The bacteria are spread mainly by the rat flea.
Depending on the form, plague can cause fever, chills, swollen lymph nodes, headache, a rapid heartbeat, cough, difficulty breathing, vomiting, and diarrhea.
Identifying the bacteria in samples of blood, sputum, or pus from lymph nodes confirms the diagnosis.
Antibiotics can reduce the risk of death, and isolating infected people helps prevent spread of plague.
(See also Overview of Bacteria.)
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 sporadically or in limited outbreaks.
In the United States, more than 90% of infections occur in rural or semirural areas of southwestern states such as Arizona, California, Colorado, and New Mexico, particularly among campers. In recent decades, plague affects an average of about 7 people a year (ranging from 1 to 17 people) in the United States.
Worldwide, most cases since the 1990s have occurred in Africa, with recent outbreaks in Madagascar.
The bacteria that cause plague usually infect wild rodents, such as rats, mice, squirrels and prairie dogs. The bacteria are spread by the rat flea. When wild rodents die, the fleas may move to rodents that live close to people, then to household pets, especially cats. The rat fleas may then bite people and transmit infection. People may also become infected through a break in their skin if they have direct contact with an infected animal.
Rarely, the infection is spread from person to person by inhaling droplets dispersed 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 bacteria are 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. Larger particles lodge in the throat (pharynx) and cause plague of the throat (pharyngeal plague).
Plague has several forms:
Symptoms vary depending on the form of plague.
Bubonic plague is the most common form.
Bubonic plague 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 near the flea bite or scratch, usually in the groin or armpit, become swollen and painful. These swollen lymph nodes are called buboes. 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.
A bump, sore, or a black scab may form at the site of the flea bite.
People with bubonic plague may become restless, delirious, confused, and uncoordinated.
Because the bacteria can spread through the bloodstream, the lungs may be affected, causing pneumonic plague.
More than 60% of untreated people die, usually between the third and fifth days.
Pneumonic plague is infection of the lungs.
Pneumonic plague 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.
Eventually, blood clots too much, blocking blood vessels and eventually causing bleeding because clotting factors are used up. This disorder is called disseminated intravascular coagulation. Because blood flow is blocked, gangrene develops in the limbs, which may turn black (hence, the name Black Death).
Without treatment, many organs malfunction, often causing death.
Rapid diagnosis of plague is important because the longer treatment is delayed, the higher the risk of dying.
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.
If doctors suspect pneumonic plague, a chest x-ray is taken.
The following measures can help control rodents and prevent flea bites:
Removing anything that might attract rodents (such as brush, rock piles, junk, cluttered firewood, and food) from around homes
Wearing gloves when handling animals that may be infected (for example, when skinning an animal)
Using repellents, such as DEET (diethyltoluamide), when exposure to rodent fleas is possible (for example, while camping, hiking, or working outdoors)
Applying products that control fleas to pets
If pets roam outside in areas where plague is present, not letting them sleep in a person's bed
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.
Doctors monitor people who have had contact with a person who has pneumonic plague. Their temperature is taken every 4 hours for 6 days.
Also, antibiotics are given to prevent the infection from developing. Adults are given doxycycline or ciprofloxacin, and children are given trimethoprim/sulfamethoxazole.
Treatment of plague begins immediately and reduces the risk of death to 11%. For pneumonic or septicemic (bloodstream) plague, antibiotics must be started within 24 hours after symptoms appear.
Streptomycin or gentamicin is given by injection for 10 days or until 3 days after temperature has returned to normal. Other antibiotics such as doxycycline, levofloxacin, 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). Respiratory isolation includes the following:
Other Yersinia species, such as Yersinia enterocolitica and Yersinia pseudotuberculosis, infect animals worldwide. These bacteria can be transmitted to people but cause a less severe illness than plague.
Infections caused by other Yersinia bacteria can also be acquired by handling infected animals (for example, by hunters), by consuming or handling raw or undercooked meat or unpasteurized milk or milk products from infected animals, or by drinking contaminated water.
These infections commonly cause diarrhea, which usually resolves on its own, and sore throat. Lymph nodes in the abdomen become inflamed, causing pain in the lower right part of the abdomen. The pain resembles the pain of appendicitis.
The bacteria may infect the bloodstream, but they rarely spread to parts of the body outside the intestinal tract.
Diagnosis of other Yersinia infections includes testing samples of blood, stool, sputum, or pus from lymph nodes.
Prevention of other Yersinia infections involves the following:
Treatment of other Yersinia infections is focused on relieving the diarrhea. These infections usually resolve on their own. However, antibiotics are needed if the infection has spread to the bloodstream or other organs.