Plague in California

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California Department of Public Health, Vector-Borne Disease Section

What is Plague?

Plague is an infectious disease caused by the bacteria Yersinia pestis.

In California, plague most often affects wild rodents, such as ground squirrels and chipmunks. However, humans, cats, and other animals can get plague if they live in or go to areas where wild rodents are naturally infected with plague. In California, the most common way plague is transmitted to humans is through the bite of infected fleas that usually live on wild rodents or in rodent burrows in the ground.

Plague was first introduced in the United States in the late 1800s and early 1900s, causing outbreaks in port cities such as San Francisco and Los Angeles. Today, plague is endemic (naturally occurring) in the rural western United States, including parts of California. Plague in humans is rare and can be treated effectively with antibiotics if diagnosed early.

Where is Plague Found?

The map on the left shows areas in California where plague is endemic, or naturally occurring, in recent decades. These are areas where there may also be an increased risk of plague transmission to humans.

These endemic areas are estimated, based on the locations of plague-positive samples in the California Department of Public Health database collected from 1983 to present.

Plague is currently found in many foothill and mountainous areas of California but is not found in the Central Valley and southeastern desert regions of the state.

Scroll down to learn about the history of plague in California and how plague affects California's wild rodent populations.

Monitoring Plague Activity

To help reduce the risk of human plague cases, the California Department of Public Health, Vector-Borne Disease Section works with state, federal, and local agencies to detect increases in plague activity in wild rodents across California.

Prevention

The best way to reduce your chances of getting plague is to avoid contact with wild rodents and their fleas. DO NOT touch, feed, or handle any rodents.

Plague warning signs posted at a recreation area with known plague activity.

Other ways to help prevent plague:

  • Be aware of areas where plague-infected rodents might be present (look for signs or postings in campgrounds that mention plague). 
  • Follow instructions on plague notices that are posted at camping and recreation areas.
  • Store food and garbage in rodent-proof containers.
  • Do not camp, rest, or sleep near rodent burrows.
  • Wear long pants tucked into boots and use insect repellent to keep fleas from biting you.
  • Do not allow pets to approach sick or dead rodents, or to explore rodent burrows, because this can expose them to fleas. 
  • While camping or exploring areas where plague has been reported, leave pets at home if possible; otherwise, keep pets on a leash.
  • Protect pets with flea control products (for more information, talk to your veterinarian).

Be careful when handling sick cats that may have had contact with wild rodents in plague areas. If your cat is sick, take it to the veterinarian for examination.

If you become sick within seven days after being in a known area with plague, talk to a physician or doctor immediately and tell them if you have been exposed to fleas or wild rodents.

Additional Information

Plague warning signs posted at a recreation area with known plague activity.