Plague is a severe infection caused by the gram-negative bacteria Yersinia pestis.
In the past, massive plague epidemics, such as the black death of the Middle Ages, killed many people. The main contributing factors were large numbers of rodents, urban crowding, and poor sanitation. Plague now occurs mostly in rural areas. In the United States, more than 90% of infections occur in southwestern states such as Arizona, California, Colorado, and New Mexico, particularly among campers. Each year, plague affects about 10 to 15 people in the United States and 1,000 to 3,000 people worldwide.
The bacteria that cause plague usually infect wild rodents, such as rats and prairie dogs. The bacteria are spread by the rat flea. When wild rats die, the fleas may move to rats that live close to people, then to household pets, especially cats. The rat fleas may then bite people and transmit infection. Rarely, the infection is spread from person to person by coughing or sneezing. The bacteria lodge in the lungs and cause a type of pneumonia (pneumonic plague). Spread between people usually happens only when people live with or care for a person with pneumonic plague.
Plague can also be acquired by handling infected animals (for example, by hunters) or eating undercooked meat from infected animals.
Plague bacteria is a potential biological weapon. The bacteria can be spread through the air and inhaled. The size of the airborne particle determines where the bacteria lodge in the respiratory tract. Small particles lodge in the lungs, causing pneumonic plague.
Plague has several forms: bubonic, pestis minor, pneumonic, or septicemic. Symptoms vary depending on the form of plague.
Bubonic plague is the most common form. Symptoms may appear a few hours to 12 days after exposure (typically, after 2 to 5 days). Chills and fever of up to 106° F (41° C) occur suddenly. The heartbeat becomes rapid and weak, and blood pressure may drop. Many people become delirious. Shortly before the fever or at the same time, lymph nodes in the groin, armpits, or neck usually swell. They are firm, red, warm, and very tender. During the second week, pus may drain from the lymph nodes. The liver and spleen may enlarge. More than 60% of untreated people die, usually between the third and fifth day.
Pestis minor is a mild form of bubonic plague. Its symptoms—swollen lymph nodes, fever, headache, and exhaustion—disappear within a week.
Pneumonic plague is infection of the lungs. Symptoms begin abruptly 2 or 3 days after exposure to the bacteria. People have a high fever, chills, a rapid heartbeat, and often a severe headache. Within 24 hours, they develop a cough that brings up clear sputum, which soon becomes flecked with blood. Then the sputum becomes pink or bright red (resembling raspberry syrup) and foamy. Breathing is rapid and labored. Most untreated people die within 48 hours after symptoms start.
Septicemic plague is infection that spreads into the blood. About 40% of people have nausea, vomiting, diarrhea, and abdominal pain. Without treatment, many organs malfunction, often causing death.
To diagnose plague, doctors take samples of blood, sputum, or pus from lymph nodes. Samples are examined under a microscope and sent to a laboratory to grow (culture) bacteria. The blood sample is also tested for antibodies to the bacteria. Tests that rapidly detect the bacteria or its genetic material (DNA), such as polymerase chain reaction (PCR), may be done.
Controlling rodents and using repellents to prevent fleabites can help. The vaccine against plague is no longer available in the United States. People who are traveling to locations with a plague outbreak may take an antibiotic such as doxycycline, trimethoprim-sulfamethoxazole, or ciprofloxacin.
Treatment begins immediately and reduces the risk of death to less than 15%. Streptomycin is given by injection for 7 to 10 days. Other antibiotics such as gentamicin, doxycycline, and ciprofloxacin are also effective.
People with pneumonic plague must be isolated so that they do not spread the bacteria through the air—called respiratory isolation. It includes the following:
Anyone who has had unprotected contact with a person or animal (such as a domestic cat) with pneumonic plague must take doxycycline or ciprofloxacin for 7 days.
Last full review/revision September 2008 by Matthew E. Levison, MD