Information on Plague

General

Plague, caused by a bacterium called Yersinia pestis, is transmitted from rodent to rodent by infected fleas.

Plague is characterized by periodic disease outbreaks in rodent populations, some of which have a high death rate. During these outbreaks, hungry infected fleas that have lost their normal hosts seek other sources of blood, thus increasing the increased risk to humans and other animals frequenting the area.

Epidemics of plague in humans usually involve house rats and their fleas. Rat-borne epidemics continue to occur in some developing countries, particularly in rural areas. The last rat-borne epidemic in the United States occurred in Los Angeles in 1924-25. Since then, all human plague cases in the U.S. have been sporadic cases acquired from wild rodents or their fleas or from direct contact with plague-infected animals.

Rock squirrels and their fleas are the most frequent sources of human infection in the southwestern states. For the Pacific states, the California ground squirrel and its fleas are the most common source. Many other rodent species, for instance, prairie dogs, wood rats, chipmunks, and other ground squirrels and their fleas, suffer plague outbreaks and some of these occasionally serve as sources of human infection. Deer mice and voles are thought to maintain the disease in animal populations but are less important as sources of human infection. Other less frequent sources of infection include wild rabbits, and wild carnivores that pick up their infections from wild rodent outbreaks. Domestic cats (and sometimes dogs) are readily infected by fleas or from eating infected wild rodents. Cats may serve as a source of infection to persons exposed to them. Pets may also bring plague-infected fleas into the home.

Between outbreaks, the plague bacterium is believed to circulate within populations of certain species of rodents without causing excessive mortality. Such groups of infected animals serve as silent, long-term reservoirs of infection.

Geographic Distribution of Plague

In the United States during the 1980s plague cases averaged about 18 per year. Most of the cases occurred in persons under 20 years of age. About 1 in 7 persons with plague died.

Worldwide, there are 1,000 to 2,000 cases each year. During the 1980s epidemic plague occurred each year in Africa, Asia, or South America. Epidemic plague is generally associated with domestic rats. Almost all of the cases reported during the decade were rural and occurred among people living in small towns and villages or agricultural areas rather than in larger, more developed, towns and cities.

The following information provides a worldwide distribution pattern:

• There is no plague in Australia.
• There is no plague in Europe; the last reported cases occurred after World War II.
• In Asia and extreme southeastern Europe, plague is distributed from the Caucasus Mountains in Russia, through much of the Middle East, eastward through China, and then southward to Southwest and Southeast Asia, where it occurs in scattered, localized foci. Within these plague foci, there are isolated human cases and occasional outbreaks. Plague regularly occurs in Madagascar, off the southeastern coast of Africa.
• In Africa, plague foci are distributed from Uganda south on the eastern side of the continent, and in southern Africa. Severe outbreaks have occurred in recent years in Kenya, Tanzania, Zaire, Mozambique, and Botswana, with smaller outbreaks in other East African countries. Plague also has been reported in scattered foci in western and northern Africa.
• In North America, plague is found from the Pacific Coast eastward to the western Great Plains and from British Columbia and Alberta, Canada southward to Mexico. Most of the human cases occur in two regions; one in northern New Mexico, northern Arizona, and southern Colorado, another in California, southern Oregon, and far western Nevada.
• In South America, active plague foci exist in two regions; the Andean mountain region (including parts of Bolivia, Peru, and Ecuador) and in Brazil.

How Is Plague Transmitted?

Plague is transmitted from animal to animal and from animal to human by the bites of infective fleas. Less frequently, the organism enters through a break in the skin by direct contact with tissue or body fluids of a plague-infected animal, for instance, in the process of skinning a rabbit or other animal. Plague is also transmitted by inhaling infected droplets expelled by coughing, by a person or animal, especially domestic cats, with pneumonic plague. Transmission of plague from person to person is uncommon and has not been observed in the United States since 1924 but does occur as an important factor in plague epidemics in some developing countries.

Diagnosis

The pathognomic sign of plague is a very painful, usually swollen, and often hot-to-the touch lymph node, called a bubo. This finding, accompanied with fever, extreme exhaustion, and a history of possible exposure to rodents, rodent fleas, wild rabbits, or sick or dead carnivores should lead to suspicion of plague.

Onset of bubonic plague is usually 2 to 6 days after a person is exposed. Initial manifestations include fever, headache, and general illness, followed by the development of painful, swollen regional lymph nodes. Occasionally, buboes cannot be detected for a day or so after the onset of other symptoms. The disease progresses rapidly and the bacteria can invade the bloodstream, producing severe illness, called plague septicemia.

Once a human is infected, a progressive and potentially fatal illness generally results unless specific antibiotic therapy is given. Progression leads to blood infection and, finally, to lung infection. The infection of the lung is termed plague pneumonia, and it can be transmitted to others through the expulsion of infective respiratory droplets by coughing.

The incubation period of primary pneumonic plague is 1 to 3 days and is characterized by development of an overwhelming pneumonia with high fever, cough, bloody sputum, and chills. For plague pneumonia patients, the death rate is over 50%.

Treatment Information

As soon as a diagnosis of suspected plague is made, the patient should be isolated, and local and state health departments should be notified. Confirmatory laboratory work should be initiated, including blood cultures and examination of lymph node specimens if possible. Drug therapy should begin as soon as possible after the laboratory specimens are taken. The drugs of choice are streptomycin or gentamycin, but a number of other antibiotics are also effective.

Those individuals closely associated with the patient, particularly in cases with pneumonia, should be traced, identified, and evaluated. Contacts of pneumonic plague patients should be placed under observation or given preventive antibiotic therapy, depending on the degree and timing of contact.

It is a U.S. Public Health Service requirement that all suspected plague cases be reported to local and state health departments and the diagnosis confirmed by the CDC. As required by the International Health Regulations, CDC reports all U.S. plague cases to the World Health Organization.

Prevention

Plague will probably continue to exist in its many localized geographic areas around the world, and plague outbreaks in wild rodent hosts will continue to occur. Attempts to eliminate wild rodent plague are costly and futile. Therefore, primary preventive measures are directed toward reducing the threat of infection in humans in high risk areas through three techniques -- environmental management, public health education, and preventive drug therapy.

Environmental Management

Epidemic plague is best prevented by controlling rat populations in both urban and rural areas. This goal has been reached in the cities, towns, and villages of most developed countries. It has not been achieved in either the rural or urban areas of many developing countries where the threat of epidemic plague continues to exist. Control of plague in such situations requires two things: 1) close surveillance for human plague cases, and for plague in rodents, and 2) the use of an effective insecticide to control rodent fleas when human plague cases and rodent outbreaks occur.

Public Health Education

In regions such as the American West where plague is widespread in wild rodents, the greatest threat is to people living, working, or playing in areas where the infection is active. Public health education of citizens and the medical community should include information on the following plague prevention measures:

• Eliminating food and shelter for rodents in and around homes, work places, and recreation areas by making buildings rodent-proof, and by removing brush, rock piles, junk, and food sources (such as pet food), from properties.
• Surveillance for plague activity in rodent populations by public health workers or by citizens reporting rodents found sick or dead to local health departments.
• Use of appropriate and licensed insecticides to kill fleas during wild animal plague outbreaks to reduce the risk to humans.
• Treatment of pets (dogs and cats) for flea control once each week.

Preventive Drug Therapy

Antibiotics may be taken in the event of exposure to the bites of wild rodent fleas during an outbreak or to the tissues or fluids of a plague-infected animal. Preventive therapy is also recommended in the event of close exposure to another person or to a pet animal with suspected plague pneumonia. For preventive drug therapy, the preferred antibiotics are the tetracyclines, chloramphenicol, or one of the effective sulfonamides.

Vaccines

The plague vaccine is no longer commercially available in the United States.

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This page last reviewed March 30, 2005

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