scratches or bites from infected pet cats

    People who are most likely to be infected include hunters, veterinarians and those who camp or hike in areas where animals are infected with plague. Domestic cats or dogs also can spread the disease to their owners by bringing infected fleas into the home. In the United States, 78 percent of plague infections occur after a flea bite, and 20 percent develop after direct contact with an infected animal (touching a carcass, skinning a dead animal). Another 2 percent occur after inhaling airborne droplets that contain Y. pestis bacteria from an animal that has plague infecting the lungs.


    Plague occurs in three different forms — bubonic, septicemic and pneumonic. In each form, Y. pestis bacteria are transmitted in a different way, and they cause slightly different symptoms:

    • Bubonic plague — This form of plague is transmitted by the bite of an infected rat flea, and it takes its name from the swollen, infected lymph nodes called buboes that are found within the lymph system draining the area of the flea bite. About two to six days after the bite, the victim develops a high fever, chills, muscle aches, headache and extreme weakness. Within another 24 hours, one or more buboes appear as painful, red, hot swellings in the groin if the flea bite was on the legs, or in the underarm or neck if the bite was on the arms, neck or upper body. Without proper treatment, the victim eventually collapses, becomes delirious and may have convulsions. Then, as plague bacteria spread through the bloodstream, symptoms of septicemic plague may begin.

    • Septicemic plague — This form of plague can develop as a complication of untreated bubonic plague, when Y. pestis bacteria spread to the bloodstream and cause a blood infection called septicemia. It also can occur if the bacteria get into the bloodstream through direct contact with the tissue or blood of an infected animal. The first symptoms of septicemic plague can mimic gastroenteritis — nausea, vomiting, diarrhea, abdominal pain. Later, the person develops severe bleeding problems, including spontaneous bleeding under the skin, scattered bruises, hematuria (blood in the urine) and abnormal bleeding from the mouth, nose and rectum. The bleeding problems usually are followed by signs of shock (severe drop in blood pressure, rapid pulse, unconsciousness), kidney failure and severe breathing difficulties.

    • Pneumonic plague — This form of plague develops when a victim has inhaled contaminated airborne droplets, usually carried in coughs, from an animal or human whose plague infection has spread to the lungs. Symptoms begin one to four days later and include high fever, chills, headaches, muscle aches, extreme weakness, chest pain, rapid breathing, severe shortness of breath and coughing up blood. Without proper treatment, the disease quickly leads to death.


    Because plague is so rare in the United States, a physician at first may miss the diagnosis, especially if the person becomes ill outside the Southwestern United States. If your doctor suspects you might have plague, he or she will ask whether you recently noticed a flea bite, whether you have been exposed to wild rodents in the desert of the Southwest, or whether you have recently traveled to a plague-infested area of the world. Your doctor also will ask whether you recently have been in contact with a dead animal, or whether you have been treating a household pet, especially a cat, that has been extremely ill. To confirm the diagnosis, blood tests or tests of other body fluids can be done to look for the presence of Y. pestis bacteria. The tests also can check for antibodies, defensive immune chemicals produced by the patient’s immune system against the F1 antigen, a foreign protein produced by Y. pestis bacteria.

    Expected Duration

    With proper antibiotic treatment, most symptoms of uncomplicated bubonic plague will subside within two to five days, although swollen buboes can remain for several weeks. Recovery from more severe septicemic plague and pneumonic plague usually takes longer, depending on the severity of the patient’s bleeding problems, respiratory failure and other potentially life-threatening symptoms.


    If you live, work or vacation in regions where plague affects local rodent populations, here are things you can do to help prevent the spread of plague:

    • Avoid contact with sick or dead animals, especially rodents, and report these animals to local health authorities.
    • Avoid contact with the nests and burrows of squirrels, chipmunks and other wild rodents.
    • Apply an insect repellent containing DEET (diethyltoluamide) to your skin and use other insect repellents on clothing.
    • Use veterinarian-approved flea collars and other flea repellents on cats and dogs.
    • Follow your veterinarian’s guidelines when handling a severely ill cat or dog, and have the animal examined promptly by your veterinarian.
    • If you have to handle a dead animal, use gloves.
    • If your home is in a high-risk area, ask local health authorities how to rodent-proof your house, and keep your property free of brush, rocks, garbage and food waste that can attract rodents.

    If someone is exposed to a person or animal infected with plague, antibiotics can be given to prevent the person from getting the disease. These include tetracyclines, chloramphenicol (Chloromycetin) or a sulfonamide.

    A plague vaccine was developed that was felt to provide some protection, but it is no longer available in the United States.


    A person who is suspected to have the plague needs to be hospitalized. Once in the hospital, the person is isolated immediately, and hospital personnel take special precautions to prevent the spread of plague bacteria. For instance, they will wear gloves, masks and protective clothing. Antibiotics will be given intravenously (into a vein) for at least 10 days. The drug of choice is usually streptomycin (sold as a generic) or gentamicin (Garamycin, G-Mycin, Jenamicin), but other antibiotics that may be used include the tetracyclines (sold under several brand names) or chloramphenicol (Chloromycetin). Patients with severe bleeding problems or respiratory failure will be treated in an intensive care unit.

    The U. S. Public Health Service requires health care professionals to report all cases of suspected plague immediately to local and state health departments. The U.S. Centers for Disease Control (CDC) will confirm the diagnosis, and report it to the World Health Organization (WHO).

    When To Call A Professional

    Call your doctor whenever you or someone in your family develops a fever or becomes severely ill following:

    • A flea bite or any unidentified insect bite
    • Exposure to a sick or dead animal, even a pet cat
    • Travel or work in areas where rodent burrows and nests are common
    • Travel to the high-risk areas of the Southwestern United States or to countries where plague is most common

    Also call your doctor if you develop a painful, hot swelling in your groin, underarm or neck, especially if you have recently been bitten by an insect.


    Without prompt antibiotic treatment, plague is fatal in 50 percent to 90 percent of cases. Even with appropriate antibiotics and hospital care, about 15 percent of plague patients in the United States die. Pneumonic plague is the most rapidly fatal form of plague, and most victims will die if they do not receive antibiotics within the first 18 hours after symptoms begin.

    Johns Hopkins patient information

    Last revised:

    Diseases and Conditions Center

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    All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.