Plague, caused by a bacterium called Yersinia pestis, is transmitted from rodent to rodent by infected fleas. 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 risk to humans and others animals frequenting the area.

For the Pacific states, the California ground squirrel and its fleas are the most common source. Many other rodent species, for instance, wood rats, chipmunks, and other squirrels, suffer plague outbreaks and their fleas occasionally serve as sources of human infection. Domestic cats (and sometimes dogs) are readily infected from 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.

In the United States during the 1980s plague cases averaged about 18 per year. Most of the cases occurred in persons under 20 years of age. About 1 in 7 persons with plague died.

Plague is transmitted from animal to animal and from animal to human by the bites of infected 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 an animal.  Transmission of plague from person to person is uncommon and has not been observed in the United States since 1924.

Pneumonic plague is transmitted by inhaling infected droplets expelled by coughing, by a person or animal, especially domestic cats. 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 pneumonia plague patients, the death rate is over 50%.

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.

In regions such 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. Prevention measures include: 

    • Eliminating food and shelter for rodents in and around homes, work places, and recreation areas by 
      making buildings rodent-proof, and by removing brush, rock piles, junk and food sources (such as pet
      food) from properties. 

    • Use of appropriate and licensed insecticides to kill fleas during wild animal plague outbreaks to reduce 
      the risk to humans. 

    • Treatments for pets (dogs and cats) for flea control once each month.


More Useful Information and Links Regarding Plague:

Centers for Disease Control and Prevention:
http://www.cdc.gov/plague/