Plague Facts (cont.)

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What causes plague?

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The bacteria that cause plague are known as Yersinia pestis. In the natural state, the bacteria infect wild rodents. Plague can still be found in many areas of the world, but 95% of cases today occur in Madagascar and sub-Saharan Africa. The World Health Organization states that between 1,000-2,000 cases are reported each year worldwide, but there are estimated to be more cases that go unreported. The bacteria are found in the U.S. in semi-arid areas of the southwest. Fleas that feed off of infected animals transmit the bacteria to other animals. Rats, ground squirrels, mice, prairie dogs, chipmunks, voles, and rabbits are examples of animals that may carry the plague bacteria. The bacteria are believed to persist at a low level in natural populations of these animals. When a large number of infected wild rodents die, fleas that have bitten these animals may bite humans and domestic animals. Cats that are bitten usually become ill, and they may cough infectious droplets into the surrounding air. While infected dogs may not appear ill, they may still carry infected fleas into the home.

The last urban outbreak of flea-transmitted plague in the U.S. occurred in the 1920s. Plague in the U.S. is rare today but occasionally occurs in the southwestern portion of the country where wild rodents may be infected. Between 1900-2012, 999 confirmed or probable cases of plague occurred in the U.S.

What are risk factors for plague?

Risk factors for plague include being bitten by fleas as well as exposure to rodents. Scratches or bites from infected domestic cats are also a risk factor. Contact with individuals with pneumonic plague (pneumonia caused by the plague bacteria, see below) is also a risk factor for acquiring the infection.

What is the incubation period for plague?

Symptoms and signs of plague usually develop between two and seven days after acquiring the infection, although they may appear after only one day in cases of exposure to pneumonic plague.

What are plague symptoms and signs? What are the different types of plague?

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The signs and symptoms of plague may take three forms:

Bubonic plague

In this form of the infection, bacteria infiltrate the lymph nodes, causing enlarged, painful, tender lymph nodes called buboes. Accompanying symptoms are fever, chills, headaches, and weakness. If not treated, the infection can spread to other areas of the body.

Septicemic plague

This form of plague is a result of plague bacteria entering the bloodstream. It can occur on its own or may develop from bubonic plague. Symptoms include fever, chills, weakness, abdominal pain, and shock. There can be bleeding and tissue death, especially of the fingers and toes. These dying tissues may appear black, hence the name Black Death.

Pneumonic plague

In this form of the illness, symptoms of other types of plague can be present, but the characteristic clinical picture of pneumonia is present. The plague bacteria spread to the lungs or infect the lungs directly when infected droplets in the air are inhaled. This is the only form of plague that can be transmitted from person to person. Shortness of breath, chest pain, and cough with watery or bloody mucus production are symptoms of pneumonic plague.

Medically Reviewed by a Doctor on 10/8/2015

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