State health officials say the plague has been detected in animals in six Colorado counties, including in ten cats that may have been infected through hunting and eating infected rodents.
John Pape is an epidemiologist with the state health department.
He says cats present a concern because pets that become severely ill could transmit the disease directly to their owners.
Dogs and cats also could bring infected fleas into the home.
Counties that have detected the plague include Archuleta, Larimer, La Plata, Mesa, Montezuma and San Miguel.
Plague was detected in animals throughout the state last year and in three humans.
Since being first documented in Colorado in 1957, nine people have died from the plague.