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
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.
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?
Bites of fleas from infected
rodents. Hungry fleas will leave a sick or dead
rodent to find another host and can bite people.
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.
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
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
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.