Plague is an infectious disease caused by a rod-shaped bacterium called Yersinia pestis, which usually is spread to humans through the bites of infected fleas. Most epidemics of plague among humans have involved fleas that live in the fur of rats and other rodents. The rats are called vectors of the disease because they carry the infected fleas to other animals, including humans.
There are three major forms of plague infection in humans: bubonic, septicemic, and pneumonic. Bubonic plague is the most familiar form; it is characterized by high fever and the appearances of buboes, or tender, swollen lymph glands, in the groin, armpit, and neck of the infected person. Septicemic plague is an infection of the bloodstream; the patient may not develop buboes but has a high fever. It has a higher mortality rate than the bubonic form of plague. Pneumonic plague, the most deadly form, can be spread directly from infected humans, or sometimes cats, to other people through coughing. The plague bacteria infect the lungs, causing chest pain, coughing up of blood, and difficulty breathing along with high fever and chills.
The last epidemic of plague in the United States that was carried by rats occurred in Los Angeles in 1924. All cases of plague reported in North America since then have been isolated incidents. According to the Centers for Disease Control and Prevention (CDC), there are on average ten cases of plague in the United States each year, almost entirely in the Southwest and California, with the local ground squirrel as the rodent reservoir. This is a decrease from the average of eighteen cases per year in the 1980s. Worldwide, there are about 2,000 cases of death from plague every year, mostly in Africa, Asia, or South America.
Although most cases reported each year occur in Caucasians, Native Americans in parts of Utah, New Mexico, and Arizona have a high risk of getting plague because the disease is endemic in those areas and many members of these tribes work as sheepherders or ranchers.
Risk factors for getting plague include having an occupation that requires working outdoors in an area where plague is endemic or hiking and camping in such areas.
Plague is caused by bacteria transmitted to humans, mostly through the bites of infected fleas. The fleas live in the fur of house rats, ship rats, and other rodents, and pick up the bacteria from biting an infected rodent.
The fleas can then infect other rats, prairie dogs, chipmunks, rabbits, rock squirrels, and similar animals by biting them. From time to time the infected rodents begin to die off at a high rate. The fleas then go looking for other sources of blood meals, and may turn to domestic animals or humans.
The Black Death
The Black Death is the name given to the pandemic (worldwide epidemic) of bubonic plague that began in China and moved across Central Asia to Europe in 1347. It is estimated to have killed 75 million people worldwide; between 25 and 50 million deaths occurred in Europe, representing 30–60 percent of the population at that time.
The Black Death, in part, spread to Europe through an early form of bioterrorism. The soldiers of a Mongol army besieging Caffa, a trading city on the Crimean peninsula, began to die from plague in early 1347. The Mongol general ordered the infected bodies to be catapulted over the city walls in order to spread the plague inside the city. Some Italian traders in Caffa fled back to Italy, taking the plague bacteria with them to Sicily and southern Europe. It spread northward from Italy, reaching France and Spain in 1348, England in 1349, and the Scandinavian countries in 1350.
This was neither the first nor the last pandemic of bubonic plague. The first major known outbreak was the Plague of Justinian, which began in Africa and killed about a quarter of the population of the eastern Mediterranean during the reign of the
Emperor Justinian in 541–542 CE. The Great Plague of London later killed a fifth of the city's population between 1665 and 1666.
When a flea sucks blood from an animal infected with plague, the plague bacteria can multiply inside the flea's upper digestive tract and cause obstruction. When the flea bites its next victim to try to feed, it injects material loaded with bacteria into the skin. Once inside the new
host, the bacteria travel to the lymph nodes, where they multiply further and produce the characteristic buboes of the bubonic form of the disease. The tissue in the lymph nodes may break down and allow the bacteria to travel through the bloodstream, producing the septicemic and pneumonic forms of plague.
The incubation period of plague in humans is two to six days for the bubonic form and as little as two days for the pneumonic form. The symptoms of each form typically include:
- Bubonic plague: Rash or blister at the site of the flea bite; muscle aches; buboes; and high fever. The buboes are extremely painful; the patient may not want to let the doctor examine them and may hold the limbs in unusual positions to relieve pressure on the buboes. As the swollen lymph glands grow in size, they may burst and release blood and bad-smelling pus.
- Septicemic plague: Patients may die quickly from septicemic plague. Symptoms include sudden drop in blood pressure, nausea, vomiting, diarrhea, fever, abdominal pain, and sudden organ failure.
- Pneumonic plague: Severe cough, chest pain, difficulty breathing, and frothy or bloody material brought up by coughing.
Diagnosis is made by a combination of history-taking and recording of the patient's fever and other symptoms. A history of exposure to rodents or fleas, the presence of flea bites on the patient's skin, and a high fever are major warning signs. Because plague should be treated as swiftly as possible—within twenty-four hours—the doctor will have the patient hospitalized for antibiotic treatment. A sample of fluid from a bubo (if one is present) or cerebrospinal fluid can be sent to the CDC laboratories for analysis; most hospital laboratories are not equipped to do the specialized testing required. If the patient appears to have septicemic plague, a blood sample will be taken as well. For pneumonic plague, a sample of the material coughed up by the patient will be sent for analysis.
Treatment requires immediate hospitalization in an isolation room. Antibiotics are given directly into the veins or into the muscles for at least
ten days. Patients dehydrated by fever will be given intravenous fluids as well. The patient may be put on a ventilator if they need help in breathing or given pure oxygen to breathe.
People who have been in contact with a patient diagnosed with pneumonic plague are often given antibiotics as a preventive measure and monitored carefully by their doctors.
Doctors are required by law to report a case of plague to the World Health Organization (WHO) as well as the CDC.
Prognosis depends partly on the patient's age, general health, and the promptness of treatment. Elderly people are at high risk of dying from plague even with prompt treatment. The mortality rate of untreated patients with the bubonic form of plague is about 50 percent if sepstemic plague ensues; however, it drops to 5 percent for those who are diagnosed and treated promptly. Almost all people with pneumonic plague will die without treatment.
Plague can be prevented by strict control of rats in urban areas, extermination of rats on ships and in harbor areas, and by surveillance (monitoring) of prairie dogs and other rodents in areas where the plague is endemic. People who notice the sudden appearance of large numbers of dead or dying rats, mice, or other rodents in their area should notify their public health department at once, as the rapid death of these animals is a warning sign of plague.
There is no commercially available vaccine against plague.
The most worrisome aspect of plague is not the possibility of an epidemic spread by fleas but rather one spread by terrorists. The CDC has identified the plague bacteria as an organism that could be turned into an aerosol form and then spread over large populations. Because people would breathe in the aerosolized bacteria, they would develop the pneumonic form of plague—the deadliest form and the form that can spread from person to person. WHO has estimated that spraying plague bacteria over a city of 5 million people would cause 150,000 cases of plague and 40,000 deaths.
In order to protect against this threat, the United States, Canada, and the United Kingdom signed an agreement in April 2005 to jointly develop a more effective vaccine against plague. Two experimental vaccines, one developed in the United Kingdom and the other by the United States Department of Defense, are presently undergoing clinical trials. The vaccines will be evaluated and the one that shows greater promise will selected for advanced development.
SEE ALSO Anthrax
WORDS TO KNOW
Bioterrorism: The use of disease agents to frighten or attack civilians.
Bubo: A swollen lymph node in the neck, armpit, or groin area.
Endemic: A term applied to a disease that maintains itself in a particular area without reinforcement from outside sources of infection.
Host: An organism that is infected by a virus, bacterium, or parasite. Rats and other rodents are common hosts of plague.
Pandemic: A disease epidemic that spreads over a wide geographical area and affects a large proportion of the population.
Surveillance: Monitoring of infectious diseases by public health doctors.
Vector: An animal that carries a disease from one host to another.
Byrne, Joseph P. Daily Life During the Black Death. Westport, CT: Greenwood Press, 2006.
Hillerman, Tony. “We All Fall Down.” In The Great Taos Bank Robbery and Other True Stories of the Southwest. New York: Harper Perennial, 2001. This chapter is an account of the death of a young biologist who caught bubonic plague during a field trip in New Mexico.
Lynette, Rachel. Bubonic Plague. Detroit, MI: KidHaven Press, 2005.
Acocella, Joan. “The End of the World: Interpreting the Plague.” New Yorker, March 21, 2005. Available online at http://www.newyorker.com/archive/2005/03/21/050321crbo_books?currentPage=all (accessed April 12, 2008). This is a review of two recent books about the Black Death that includes a discussion of modern outbreaks of plague.
Maugh, Thomas H., II. “An Empire's Epidemic.” Los Angeles Times, May 6, 2002. Archived on the Web site of the UCLA School of Public Health at http://www.ph.ucla.edu/EPI/bioter/anempiresepidemic.html (accessed April 12, 2008). This is a news article about the use of DNA to study the strain of plague that caused the Plague of Justinian.
Centers for Disease Control and Prevention (CDC). Plague Home Page. http://www.cdc.gov/ncidod/dvbid/plague/index.htm (accessed April 11, 2008).
NOVA Online. Bioterror. http://www.pbs.org/wgbh/nova/bioterror/ (accessed April 12, 2008). This is the companion Web site to a television show first aired in November 2001. It contains a segment on plague as an agent of bioterrorism.
Public Broadcasting Service (PBS). Secrets of the Dead: The Mystery of the Black Death. http://www.pbs.org/wnet/secrets/case_plague/index.html (accessed April 12, 2008). This is the companion Web site to a television broadcast that was first aired in 2002.