The Centers for Disease Control and Prevention (CDC) recently released their Morbidity and Mortality Weekly Report, which discussed a mini-outbreak of plague that occurred here in Colorado last year. Four people fell ill with the pneumonic form of the plague after coming in contact with a sick dog. The dog was euthanized before veterinarians could determine exactly what was going on (a full necropsy was not initially permitted), and it was only after one of the human patients was diagnosed with plague that follow-up testing revealed the cause of everyone’s illnesses.


The word “plague” conjures up pictures of the Middle Ages and epidemics that killed millions of people, but it is simply the name given to the disease that results from infection with Yersinia pestis bacteria. Plague can take three forms:


  • Bubonic — infection is localized in nearby lymph nodes after exposure occurs through the skin.
  • Pneumonic — infection involves the lungs resulting in pneumonia and can develop as a result of untreated bubonic plague or breathing in Yersinia pestis that has been coughed up by a sick person or animal.
  • Septicemic — infection of the blood that leads to shock and death if not treated quickly and aggressively.


In animals, plague is usually spread by fleas that feed on infected rodents, like prairie dogs (the most commonly affected species), rabbits, squirrels, mice, and rats. When an animal dies from the plague, the fleas leave the carcass to find another host, thus spreading the disease. Animals can also become sick after coming in contact with blood or tissues from another infected animal. This often happens through hunting or scavenging.


Treatment for plague is straightforward and includes appropriate antibiotic therapy, supportive care, and isolating patients until they are no longer contagious.


The CDC report on the cases in Colorado is notable for several reasons:


  • Symptomatic plague infections in dogs are quite rare. Unlike cats, dogs are naturally quite resistant to the plague. Feline behavior (hunting rodents) also puts them at greater risk for coming in contact with Yersinia pestis bacteria.
  • The possibility exists that one of the human cases resulted from direct human-to-human transmission. The last time this is thought to have happened in the United States was in 1924.
  • Dog-to-person transmission of the plague is even rarer. Only one other case like this has been reported.


I found the CDC’s timeline absolutely fascinating, particularly the fact that Patients B and C (both employees of the veterinary hospital that took care of the sick dog) put themselves on antibiotics without consulting their doctors. This practice is more common in the veterinary profession than we’ll ever admit to.



Now, plague is not a nationwide problem, but if you live or travel to the Western United States (especially the Four Corners region) you should take precautions, like these put forth by the Colorado Department of Public Health and Environment, to protect yourself and your pets: 


  • Do not directly handle any dead rodents, including prairie dogs, rabbits, squirrels, mice and rats.
  • Keep pets away from wildlife, especially dead rodents.
  • Don’t let dogs or cats hunt prairie dogs or other rodents.
  • Don’t allow pets to roam freely.
  • Treat pets for fleas according to a veterinarian's advice.
  • Do not feed prairie dogs or other rodents. This attracts them to your property, brings them in close contact with other rodents, and increases the risk of disease transmission.
  • Be aware of rodent populations in your area and report sudden die-offs or multiple dead animals to your local health department.



Dr. Jennifer Coates



Image: Patrick O'Connor / Shutterstock





CDC Confirms Human Plague Infection from Dog in Colorado


The Plague is Alive and Well in the American West


Cat Infects Colorado Man with Bubonic Plague


Plague in Dogs


Plague in Cats