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Plague is an acute bacterial infection caused by Yersinia pestis. It is transmitted from rodent to rodent by infected fleas.

The natural hosts are small animals such as wild rats, and man is an incidental host. In certain parts of the world the organism is known to circulate in animals. 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.

Animals which are common sources of human infection include: rock squirrels and their fleas, prairie dogs, wood rats, chipmunks, and other ground squirrels and their fleas. 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.

Plague is still reported consistently from several countries in Africa, Asia, South America and rural parts of the USA. There is no plague in Europe and Australia.

The organism is usually passed to humans through the bite of a flea which has previously fed on an infected animal. In other words, the flea acts as a "vector" which passes the organism from animals to man. 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 but does occur as an important factor in plague epidemics in some developing countries. Bubonic plague is generally not spread from person-to-person, except through direct contact with any fluids from the swellings.

Signs and symptoms
There are 5 forms of plague, all of which are uncommon and 3 of which are very rare.
Usually, plague takes the form known as bubonic plague. The bacteria invade the body and cause fever. The lymph nodes swell up and become very painful and hot-to-the-touch (this is the bubo). This bubo is pathognomonic for plague and can range in size from1-10cm in length. 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.
Less frequently, the organism causes pneumonic plague, a form of pneumonia with severe respiratory symptoms, rapidly developing cough and difficulty in breathing. This form of the plague can be transmitted to others through the expulsion of infective respiratory droplets by coughing.
The other very rare presentations of plague include meningitis, septicaemic and pharyngeal plague.

Incubation Period
The incubation period for plague is usually between 1 and 7 days. For plague pneumonia following inhalation it is shorter at 1-4 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%.
The onset of bubonic plague is usually 2-6 days after a person is exposed.

Antibiotics are the treatment for plague. They should be started as soon as possible. The drugs of choice are streptomycin or gentamycin, but a number of other antibiotics are also effective. Plague can be very effectively treated with antibiotics, meaning that in treated cases deaths occur in fewer than 5% of cases. If left untreated the infection can result in death in 90% or more of cases.

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.

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.

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.

There is no vaccine commercially available.