SALT LAKE CITY -- A campground at Natural Bridges National Monument has been closed because of bubonic plague detected among field mice and chipmunks.

Plague also has been found this spring in rodent populations at Mesa Verde National Park and Colorado National Monument.

National Park Service officials said there never has been a reported human case of bubonic plague originating from the parks or national monuments.

"We come down on the conservative side when it comes to closing campgrounds," said Joe Winkelmaier of the U.S. Public Health Service. "We just like to be sure when it comes to plague."

Several weeks ago, park rangers noticed a large number of dead field mice at Natural Bridges, about 40 miles west of Blanding. Chief Ranger Ralph Jones showed that tests indicated they died from the plague.

Rangers plan to insecticides to kill fleas in the campground area. Humans usually contract bubonic plague after being bitten by fleas that have bitten infected rodents. The campground could be reopened as soon as next week.

Plague occurs throughout the West, but is concentrated in the Four Corners area of Utah, Colorado, New Mexico and Arizona. An average of 18 cases involving humans are reported each year in the United States, according to the federal Centers for Disease Control and Prevention. About one in seven victims die.