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Plague, is caused by bacteria called Yersinia pestis. Onset of plague is usually
2 to 6 days after a person is exposed. Initial symptoms include fever, headache,
and general illness, followed by the development of painful, swollen regional
lymph nodes. The disease progresses rapidly and the bacteria can invade the
bloodstream, producing severe illness, called plague septicemia. Once a human
is infected, a progressive 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 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%.
Geographic Distribution of Plague
In the United States, most of the human plague cases occur in two regions:
- Northern New Mexico, northern Arizona, and southern Colorado.
- California, southern Oregon, and far western Nevada.
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 infected
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.
Human plague cases in the U.S. have been sporadic cases acquired from wild
rodents or their fleas. 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, wild carnivores, and even antelopes, which 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.
Plague will probably continue to exist in its many localized geographic
areas in the southwest since attempts to eliminate wild rodent plague
are impractical 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
- Preventive drug therapy
Preventing epidemic plague requires the reducing or eliminating house rat
populations in both urban and rural areas.
Control of plague in such situations requires two things:
- Close surveillance
for human plague cases, and for plague in rodents.
- 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
- Eliminating of food and shelter for rodents around homes, work places, and recreation areas by removing brush, rock piles, junk, and food sources (such as pet food), from the site.
- Surveillance for plague activity in rodent populations in and surrounding high risk areas 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.
If you have any questions, please contact your nearest Regional Point of Contact,
park sanitarian or call WASO Public Health for more information.
Health Information, National Center for Infectious Diseases, Centers for
Disease Control and Prevention, Department of Health and Human Services.
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