Plague is caused by Yersinia pestis bacteria. It can be a life-threatening infection if not treated promptly. Plague has caused several major epidemics in Europe and Asia over the last 2,000 years. Plague has most famously been called "the Black Death" because it can cause skin sores that form black scabs. A 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% of the population died within days, with fever and swollen skin sores.
Worldwide, up to 3,000 cases of plague are reported to the World Health Organization (WHO) each year, mostly in Africa, Asia and South America.
Plague is primarily an infection of animals including many species of rodents (including mice, rats, ground squirrels, prairie dogs, chipmunks and rabbits). In the United States, it is most commonly transmitted to humans by the bite of an infected rat flea (Xenopsylla species). People are most at risk of infection when they are in areas where these rodents and their fleas are plentiful. Less commonly, humans can become infected in other ways:
- When Y. pestis bacteria enter the body through a break in the skin after direct contact with the meat or blood of an infected animal (could happen, for example, when a hunter skins a carcass)
- By breathing in droplets of Y. pestis bacteria if a person is in close contact with a human or animal with plague infection of the lungs (pneumonic plague)
- From scratches or bites by infected domestic 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.
Plague occurs in different forms: bubonic, septicemic and pneumonic are the most common.
- Bubonic plague. This form of plague is the most common of all (more than 80% of all cases). It takes its name from the infected lymph nodes called "buboes." Buboes are very painful, red and swollen lymph nodes that develop very quickly near the area of the flea bite. If the bite was on the leg, a bubo would probably appear in the groin. If the flea bite was on the arm, buboes might appear in the underarm or in the neck. About 2 to 6 days after the flea bite, a person with bubonic plague develops a high fever, chills, muscle aches, headache and extreme weakness and within another 24 hours, 1 or more buboes appear. With prompt treatment of appropriate antibiotics, over 90% of people will survive. Without proper treatment, the Y. pestis bacteria could spread through the bloodstream and a person could develop septicemic plague.
- Septicemic plague. This form of plague is the second most common. It can develop when Y. pestis bacteria spread through the bloodstream and cause a blood infection called septicemia. It can also happen if Y. pestis spreads from a bubo or from the lungs into the bloodstream. It can also happen if the Y. pestis bacteria get into the bloodstream after a person has direct contact with the meat or blood of an infected animal. The first symptoms of septicemic plague can include nausea, vomiting, diarrhea and abdominal pain. The person can also develop severe bleeding problems, including sudden bleeding under the skin, scattered bruises, blood in the urine and abnormal bleeding from the mouth, nose and rectum. The bleeding problems can be followed by signs of shock (severe drop in blood pressure, rapid pulse, unconsciousness), kidney failure, severe breathing difficulties and even death. With appropriate treatment, however, 75% to 80% of people survive.
- Pneumonic plague. This form of plague is currently very rare. It happens when Y. pestis bacteria infect the lungs and cause pneumonia. It can develop when a person breathes in droplets of Y. pestis from an animal or person who has plague infection in the lungs. People who have bubonic or septicemic plague can also develop Y. pestis infection in the lungs. Symptoms include high fever, chills, headaches, chest pain, rapid breathing, severe shortness of breath and cough that might bring up blood. Without proper treatment, the disease can quickly lead to death.
If your doctor suspects you might have plague, he or she will ask whether you:
- Recently noticed a flea bite
- Have been around wild rodents
- Have recently traveled to an area of the world where plague is known to occur
- Have been in contact with a dead animal
- Have been treating a pet that has been extremely ill
To confirm the diagnosis, blood or other body fluids can be tested to look for evidence of Y. pestis bacteria infection.
With proper antibiotic treatment, most symptoms of uncomplicated bubonic plague will subside within two to five days. However, swollen buboes can remain for several weeks. Recovery from more severe septicemic plague and pneumonic plague usually takes longer.
If you live, work or vacation in regions where plague affects local rodent populations, here are things you can do to help prevent getting the infection and allowing plague to spread:
- Avoid contact with sick or dead animals, especially rodents. Report any observations of sick or dead animals to the local health department or law enforcement officials.
- Avoid contact with the nests and burrows of squirrels, chipmunks and other rodents.
- Eliminate sources of food and nesting places for rodents around homes, work places and recreation areas. Remove brush; rock piles; junk; cluttered firewood; and potential-food supplies, such as pet and wild animal food. Ask local health authorities how to rodent-proof your house.
- If you live in areas where rodent plague occurs, use veterinarian-approved flea collars and other flea repellents on cats and dogs. Treat pet dogs and cats for flea control regularly and not allow these animals to roam freely. Follow your veterinarian's guidelines when handling a severely ill cat or dog, and have the animal examined promptly by your veterinarian.
- If you anticipate being exposed to rodent fleas, apply an insect repellent containing DEET (diethyl toluamide) to your skin and use other insect repellents on clothing to prevent flea bites.
- Wear gloves when handling potentially infected animals or a dead animal.
If someone is exposed to a person or animal infected with plague, antibiotics can be prescribed by a doctor to prevent the person from getting the disease.
A plague vaccine is no longer available in the United States.
A person who is ill and is suspected of having the plague needs to be hospitalized. Hospital staff will take special steps to prevent the spread of the plague bacteria to other people. For example, the infected person will be put in a private room and the hospital staff will wear gloves, masks and protective clothing when coming into the room.
Antibiotics will be given intravenously (into a vein). Patients with severe bleeding problems or difficulty breathing will be treated in an intensive care unit. In the United States, 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 Centers for Disease Control (CDC) will confirm the diagnosis and report it to the World Health Organization (WHO).
Call your doctor whenever you or someone in your family develops a fever or becomes very sick (fever, severe weakness, severe headache) after:
- 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, swollen lymph node, especially if you have recently been bitten by an insect.
Without quick antibiotic treatment, plague can be fatal. Even with the right antibiotics and good hospital care, as many as 10% of plague patients in the United States die.
Centers for Disease Control and Prevention (CDC)
Division of Vector-Borne Infectious Diseases