The plague has been identified in three additional Colorado residents by the Colorado Department of Public Health and Environment, bringing the total number to four cases. The initial investigation of the original case identified the three individuals, all of whom had direct contact with a dog that was killed by the plague.

In each case, the patients had mild symptoms and were treated with the correct antibiotics, allowing them to recover and to no longer be contagious. The initial infected patient remains in the hospital.

It is believed that the infected dog was exposed to a prairie dog or rabbit with plague-infected fleas in Adams County.

Person-to-person transmission of the illness is very rare, although those who may have been exposed to the four cases through close contact have been identified and provided with antibiotic treatment or are being closely monitored for symptoms.

According to the Colorado Department of Public Health & Environment, Colorado has identified 60 cases of human plague, of which nine, or 15 percent, were fatal.

The plague is typically spread via fleas from rodents, usually prairie dogs. Staff members from the Tri-County Health Department have gone door-to-door around the area with information about the plague and in an effort to assess prairie dog populations.