What is plague?

Plague is a highly infectious bacterial disease that can cause serious symptoms and is often fatal if left untreated. There are three forms of the disease:

  • Bubonic plague, in which lymph nodes close to the site of infection become swollen and painful
  • Septicemia plague, in which the bacteria spreads throughout the body in the bloodstream
  • Pneumonic plague, in which the lungs become infected. Pneumonic plague can be transmitted directly from one patient to another through coughing.

Plague ravaged Europe during the 6th century (the plague of Justinian) and again in the 14th century (the Black Death). In California, plague exists among wild mice in specific “foci” or localities. The bacteria is transmitted between animals by certain types of fleas. It does not cause any serious illness in mice. However, ground squirrels and domestic rats are rapidly killed by the disease and leave behind hordes of hungry, infected fleas. Humans become infected when they are exposed to these fleas. Humans can also be infected through their pets. Cats, in particular, tend to develop pneumonia when infected and can spread the disease to their owners or veterinarians through sneezing or coughing.

What are the symptoms of plague?

At the present time, plague in humans is relatively rare and can be treated successfully with modern antibiotics. However, it is vital that the disease be recognized and treated in its early stages. If not, it is often fatal and, if lung infection (plague pneumonia) develops, it can be transmitted directly and rapidly to others.

Is there plague in San Mateo County?

Yes! Plague occurs in the wildlife population in San Mateo County. The San Francisco Peninsula is noteworthy as the location of the first and largest outbreak of urban plague in the United States. In 1900, there was an epidemic of plague among San Francisco residents which took 118 lives and lasted for four years. A second plague epidemic occurred in 1907, following the 1906 earthquake. Plague was detected among mice on San Bruno Mountain in the 1940s and is still present among wild rodents at there and other localities in San Mateo County. The presence of plague in these areas does not present an extreme risk to humans unless it moves into domestic rats or ground squirrels.

See more information about ground squirrels.

In conjunction with the San Mateo County Health Department, the District regularly conducts surveys for plague and other rodent-borne diseases. When plague is detected, information is distributed to inform visitors of the area.

How can I get plague?

  1. Bites of fleas from infected rodents. Hungry fleas will leave a sick or dead rodent to find another host and can bite people.
  2. Direct contact with sick animals. Bacteria from an infected animal can be transmitted if blood or other body fluids come into contact with cuts on an individuals skin or a mucous membrane, such as the eyes or mouth.
  3. Pet involvement. (1) Infected rodent fleas brought into the home or campsite by a dog or cat. (2) Plague pneumonia can be caught from a sick cat that is coughing or sneezing.

What is the risk of becoming infected with plague?

In California, plague occurs in the foothills, plateaus, mountains, and foggy coastal belt. Plague is absent from the southeastern desert region of the state and the Central Valley. Human cases in California occur in semi-rural areas when people or their pets are exposed to rodent fleas following an epidemic among ground squirrels or chipmunks. In recent years, several people have been exposed to plague through domestic cats that developed plague pneumonia.

Plague is most often associated with wild rodents; however, it may be transferred to rats in urban areas, increasing the risk of plague transmission to humans. Therefore, the overlap of wild and domestic rodents in areas such as San Bruno Mountain has always been a concern to public health authorities.

Due to the potential danger of pneumonic plague epidemics, public health authorities place a high priority on prevention of human plague cases. You can minimize your exposure to plague by carefully following the precautions listed here:

  • Avoid all contact with wild rodents and their fleas
  • Report dead squirrels to the District
  • Use caution when handling a sick pet, especially cats. Avoid face-to-face contact
  • Consult a veterinarian if your cat is sneezing or coughing
  • Keep rodent populations down around homes. Prevent rodents from entering buildings.
  • Minimize pet contact with rodents and their fleas. Protect pets with flea powder.