Dog Found to be Source of Colorado Plague Outbreak

May 11, 2015 | Marie Killerby and Emily Cohn | Outbreak News

Recently, the Centers for Disease Control (CDC) reported that in July 2014 a Colorado man contracted plague from his dog [1]. Furthermore, three other persons who had come into contact with the dog, and one of whom also had contact with the owner, were confirmed positive for plague [1]. All four human cases were treated with antibiotics and made a full recovery [1]. As one individual had contact with both the owner and the dog, it is possible that human-to-human transmission of the plague could have occurred, but this has not yet been confirmed [1]. After this cluster of human infections the CDC highlighted the need to consider plague as an infection of domestic animals in endemic areas [1].


What is plague?

Plague is a zoonotic disease cause by the bacterium Yersinia pestis. It is the same bacterium responsible for the “Black Death” in the 14th century where it was claimed to have killed 60% of the European population [2]. The bacterium predominantly exists in rodents, and is transmitted by rodent fleas (or contact with infected tissues) from one rodent to another [3]. In certain rodent species plague continuously cycles between rodents without causing many deaths, enabling the bacterium to exist continuously in the population, known as the enzootic cycle [3]. Other species can catch plague but they tend to die out - for example in rabbits and susceptible rodents, mortality rates can approach 100% [4]. Humans can become infected from animals via bites from rodent fleas, contact with infected tissue, or inhalation of infectious droplets [3]. Humans can also transmit plague between one another if they develop the pneumonic form of the disease [5].


Where does plague exist?

Plague exists across many regions of the world, but most human cases within the last 25 years have occurred in Africa [6]. Within the United States, most plague causes occur in northern New Mexico, northern Arizona, southern Colorado, California, southern Oregon and western Nevada [6]. A variety of different wildlife reservoirs exist across the United States, including prairie dogs, ground squirrels, antelope ground squirrels, chipmunks, woodrats and mice. [4]. The principle vectors for plague include prairie dog fleas, ground squirrel fleas and various species of woodrat and mouse fleas. [4]. Despite the abundance of potential reservoir hosts, human cases in the United States are rare; In recent years there has been on average only seven cases of plague per year in humans [4].


What are the symptoms?

 In humans, three main forms of plague infection occur: Bubonic plague, which is characterised by fever and swollen lymph nodes, septicemic plague, characterised by fever, chills, abdominal pain, shock, and possibly bleeding, and pneumonic plague with fever, weakness and pneumonia (inflammation of the lung tissues) [5]. Pneumonic plague is the only form transmissible from person to person, this occurs via coughing and sneezing of infected droplets [5]. Over 80% of United States plague cases have been the bubonic form, which makes transmission less likely [6]. Plague is treatable with commonly available antibiotics [7], however deaths still occur across the world [6].


In dogs, plague usually causes a mild illness with self-limiting fever and swollen lymph nodes, with dogs often becoming infected via eating infected rodents [8]. The canine case in this report showed an unusually severe form of the disease with fever, difficulty in breathing and coughing up blood, before being humanely euthanized [1].


Are you or your pets at risk?

Although dogs can catch and transmit plague, they are less likely to do so than cats, which are highly susceptible to the disease [8]. It is thought that whilst some cats may become infected through flea bites, the more common route is through ingestion of infected rodents [8]. Cats show more severe clinical signs than dogs and can develop bubonic, septicemic and pneumonic forms [8].  Cats with pneumonic plague pose the highest risk to humans as they can transmit pneumonic plague via coughing and sneezing of infectious droplets [8]. From 1977 to 2006, 29 people in the United States were diagnosed with cat-associated plague and at least five of these patients developed pneumonic plague [8]. Overall plague is a rarely contracted by people in the United States but it is worth remembering that both cats and dogs can potentially transmit infection via bites, scratches, owner contact with infected fluids, or pets transporting infected fleas into the house [9].



Although contracting plague from your pet is very unlikely, there are steps that can be taken to minimize the risk. In order to prevent your dog or cat becoming infected from fleas, the CDC recommends that If you live in areas where plague occurs, treat pet dogs and cats for flea control regularly and do not allow these animals to roam freely [9]. The CDC also recommends performing rodent control measures, taking any ill animals (especially cats) to a veterinarian, using repellent if you think you will exposed to rodent fleas when outdoors in an endemic areas, and avoiding contact with sick or dead animals [9, 10]. Finally, it is important that veterinarians and physicians in endemic areas are aware of both cats and dogs as a potential source of infection.


  8. Abbott, R.C., and Rocke, T.E., 2012, Plague: U.S. Geological Survey Circular 1372, 79 p., plus appendix, available at
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