State public health officials are conducting an environmental evaluation in the Stanislaus National Forest, Yosemite National Park and surrounding areas after a child contracted the plague during a July visit.
The child, who lives in Los Angeles County, is recovering but fell ill and was hospitalized after a family trip to the Northern California forest and camping at Crane Flat Campground in Yosemite National Park in mid-July, according to the California Department of Public Health. No other family members reported symptoms.
"Human cases of plague are rare, with the last reported human infection in California occurring in 2006," Public Health Director and State Health Officer Karen Smith said in a statement.
State officials are working with the Los Angeles County Department of Public Health, the Centers for Disease Control and Prevention and park and forest officials to determine the source of the child's infection.
Health officials are looking at the child's travel history and activities before becoming sick.
The Yosemite National Park will provide visitors with information about how to prevent plague exposure. Signs also will be posted at the Crane Flat campground and nearby campgrounds.
The infectious disease is carried by squirrels, chipmunks, other rodents and their fleas. After an infected rodent becomes sick and dies, its fleas can carry the disease to other animals and humans.
A person infected with the disease will experience high fever, chills, nausea, weakness and swollen lymph nodes in the neck, armpit or groin. If left untreated, plague can be fatal.
In California, the last reported cases of human plague occurred in Mono, Los Angeles and Kern counties in 2005 and 2006.
The three patients in those cases received treatment with antibiotics and survived. In California, there have been 42 human cases of plague since 1970. Nine were fatal.
Plague-infected animals usually are found in the foothills, mountains and sometimes along the coast of California.
In 2014, plague activity was detected in animals in El Dorado, Mariposa, Modoc, Plumas, San Diego, Santa Barbara and Sierra counties.
On Tuesday, a Colorado resident died of the plague after contracting the disease from a rodent, fleas or dead animal in a rural area of southwestern Pueblo County.
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