What is Plague?

Plague is a disease caused by Yersinia pestis. This bacterium is found in mammals (prairie dogs, rats, squirrels, rabbits, cats) and their fleas in many areas around the world. Occasionally this organism infects humans. Usually fleas from the infected rodents carry the disease to humans. The World Health Organization reports 1,000 to 3,000 cases of plague worldwide every year. An average of 5 to 15 cases occur each year in the western United States (Figure 7). About 14% of all cases of plague are fatal (9, 47). 

There are two different forms of this disease; bubonic plague and pneumonic plague. Most naturally occurring cases of plague are the bubonic form. Pneumonic plague is very rare. Figure 9 shows the life cycle of this organism and how it can be acquired.   

Yersinia pestis is primarily a rodent pathogen, with humans being an accidental host when bitten by an infected rat flea. The flea draws viable Y. pestis organisms into its intestinal tract. These organisms multiply in the flea and block the flea's proventriculus.

Some Y. pestis in the flea are then regurgitated when the flea gets its next blood meal thus transferring the infection to a new host. While growing in the flea, Y. pestis loses its capsular layer. Most of the organisms are phagocytosed and killed by the polymorphonuclear leukocytes in the human host. A few bacilli are taken up by tissue macrophages. The macrophages are unable to kill Y. pestis and provide a protected environment for the organisms to synthesize their virulence factors.

The organisms then kill the macrophage and are released into the extracellular environment, where they resist phagocytosis (YopH and YopE; Yersinia outer membrane protein) by the polymorphs. The Y. pestis quickly spread to the draining lymph nodes, which become hot, swollen, tender, and hemorrhagic. This gives rise to the characteristic black buboes of bubonic plague
(Figure 8). The incubation time of this form of plague is 2-6 days. Other symptoms of this form of plague include extreme exhaustion, high fever, and low blood pressure resulting in shock, convulsions, and death.

Within hours of the initial flea bite, the infection spills out into the bloodstream, leading to infection of the liver, spleen, and lungs. The patient develops a severe bacterial pneumonia, exhaling large numbers of viable organisms into the air during coughing fits. 50 to 60 percent of untreated patients will die if untreated. As the epidemic of bubonic plague develops (especially under conditions of severe overcrowding, malnutrition, and heavy flea infestation), it eventually shifts into a predominately pneumonic form (pneumonic plague)


Once a person develops pneumonic plague they can infect other people. When someone with pneumonic plague coughs organisms in the lungs are expelled into the air. If an uninfected person inhales enough organisms they will develop pneumonic plague in 1-3 days. If not treated early the death rate can be as high as 70%. If untreated the mortality rate is 100 percent.


While in the bloodstream the bacteria also stimulate the immune system resulting in massive clotting of the blood and fever. The blood clots lodge in small blood vessels causing damage to the personís organs, fingers, toes and nose. In time the person will run out of clotting factors and other responses will cause the fluid part of the blood to leak out of the blood vessels. When this happens the person will go into shock due to low blood pressure. Symptoms include shock, confusion, convulsions, bleeding from the nose and bowel, small red spots that enlarge bluish discoloration of the limbs, toes, fingers and ears, gangrene of the fingers, toes, ears and nose and death.   

Why is pneumonic plague an attractive BW?
Yersinia pestis used in an aerosol attack could cause many cases of pneumonic plague. Unlike anthrax spores the plague bacterium is easily destroyed by sunlight and drying. Even so, when released into air, the bacterium can survive for up to one hour, depending on conditions. One to six days after infection with the bacteria, people would develop pneumonic plague. Once people have the disease, the bacteria can spread to others who have close contact with them. Because of the delay between exposure to the bacteria and illness, people could travel over a large area before becoming contagious. Many others in distance locations could also be infected. Following all the victims and potential victims would become a very difficult task. A worst-case scenario calculated by the World Health Organization indicates that a 50-kg (about 110-lb) release of Yersinia pestis over a city of 5 million would infect 150,000 with 30,000 of those infected dying of the infection (49). 

Laboratory Diagnosis

Yersinia pestis grows quite well on most laboratory media. Blood samples, sputum samples and samples from the swollen lymph node can be placed on media to grow the organisms and then identify them. Growth and identification of the organism will take from 24 to 48 hours. The blood and lymph node samples can be smeared on glass slides and stained with special antibodies to speed the diagnosis.


Antibiotics can be used to treat plague and should begin as soon as possible after laboratory specimens are taken. Streptomycin is the antibiotic of choice. Gentamicin is used when streptomycin is not available. Tetracycline and chloramphenicol are also effective.  


Avoid contact with wild animals that are common carriers of the plague. Treat all mammalian pets to prevent flea infestations. People who have been bitten by an infected animal or flea oftentimes are given antibiotics (prophylaxis).  

Persons who have been in close contact with a plague patient, particularly a patient with plague pneumonia, should be identified and evaluated. In many cases these people will be given antibiotics to prevent the disease from occurring. The preferred antibiotics for prophylaxis against plague are the tetracycline or sulfonamides. 

A vaccine is currently being developed and could in the near future be available. It is likely that at first it will only be available for military personnel.


© 2005 Neal Chamberlain. All rights reserved. 
Site Last Revised 5/5/05
Neal Chamberlain, PhD. A. T. Still University of Health Sciences/Kirksville College of Osteopathic Medicine.

Site maintained by: Neal R. Chamberlain PhD.: