(Adds details of health advisory, quote; changes byline;previous UNDATED)
By Dan Whitcomb
LOS ANGELES, July 25 (Reuters) - Authorities have evacuatedand shut down a section of a national forest outside Los Angelesfor at least a week after a ground squirrel was found thereinfected with the plague, county public health officials said onThursday.
The squirrel tested positive for plague after it was trappedin the Angeles National Forest during "routine surveillanceactivities," the Los Angeles County Department of Public Healthsaid in a written health advisory.
Health officials said that as a precaution, visitors wereordered to leave three campgrounds and a recreation area of theAngeles National Forest, which encompasses some 655,000 acres inthe San Gabriel Mountains just north of metropolitan LosAngeles.
Plague, known as the "Black Death" when it was blamed forkilling some 25 million Europeans during the Middle Ages, is abacterial infection that can be transmitted to humans throughthe bites of infected fleas.
A health department spokesman said no people were believedto have been infected.
"It is important for the public to know that there have onlybeen four cases of human plague in Los Angeles County residentssince 1984, none of which were fatal," the health departmentchief, Dr. Jonathan Fielding, said in a statement.
An average of seven cases of plague are reported each yearin the United States, according to the U.S. Centers for DiseaseControl and Prevention, mostly in western states. The disease istypically not fatal if treated with antibiotics.
Officials said further testing of squirrels would beconducted in the Angeles National Forest before the BrokenBlade, Twisted Arrow and Pima Loops campgrounds were re-openedto the public.
According to the health department, plague has been known toreside in the San Gabriel Mountains ground squirrel population.
Previous surveillance efforts have identified five othersquirrels carrying the disease since 1996.
The most recent squirrel found to be infected was trapped onJuly 16 during routine checks in the park and confirmed to carrythe disease on July 23, according to the health advisory.
Last year, an Oregon man survived a bout with plague, one ofthree reported in that state since the mid-1990s, none of whichwere fatal. (Additional reporting by Karen Brooks; Editing by Steve Gormanand Leslie Adler)