Three ground squirrels found at Palomar Mountain campgrounds have tested positive for plague, a deadly bacterial infection that wiped out millions of people in the Middle Ages, San Diego County officials said Wednesday.
Plague infections are now rare among humans, but at least a few squirrels in higher elevations around Palomar Mountain or Julian are found during routine testing each year, said Chris Conlan, supervising vector ecologist for the San Diego County Vector Control program.
The county posts plague warning signs in areas where the disease is found, officials said.
There has never been a recorded human plague infection in San Diego County, although there have been several in California, officials said. Fewer than 100 cases were reported over the last decade in the U.S., mostly in California, Arizona, New Mexico and Colorado, according to the Centers for Disease Control and Prevention.
Two of the infected squirrels found recently were in the Cedar Grove Campground, according to a county statement. The third was trapped at the Doane Campground.
Earlier this year, officials found another plague-infected squirrel at the Cedar Grove Campground.
The squirrels had relatively few fleas and presented a low risk for passing the bacteria to humans, the statement said.
Humans can contract the plague from direct contact with rodents. However, it's more often the case that fleas pass the disease to humans after feeding on the blood of an infected rodent, according to the National Institutes of Health.
Once contracted, the plague most often takes one of three forms: bubonic plague, which infects the lymph nodes and is the most common; pneumonic plague, which infects the lungs and is the deadliest form of plague; and septicemic plague, which attacks the blood.
Symptoms typically appear within a few days of exposure, according to the National Institutes of Health. Treatment within 24 hours of the first symptoms lowers the possibility of death.
Bubonic plague symptoms may include chills, fever, a general ill feeling, headache, muscle pain, seizures and painful lymph gland swelling in the groin, armpits or neck.
Pneumonic plague symptoms might include coughing, difficulty breathing, fever, coughing up blood and severe chest pain when breathing deeply.
Septicemic plague could cause death before symptoms occur. Symptoms can include abdominal pain, excessive bleeding, diarrhea, fever, nausea and vomiting.
County officials said those in rural mountain areas should avoid contact with wildlife ---- including sick or dead animals ---- and stay away from animal burrows, especially while sleeping or resting. Pets should be kept on a leash or left at home, officials said.
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