INFECTIOUS DISEASE Bacterial • Lyme disease • Toxic shock syndrome • Tetanus • Strep throat • Tuberculosis • Whooping cough • Plague • Diphtheria • Typhoid fever • Cholera • Rocky Mountain spotted fever • Botulism • Ehrlichiosis • Tularemia • MRSA infection • Whipple's disease • Brucellosis

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Plague is a life-threatening infection caused by the organism Yersinia pestis, the bacterium that caused the 14th-century Black Death plague pandemic.

Plague-causing bacteria still exist in the environment today, although their effect has abated dramatically. The major threat of plague these days comes not through natural transmission, but through intentional transmission, perhaps by terrorists as an agent of biological warfare.

In nature, infected fleas transmit Y. pestis primarily among rodents. When a plague outbreak among rodents kills many of them in short order, infected fleas that were feeding on the rodents' blood jump to other animals and humans, spreading the infection.

During the Black Death, plague caused 20 million to 30 million deaths in Europe. More recent pandemics through the late 19th century killed millions of people worldwide. Improved living conditions and health services have made such large-scale outbreaks of natural plague unlikely, but occasional isolated plague cases continue.

These days, weapons may be more likely than rats to cause a plague pandemic. Plague bacteria could conceivably be put into a form that could be sprayed through the air, infecting anyone inhaling the bacteria and causing pneumonic plague. This form of plague affects your lungs and can spread from person to person. Fortunately, when given promptly, antibiotics can effectively treat plague most of the time.

There are three types of plague: bubonic, septicemic and pneumonic. Signs and symptoms of plague vary depending on the type and on how you contract it. It's possible to develop more than one type of plague.

Bubonic plague
This is the most common type of plague in humans, accounting for the majority of naturally occurring cases. Bubonic plague is caused by a bite from an infected flea and is characterized by an enlarged, infected lymph node called a bubo.

Signs and symptoms of bubonic plague generally appear within two to eight days after a plague-infected flea bites you. After you're bitten, the bacteria travel through your lymphatic system, infecting the first lymph node they reach. The resulting bubo is usually 1 to 10 centimeters in diameter, swollen, painful and warm to the touch. It can cause so much pain that you can't move the affected part of your body. The bubo usually develops in your groin, but may also appear in your armpit or neck, depending on where the flea bit you. More than one bubo can develop, but typically buboes affect only one area of your body.

Buboes may not be noticeable until a day or more after other symptoms appear. Other signs and symptoms of bubonic plague include:

• Sudden onset of fever and chills
• Headache
• Fatigue or malaise
• Muscle aches

Septicemic plague
Septicemic plague occurs when plague bacteria multiply in your bloodstream. You can contract this form of plague when bacteria transmitted by a fleabite enter directly into your bloodstream, or as a complication of bubonic or pneumonic plague. If septicemic plague occurs as a complication of bubonic plague, buboes may be present.

Signs and symptoms of septicemic plague include:

• Fever and chills
• Abdominal pain, diarrhea and vomiting
• Bleeding from your mouth, nose or rectum, or under your skin
• Shock
• Blackening and death of tissue (gangrene) in your extremities, most commonly your fingers, toes and nose

The gangrene associated with septicemic plague inspired the nickname Black Death for the 14th-century pandemic.

Pneumonic plague
Pneumonic plague is the least common form of plague — accounting for 12 percent of U.S. cases in the last 50 years — but the most rapidly fatal. Primary pneumonic plague can occur when you inhale infectious droplets coughed into the air by a person or animal with pneumonic plague. Early signs and symptoms, which generally occur about two days after inhaling contaminated droplets, include:

• High fever
• Weakness
• Signs of pneumonia, including chest pain, difficulty breathing and a cough with bloody sputum
• Nausea and vomiting

You can also develop pneumonic plague as a complication of bubonic or septicemic plague if the bacteria spread to your lungs. This is called secondary pneumonic plague.

Pneumonic plague progresses rapidly and may cause respiratory failure and shock within two days of infection. If antibiotic treatment isn't initiated within a day after signs and symptoms first appear, the infection is likely to be fatal.

Plague resulting from a bioterrorist attack
It's possible that plague bacteria could be turned into an aerosol and then might be spread over large populations as a bioterrorist weapon. An attack of this kind would cause pneumonic plague — the most deadly and most contagious type. In 1970, the World Health Organization estimated that if plague bacteria were sprayed over a city of 5 million people, up to 150,000 people could be infected and 36,000 might die.

According to a consensus statement by a group of scientists published in the Journal of the American Medical Association, the incubation period for pneumonic plague following a bioterrorist attack might last from one to six days, but more often from two to four days. Signs and symptoms would mirror those of naturally occurring pneumonic plague, but might also include nausea, vomiting, abdominal pain and diarrhea.

Other indications that a bioterrorist event or germ warfare is behind a pneumonic plague outbreak include a high incidence of pneumonic plague in humans in regions of the country that haven't had outbreaks among animals or rodents, or when plague occurs in people without any known risk factors.

Plague has afflicted humans throughout history. The first recorded plague outbreak began in Egypt in A.D. 541.

The Black Death pandemic in the 14th century killed one-third of Europe's population. Europeans living during early pandemics believed the disease was a punishment from the gods or an unlucky confluence of astrological or supernatural elements.

The most recent plague pandemic began in China in the late 1800s and, due to booming international trade and ships with high rat populations, spread quickly throughout Asia and other parts of the world. That outbreak caused more than 12 million deaths in India and China alone.

The cause of plague, the Yersinia pestis bacterium, was discovered in 1894 by Alexandre Yersin. Soon after, scientists realized that fleas transmitted the bacteria.

In World War II, the Japanese army released plague-carrying fleas over a part of China, causing outbreaks of the disease. After World War II, both the United States and the former Soviet Union pursued biological weapons programs that developed means of exposing large populations to plague bacteria. Today, plague is one of a number of feared potential agents of bioterrorism, along with anthrax, smallpox, botulism, tularemia and nerve gases.

Naturally occurring plague outbreaks are most common in rural areas and in urban areas characterized by overcrowding, poor sanitation and a high rat population. Outbreaks can happen at any time of year.

In the United States, plague outbreaks occur most often between April and November. Most U.S. cases occur in Western states, including New Mexico, Arizona, Colorado and California.

Rock squirrels and ground squirrels are the most common sources of infection in the United States. Other rodents, including chipmunks and prairie dogs, may host plague-carrying fleas. Animals that may be infected and pose a transmission risk to humans include wild rabbits and domestic cats that have contact with wild rodents.

The disease usually spreads through fleabites, but you can also contract plague after being exposed to an infected animal that may have coughed infectious droplets into the air or through a break in your skin after handling an animal with plague. Groups at increased risk include veterinarians, cat owners, hunters, campers and hikers in areas with recent plague outbreaks among animals.

Call your doctor if you or someone close to you develops signs or symptoms of plague within a week of any of the following:

• Being exposed to a sick or dead animal
• Being bitten by a flea or by an unknown insect
• Spending time in an area with a known, recent plague outbreak or with a large number of dead or dying animals
• Having close contact — within three feet — with a person or animal with pneumonic plague
• Traveling to a high-risk region of the United States or another country with high plague rates

Your doctor may suspect plague if you live in a high-risk region. With the exception of a visible bubo, signs and symptoms often mimic other, more common infectious diseases.

You'll likely be asked to describe the type and severity of your symptoms and tell your doctor about your recent history, including whether you've been exposed to sick animals or traveled to areas with plague outbreak.

If your doctor suspects plague, he or she may confirm the diagnosis through microscopic examination of fluid extracted from your bubo, bronchi or trachea. Needle aspiration is used to obtain fluid from your bubo. Fluid is extracted from your airways using endoscopy. In this procedure, a thin, flexible tube is inserted through your nose or mouth and down your throat. A suction device is sent down the tube to extract a fluid sample from your airways.

Your doctor may also test blood drawn from your veins to diagnose plague. Y. pestis bacteria generally are present in your bloodstream only if you have septicemic plague.

Complications of plague may include:

• Gangrene of your fingers and toes resulting from clots in the small blood vessels of your extremities
• Severe shock
• Sudden, severe lung failure (acute respiratory distress syndrome)
• Bloodstream infection (septicemia)
• Inflammation of the membranes and fluid surrounding your brain and spinal cord (meningitis)
• Death

With prompt treatment, the overall fatality rate from plague is between 5 percent and 14 percent in the United States. Without treatment, mortality rates can be as high as 60 percent for bubonic plague and 100 percent for pneumonic plague. Death can occur within days after symptoms first appear if treatment doesn't begin promptly.

As soon as your doctor suspects that you have plague, you'll need to be admitted to an isolation room in a hospital. There, you'll receive powerful antibiotics directly into your veins (intravenously) or your muscles (intramuscularly) for at least 10 days. Streptomycin and gentamicin are the most effective drugs against plague. Other alternatives include intravenous doxycycline (Vibramycin), ciprofloxacin (Cipro) and chloramphenicol (Chloromycetin).

If you have serious complications, such as bleeding abnormalities, organ failure and respiratory distress, then respiratory support, intravenous fluids and oxygen may be necessary. Your doctor is required by law to report documented plague infection to local health officials.

Even if you don't have signs or symptoms, you'll need treatment with preventive, oral antibiotics for seven days after direct exposure to a person with pneumonic plague.

Previously, a vaccine was available for bubonic plague, but its efficacy was never well studied and the manufacturer stopped producing it in 1999. Clinical trials on a new plague vaccine are in the earliest stages.

Although no effective vaccine is available, antibiotics offer effective preventive therapy if you're at risk or have been exposed to plague. Ask your doctor immediately about preventive antibiotics if you:

• Have had close contact with a person or animal with known or suspected pneumonic plague
• Have been bitten by a flea or unknown insect in an area known to have recent plague cases
• Are planning to spend time in a region with recent plague outbreak

Take the following precautions if you live or spend time in regions where plague outbreaks occur:

• Avoid contact with sick or dead animals. If you hunt, wear gloves when handling dead animals.
• Rodent-proof your home. Remove potential nesting areas, such as piles of brush, rock, firewood and junk. Don't leave pet food or any other foods in areas that rodents can easily access.
• Prevent your pets from contracting fleas. Use flea-control products and don't allow pets to wander unsupervised. Ask your veterinarian for recommended flea-control brands and guidelines.
• Take precautions when outdoors. Closely supervise your children and pets when spending time outside in areas with large rodent populations. Use insect repellent on your skin and clothing.

Know the risk factors and the symptoms of plague so that you can identify it early and contact your doctor immediately. If you know of recent plague cases in your area, report sick or dead animals to your local health department or to police.