Carol McInnes, an environmental health specialist with the Boulder County Health Department, said she received a report Friday of a possible die-off at the colony via city open space rangers.
Some trail users saw dead prairie dogs on the trail last week.
The fleas McInnes collected Friday went by courier to a state lab for testing for signs of bubonic plague Wednesday morning. The results should be available later this week or early next week, she said.
Plague occurs naturally in Colorado and is an infectious disease spread by fleas to wild rodents and other small mammals, such as squirrels, rats, prairie dogs and rabbits.
Symptoms of plague infection include high fever, extreme fatigue and painful swollen lymph nodes. Anyone who notices these symptoms on themselves or on their pet should call their doctor or veterinarian immediately. Plague can be treated with antibiotics, but this treatment is most successful when the disease can be diagnosed quickly.
People are advised to stay on the trail and keep their dogs and cats out of the colony.
Household pets can spread infected fleas to humans, and cats can get sick themselves with plague. Pets that go outside should also have flea collars.
People should never touch sick or dead animals with their bare hands, health department officials said.