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Bubonic Plague is an infectious disease of animals and humans caused by a bacterium named Yersinia pestis. People usually get plague from the bite of a rodent flea that is carrying the plague bacterium or by handling an infected animal. Millions of people in Europe died from plague in the Middle Ages, when flea-infested rats inhabited human homes and places of work. Today, modern antibiotics are effective against the plague, but if an infected person does not seek treatment promptly, the disease is likely to cause illness or death. Despite its epidemic history and the deaths of millions of people, the public still knows little about the disease.

Due to its bacterial nature, Yersinia pestis has proven difficult to classify; however, new technologies have allowed researchers to classify it more accurately in the past several years. Y. pestis is a prokaryotic bacterium and, therefore, is of the kingdom Monera. Taxonomists formerly classified its genus, Yersinia, in the family Pasteurellaceae, but based on DNA similarities to Escherichia coli, the Yersinia group became members of the Enterobacteriaceae family. Numerous biochemical differences among its members are the basis of differentiation among the members of the family Enterobacteriaceae. More recently, biologists have applied nucleic acid techniques to assist in the definition of genera and species within this family; hence, as more techniques are applied, newly defined genetic relationships have lead to changes in classification. Though there are 11 named species in the genus Yersinia, doctors consider only three significant human pathogens: Y. pestis, the agent of plague, Y. pseudotuberculosis, and Y. enterocolitica. Y. pseudotuberculosis is the closest genetic relative to Y. pestis but can be distinguished from the plague bacteria by its clinical manifestations and by laboratory test results. Both Y. pestis and Y. pseudotuberculosis do not frequently infect humans in contrast to Y. enterocolitica, which scientists find more frequently in clinical specimens.

The history of bubonic plague is remarkable. For centuries, plague represented certain death for many people living in Asia, Africa and Europe, where, it has been said, populations were so affected that sometimes there were not enough people left alive to bury the dead. Because the cause of plague was unknown, plague outbreaks contributed to massive panic in cities and countries where it appeared. The people believed that the displeasure of the gods or a heavenly disturbance was the cause of the disease. In response, some Christians, known as flagellants, began to beat themselves ritually as penance for their own and for others sins. The public blamed innocent groups of people for spreading the plague and persecuted them for doing so. Numerous references in art, literature, and monuments attest to the horrors and devastation of past plague epidemics. So imprinted in our minds is the fear of plague that, even now, in the 21st century, a suspected plague outbreak can incite mass panic and bring much of the world’s economy to a temporary standstill. The number of human plague infections is low when compared to diseases caused by other agents, yet plague invokes an intense, irrational fear, disproportionate to its transmission potential in the antibiotic/vaccination era.

Several research scientists have proven key in identifying the plague agent and in determining how it is spread. In 1894, the individual research of Yersin and Kitasato in Hong Kong on the discovery of the infectious agent of plague opened the way for others research on the disease and on how it spreads. Kitasato and Yersin described, within days of each other s findings, the presence of bipolar staining organisms in the swollen lymph node (bubo), blood, lungs, liver, and spleen of dead patients. Though both investigators reported their findings, there were a series of confusing and contradictory statements by Kitasato that eventually led to the acceptance of Yersin as the primary discoverer of the organism now named after him, Yersinia pestis. Yersin recorded that the plague affected rats not only during plague epidemics but also preceding such epidemics in humans. In fact, plague was designated, in local languages, as a disease of the rats: villagers in China, India, and Formosa (currently Taiwan) claimed that when hundreds and thousands of rats lie dead in and out of houses, plague outbreaks in people soon followed. Simond described the transmission of plague was in 1898. He noted that persons who became ill did not have to be in close contact with each other to acquire the disease. Numerous observations led Simond to suspect that the flea might be an intermediary factor in the transmission of plague since people acquired plague only if they were in contact with recently dead rats and were not affected if they touched rats that were dead for more than 24 hours. He demonstrated that the rat flea (Xenopsylla cheopis) transmitted the disease from one rat to another and hypothesized that it could do likewise between rats and humans.

The plague is an ancient disease that is not likely to disappear; its continued outbreaks throughout the world attest to its persistent presence. Plague cycles naturally, circulating between small mammals and fleas without human involvement. These periods, during which people report few or no human cases, may last for years, leading to mistaken declarations of plague eradication. However, despite these long silent periods, plague may suddenly reappear. The combination of false assurance of its eradication, and the public s failure to use proper prevention methods, sets the stage for the panic that may result when plague spills over from its natural cycle into rodent populations (and their fleas) with close human contact. This spillover brings the plague into closer human contact. Poor sanitation, overcrowding, and high numbers of rodents are conditions that enhance plague transmission. Thus, a plague outbreak has come to represent particular social, environmental and political conditions.

Most mammals are susceptible to infection by the plague. The bites of infected rodent fleas are the most common sources of plague bacteria among humans. During rodent plague outbreaks, many animals die and their hungry fleas seek other sources of blood to survive. Persons and animals that visit places where rodents have recently died from plague risk getting the disease from fleabites. By handling infected rodents, rabbits, or wild carnivores that prey on these animals, people can also become infected House cats also are susceptible to plague. Infected cats become sick and may directly transmit plague to persons who handle or care for them. Additionally, dogs and cats may bring plague-infected fleas into the home. Inhaling droplets expelled by the coughing of a plague-infected person or animal (especially house cats) can result in pneumonic plague. Transmission of pneumonic plague from person to person is uncommon but sometimes results in dangerous epidemics that can spread quickly due to the diseases airborne nature.

Initial diagnosis of bubonic plague is simple. The typical sign of the plague is a swollen and very tender lymph gland, accompanied by pain. In the Middle Ages, people called the swollen gland a bubo, and it is from this term that the disease draws its name. One should suspect bubonic plague when a person develops a swollen gland, fever, chills, headache, and extreme exhaustion or when that person has a history of possible exposure to infected rodents, rabbits, or fleas. A person usually becomes ill with bubonic plague 2 to 6 days after being infected.

Laboratory confirmation of a plague diagnosis is also an easy process. Doctors must examine specimens for evidence of the plague and they should obtain the specimens from appropriate sites for isolating the bacteria. The preferred specimen for microscopic examination and isolation from a bubonic case is material from the affected bubo, which should contain numerous organisms. Doctors must also take blood cultures whenever possible in order to test for another form of the plague, septicemic plague, which infects the blood and almost invariably results in death.. Additionally, affected lymph nodes may intermittently release bacteria into the bloodstream; therefore, a series of blood specimens taken 10-30 minutes apart may be productive in the isolation of Y. pestis. Sputum/throat smears taken from pneumonic plague patients may reveal Y. pestis when subjected to the fluorescent-antibody (FA) test. In cases where live organisms are not culturable, lymphoid tissues, lung, and bone marrow samples may yield evidence of plague infection by FA test or by detection of Y. pestis DNA.

If plague patients do not receive specific antibiotic therapy, the disease can progress rapidly. Individuals who have contracted the disease are isolated, put to bed, and fed fluids and easily digestible foods. The afflicted may receive sedatives to reduce pain and to quiet delirium. During World War II, scientists using sulfa drugs were able to produce cures of the plague; subsequently, researchers found antibiotics, such as streptomycin and tetracyclines, to be more effective in curing the disease.

Plague often results in death when it left untreated. Occasionally, plague bacteria invade the bloodstream, and as the plague bacteria multiply in there, they spread rapidly throughout the body and cause a severe and often fatal condition known as septicemic plague. In primary septicemic plague, the victim has a sudden onset of high fever and turns deep purple in several hours, often dying within the same day that symptoms first develop. The purple color, which appears in all plague victims during their last hours, is due to respiratory failure; the popular name Black Death that is applied to the disease is derived from this symptom. Infection of the lungs with the plague bacterium may also result in death because it causes the pneumonic form of plague, a severe respiratory illness. The infected person may experience high fever, chills, cough, and breathing difficulty and may expel bloody sputum. About 14% (1 in 7) of all plague cases in the United States are fatal.

Statistics reveal that only geography isolates the plague and that other factors do not hinder its spread. Outbreaks in people occur in highly rural communities and in inner cities. They are usually associated with infected rats and rat fleas that live in these areas. In the United States, the last urban plague epidemic occurred in Los Angeles in 1924-25. Since then, human plague in the United States has occurred mostly as scattered cases in rural areas (an average of 10 to 15 persons each year). Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year. In North America, biologists have found affected animals from the Pacific Coast to the Great Plains, and from southwestern Canada to Mexico. Most human cases in the United States occur in two regions: 1) northern New Mexico, northern Arizona, and southern Colorado; and 2) California, southern Oregon, and Far Western Nevada. Plague also exists in Africa, Asia, and South America. The charts below show distribution in both the U.S. and across the world. In modern times, plague has shown no preference among individuals of either gender or of any race.

Prevention is the only proven method to control infection with bubonic plague. Attempts to eliminate fleas and wild rodents from the natural environment in plague-infected areas are impractical; however, controlling rodents and their fleas around places where people live, work, and play is very important in preventing human disease. Therefore, doctors encourage preventive efforts in home, work, and recreational settings where the risk of acquiring plague is high. They recommend a combined approach using these methods: environmental sanitation, educating the public on ways to prevent plague exposures, preventive antibiotic therapy, and, in certain special circumstances, vaccines.

The bubonic plague is one of a variety of forms of disease caused by the bacterium Yersinia pestis. The result of the disease is an infection and inflammation of the lymph glands and, ultimately, the blood and lungs. Many people in medieval times died of the disease; however, today it is curable with antibiotics. Through prevention, humanity has reduced the number of bubonic plague cases around the world. The plague is a devastating disease that remains a problem today.


“Plague,” Microsoft Encarta Encyclopedia 99. 1993-1998 Microsoft Corporation.

“Black Death,” Microsoft Encarta Encyclopedia 99. 1993-1998 Microsoft Corporation.

Centers for Disease Control and Prevention: Division of Vector-Borne Infectious Diseases, Plague, World Wide Web Internet Website, http://www.cdc.gov/ncidod/dvbid/plagindex.htm, last update: 7-2-99.

Plague and Public Health in Renaissance Europe, World Wide Web Internet Website, http://jefferson.village.virginia.edu/osheim/, last update: 11-1-94.

Stedman s Medical Dictionary, 25th Edition, William R. Hensyl, copyright 1990, Williams & Wilkins: Baltimore.

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