Plague from dog

Plague… it’s a term that typically conjures up images of the devastating "Black Death", the pandemic that killed 75-200 million people in Europe back in the 14th century. Yet, it’s not just a historical disease. Plague is still present in a variety of small mammals in different regions worldwide (see map), including parts of the US, with periodic reports in Canada.

A recent case of plague in a Colorado man has attracted a lot of attention.  The individual developed the pneumonic form of the infection after his dog died of the same disease. It’s suspected that he was infected from a flea that fed on the infected dog, and then bit the man. However, I don’t think you can really rule out the potential for direct transmission of the bacterium, Yersina pestis, from the dog. Fortunately, despite developing pneumonic plague (the form in which the bacterium infects the lungs, and the deadliest form of Y. pestis infection), it seems that he’s recovering.  Plague is treatable with antibiotics, but it is critical that treatment be started as soon as possible or it can be fatal.

Transmission of plague from pets to people isn’t new. However, most often it involves cats that get infected while hunting rodents carrying infected fleas. Cats can develop plague, and then people caring for them (e.g. owners, veterinarians) can acquire the infection.

This case highlights a few important points:

  • Plague is still around. People living in areas where plague is present need to be aware of the risk, even though it's very low.
  • Pets get infected from contact with infected rodents, either directly or from their fleas. Keeping pets away from wildlife (e.g. keeping cats indoors, limiting free-roaming of dogs) can reduce the risk of exposure.
  • Sometimes, knowing the cause of an animal’s illness is very important for human health. Knowing that a pet had plague would greatly speed up consideration of plague in anyone who became sick and had contact with the animal.
  • Flea control can help reduce the risk of many diseases, including plague.

(Click image for source)

Plague from pet cat

Plague cases tend to get a lot of press. The fact that this disease killed a large percentage of the human population in a few different pandemics (albeit centuries ago for the most part) probably plays a role in that. Despite the impression by some that it's just a historical disease, plague is alive and well in certain parts of the world, including parts of the US, and infects a few thousand people every year.

Plague is caused by the bacterium Yersinia pestis, which lives in various wild rodents and is circulated by fleas. Transmission to people historically has been via fleas that jump from rats to people. However, plague isn't just a rat-human disease, as it can infect other animal species. Among domestic animals, cats seem to be most commonly infected, probably because of exposure while hunting.

The problem with plague and pets has been shown once again a case of plague in an Oregon man who likely picked up the infection from his cat. (Oregon is outside of the main range of plague in the US, where the disease is most common in the southwest). The man was bitten by the cat while retrieving a dried, decayed mouse carcass from its mouth. He developed septicemic plague (infection of the bloodstream), and then pneumonic plague (infection of the lungs), which is the worst case scenario. At last report, he was in critical condition and the prognosis for survival is probably guarded.

There's no mention of the cat's health. Most cases of cat-human plague occur in people taking care of sick cats (especially veterinarians). If a person is infected by a cat bite, I would expect the cat to have been sick with plague, although transmission has been reported from apparently healthy cats. Some other possible routes may need to be considered. If the cat in this case was exposed to plague, then plague's obviously in wildlife in the area, so you have to consider that the infected man might have been bitten by an infected flea (that came directly from an infected wild animal or that the cat tracked in) or from direct contact with wildlife, especially if his house had a rodent infestation.

Regardless, it's important for people in plague-endemic (and neighbouring) areas to be aware of plague and take measures to reduce the risk of exposure for themselves and their pets, such as:

  • Avoid contact with wild rodents (and wildlife in general, since larger wildlife species can also be infected).
  • Keep cats inside.
  • Don't let pets with outdoor access roam unobserved, where they might be more likely to encounter wildlife.
  • Have a flea control program for pets.
  • Address any animal/household flea infestations promptly and aggressively.
  • Make sure sick pets get prompt and appropriate medical attention, since diagnosing plague in a pet may be a critical factor in prompt treatment of people infected by the pet. Plague is an example of a disease for which diagnosing infection in the pet might save the owner's life.

Plague in New Mexico cat and dog

Plague has been identified in a dog and cat from New Mexico. It’s not surprising, since plague is present in some wild animal populations in that region, but it’s still noteworthy because of the serious nature of the disease and the potential for transmission to humans.

Plague is a bacterial infection caused by Yersinia pestis. It’s carried mostly be certain types of rodents in different regions of the world, including parts of the southwestern US. It’s usually spread by fleas that bite an infected rodent and then bite a person or other animal, but it can also be spread by close contact with an infected animal. Cases in cats and dogs are uncommon, but occur in areas where plague is present in rodents, when pets have contact with infected fleas or close encounters with infected rodents (or rodent carcasses).


The latest two cases were in Santa Fe and Rio Arriba Counties in New Mexico. No details were provided about the form of plague (e.g. bubonic, pneumonic), the suspected source of infection or whether there is concern about human exposure. Finding plague in a pet is a concern for a few reasons. It indicates that plague is present in wildlife in the area, and people could be exposed from the same sources as the pets (i.e. fleas, contact with live or dead wildlife). Also, transmission of plague from pets to their caretakers can occur, particularly from cats with pneumonic plague (respiratory tract infection). Knowing that a person has had contact with a pet with plague is critical to making a prompt diagnosis. According to the World Health Organization, plague continues to infect more than 2000 people every year.

The New Mexico Department of Health has made the following recommendations:

  • Avoid sick or dead rodents and rabbits, and their nests and burrows.
  • Keep your pets from roaming and hunting and talk to your veterinarian about using an appropriate flea control product.
  • Clean up areas near the house where rodents could live, such as woodpiles, brush piles, junk and abandoned vehicles.
  • Sick pets should be examined promptly by a veterinarian.
  • See your doctor about any unexplained illness involving a sudden and severe fever.
  • Put hay, wood, and compost piles as far as possible from your home.
  • Don’t leave your pet’s food and water where mice can get to it.
  • Veterinarians and their staff are at higher risk and should take precautions when seeing suspect animal plague cases.

 Photo: The vector of Yersina pestis: a flea (click image for source)

The dog in your bed probably won't kill you

A month or two ago, there was a lot of press about the risks of pets sleeping in beds. It was in response to an article in the journal Emerging Infectious Diseases that didn't put forth any new information, but summarized a few diseases that could potentially be transmitted by pets. Unfortunately, the relative risk of those diseases wasn't really explored, and some media reports latched onto diseases like the plague, transmission of which can occur between pets and humans but the likelihood of this in most areas is essentially nil.

Anyway, an article at has taken a more balanced approach towards the subject. One sentence perhaps say it best: "Many of those scare headlines, however, missed the main point of Chomel’s work: For most people, the risks are minimal, and there are easy ways to go about preventing pet-to-owner disease sharing."

I won't go into details here, since you can read the article yourself, but a key component is that pet ownership is never no-risk, but is usually low-risk. Basic hygiene practices and common sense can reduce the risks further. The cost-benefit needs to be considered, and while we can never completely eliminate the "cost" aspect, the benefits of pet ownership certainly outweigh the costs in the vast majority of households.

Plague in Oregon

The CDC's Morbidity and Mortality Weekly Reports has a short report about two plague cases in the US. Plague, while often thought of as a historical disease (the Black Death), is alive and well in wild rodents in some areas of the world, including parts of North America, and human cases continue to occur.

Here are highlights of the CDC report (in italics) with some extra comments.

Plague, caused by Yersinia pestis, is enzootic (present in the population, typically at a low level) among rodents in the western United States. Humans can be infected through 1) the bite of an infected flea carried by a rodent or, rarely, other animals, 2) direct contact with contaminated tissues, or 3) in rare cases, inhalation of respiratory secretions from infected persons or animals. In September 2010, the Oregon Health Authority reported the first two cases of human plague in Oregon since 1995 and the only two U.S. cases in 2010.

Both illnesses began on August 21. The patients, aged 17 and 42 years, lived in the same household and might have been exposed to plague by infected fleas from one of their dogs; that dog was found to be seropositive for Y. pestis by the passive hemagglutination-inhibition assay (dilution of 1:64). One patient acknowledged sleeping in the same bed with the dog during the 2 weeks before illness onset. Both patients had high fever and multiple bilateral inguinal buboes; one patient had hypotension, tachycardia, and acute renal failure and was hospitalized. A gram-negative rod with bipolar staining was isolated from a specimen of that patient's blood.

...25 days after specimen collection, the isolate was identified as Y. pestis... Both patients recovered uneventfully after empiric therapy with doxycycline and amoxicillin clavulanate potassium, respectively, although the latter is not considered effective in treating plague.

Plague is a Category A potential bioterrorism agent. Human infections are rare but can be life-threatening. The plague case-fatality rate depends on the clinical presentation (i.e., bubonic, septicemic, or pneumonic) and timing of antibiotic therapy initiation; if untreated, the case-fatality rate is >50% for bubonic plague and approaches 100% for pneumonic plague. Rapid laboratory identification can help guide therapy.

Sleeping in the same bed with dogs has been associated with plague in enzootic areas.  Plague patients with no history of exposure to rodents can be infected by Y. pestis if their pets carry infected rodent fleas into the home. Veterinarians always should recommend flea control to dog and cat owners.

This is an example of a situation where pets can play a role in human infection while not being the direct source of infection. While direct pet-human transmission can occur, this typically involves situations where someone has close contact with a pet that is sick with the plague. Most often, this kind of transmission is associated with close contact with cats with pneumonic (respiratory) plague.

Key aspects of reducing the risk of pet-associated plague in areas where plague is, or may be, present, are:

  • Preventing contact of pets with wildlife, living or dead.
  • Preventing roaming of pets in the wild.
  • Discouraging wildlife from living in or around homes.
  • Keeping cats indoors.
  • Routine flea control.

More information on plague and pets is available in our archives.

Prairie dog flea control

In response to a case of plague in prairie dogs in Saskatchewan's Grasslands National Park, park officials are dusting prairie dog burrows with insecticide to try to control fleas. A single case of plague, a serious bacterial infection caused by Yersinia pestis, was identified in a prairie dog in the park earlier this summer. Plague is present in some areas of North America, but it's rare in Saskatchewan. It circulates in wild small mammals, and prairie dogs are particularly susceptible to infection. The main mode of transmission is via fleas, which feed off infected animals, then bite and infect other animals.

Prairie dog numbers at the park have dropped by 50-70% this summer, however it's not known whether plague is involved in this, as there has also been a drought. It's fair to assume, though, that if there has been one case of plague found in the park, there have probably been many other undiagnosed cases. Whether or not plague is responsible for the large drop in prairie dog numbers, measures to try to reduce plague transmission are a good idea because of the impact it can have on the prairie dog population (and those of other wild mammals), as well as people or pets that may venture into the area. Anyone or anything walking through the area could plausibly be bitten by an infected flea. The odds are probably pretty low, but park officials are trying to keep people and pets out of the park to reduce this risk.

Plague in Saskatchewan

Plague has been identified in a dead prairie dog in Grasslands National Park in Saskatchewan, Canada. This disease, caused by the bacterium Yersinia pestis, circulates in small wild mammals in some regions, and is spread by fleas. While plague is endemic in some parts of North America, it's very rare in Canada. (The last reported human case was in 1939).

Prairie dogs are highly susceptible to plague.  The most likely problem with plague occurring in prairie dogs in Grasslands National Park will be the impact on the prairie dog population itself. Outbreaks of plague in prairie dogs can be devastating, virtually wiping out entire colonies.

  • Plague can be transmitted to people, but the risk here is quite low. For people to become infected, they have to be bitten by a flea that was infected by biting an infected prairie dog (or other animal). The risk of exposure to a flea infected by wildlife is pretty low if people aren't crawling around prairie dog holes and take some basic precautions.
  • The risk to pets is similarly low. Cats are more susceptible to plague than dogs, but they have to be exposed via a flea or, more commonly, from hunting and eating infected wildlife. There presumably aren't too many pet cats in Grasslands National Park, so the risk of exposure is probably limited. Dogs are rather resistant to plague, but they are probably at increased risk of exposure in a situation like this because they are more likely to be taken into areas where infected animals and fleas may be present (e.g. with people going hiking).

The greatest public health and domestic pet concern would be if plague spread beyond prairie dogs and into other small mammal populations that live closer to people or that have more contact with pets. The likelihood of this becoming a major problem is pretty low, but it's a serious disease and this situation certainly needs to be monitored.

In response to this case, park personnel are monitoring prairie dog colonies to look for more cases. They are presumably also keeping a close eye out for any other unexpected deaths of small mammals. Park officials have recommended that people stay away from prairie dog colonies, tuck their pants into their socks (to keep out fleas) and use insect repellent on their shoes. They have also closed some areas to domestic pets.

Photo: Black-tailed Prairie Dogs atop their burrows in Grasslands National Park in Saskatchewan, Canada (source:

Plague in Montana cat

A cat from Ennis, Montana, has been identified as the area's first case of plague in a pet cat this summer. Plague is a disease that conjures up images of medieval pandemics and calls of "bring out your dead" (along with calls of "I'm not dead yet" from Monty Python fans).This bacterial disease, caused by Yersinia pestis, is not just of historical interest, however. It is still an important disease in some regions. In the US, it is present in wildlife in parts of the southwestern US and circulates in wild rodents. Domestic pets mainly become infected through eating infected rodents, but the bacterium can also be spread by fleas.

Cats are quite susceptible to plague, and can develop classical bubonic plague, septicemic plague or pneumonic plague, and only about 33% of infected cats survive. Dogs are relatively resistant to the disease.

Transmission of plague from pets to people is uncommon but most often involves cats. Veterinary personnel and pet owners that care for sick cats are at highest risk. While transmission from cats to people is rare, about 20% of people infected from cats die, so it certainly warrants some precautions.

In areas where plague is present, cat owners should consider the following:

  • Keep cats indoors to prevent them from hunting and eating rodents.
  • Keep rodents and other wildlife out of the house.
  • Have a flea control program in place to prevent or treat flea infestations in pets.
  • Never catch and keep wildlife (e.g. prairie dogs) as pets.
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Plague in a Colorado cat

A house cat in the Eagle, Colorado area has been diagnosed with pneumonic plague. Plague, caused by the bacterium Yersinia pestis, is a disease that still strikes fear into people. While we are long since removed from the period where the "black death" killed a large percentage of the population in Europe, plague still has a foothold is some regions of the world like the southwestern US. It is present in some wildlife (mainly rodents) and periodically infects people or domestic animals through transmission by infected fleas or direct contact.

Plague is periodically identified in cats - it's almost always outdoor cats that are affected since they have more interaction with wildlife and are at greater risk of flea infestation. Several forms of the disease can occur, including pneumonic, septicemic and bubonic plague. Pneumonic plague is a severe lung infection caused by the plague bacterium which is highly fatal.  This form is of particular concern because infected cats can spread the infection to people through aerosols produced by coughing and sneezing, or through contact with respiratory secretions. People caring for sick cats are at risk of developing plague (especially pneumonic plague, which is almost invariably fatal if untreated). Veterinary personnel are at particularly high risk. One study reported that 20% of people who contracted plague from cats worked in vet clinics. Of these, 25% of them died. 

If you live in an area where plague is present in wildlife, keep your cat indoors, avoid contact with wild rodents, keep wild rodents out of your house and make sure that you have a flea prevention program for you pets. If these things are done, the risk of disease transmission is very low.

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Plague in a rabbit: New Mexico

Plague has been diagnosed in a dead rabbit found on a private residence in New Mexico. Plague, also known as the black death, is a highly fatal disease of humans and many animals caused by the bacterium Yersina pestis. While often considered a disease of mainly historical interest - having killed a large percentage of people on the planet during a few pandemics over the centuries - plague is actually still alive and well in some regions. In North America, most cases occur in New Mexico, Arizona, Colorado and California.

Yersinia pestis primarily lives in wild rodents and is transmitted by fleas. Many different animal species, including cats, dogs, rabbits and people, can be infected if bitten by a flea from an infected rodent (hence the historical association of the disease with rats). Predatory species (like dogs and cats) can also become infected by eating infected animals. Dogs are relatively resistant to plague and usually only develop mild disease, while cats and rabbits are as susceptible as people, and can develop bubonic, septicemic or pneumonic plague. Transmission of plague from pets to people can occur, and most often involves cats. People can become infected by close contact with sick pets, or being bitten by a flea from such a pet.

Preventing plague in animals involves flea control and reducing exposure to infected wildlife. In areas where plague is active, all pets should be on a flea control program. Cats should be kept indoors to reduce the risk of exposure (e.g. keeps them from hunting infected rodents). Dogs and cats should not be allowed to have contact with dead animals of any kind. Measures to reduce rodent infestations in and around the house are also important.

More information on plague is available in the Worms & Germs archives.

Plague isn't back... It never went away

Plague (aka the "black death") is a fascinating disease. It is one of the most important diseases in human history because it had a devastating impact of the human population during various outbreaks. Many people may not realize it, but plague is not just a historical problem - it is still alive and well in some areas of the world. Plague is caused by the bacterium Yersinia pestis, which tends to circulate in rodent populations and can be spread by fleas. In North America, plague is most common in some regions of the southwestern US, particularly New Mexico, Arizona and Colorado. We're heading into the high-risk season for plague in those areas: March to October.

Plague can infect domestic pets, and pets can be a source of human infection.  Cats are quite susceptible to plague, whereas dogs are quite resistant.  Cats can transmit plague to people.  Pneumonic plague (infection of the lungs with Y. pestis, not to be confused with bubonic plague which is primarily infection of the lymph nodes with Y. pestis (see picture left)) in cats is of particular concern, because in this form the bacterium can be spread through the air over short distances.

Prairie dogs (which some people keep as pets) are also very susceptible to plague.

A paper in Clinical Infectious Diseases a few years ago (Gage et al, 2000) described 23 cat-associated cases of plague in people, five of which were fatal. People were infected by face-to-face contact, bites, scratches or simply caring for an infected cat. Most affected people were cat owners, but some were veterinary clinic personnel.  Plague is treatable with antibiotics, but the disease can progress rapidly, so it's important to determine the diagnosis and start treatment as soon as possible.

Here are some things to consider if you live in an area where plague exists:

  • Keep pets indoors as much as possible to help prevent exposure to infected wildlife.
  • Use routine flea control measures as directed by your veterinarian.
  • Consider any cat that develops a fever of unknown origin or enlarged lymph nodes a plague suspect.
  • Don't let cats and dogs hunt wild rodents, and don't let them have access to rodent burrows.
  • If your pet has been diagnosed with plague, you need to seek medical attention promptly in case you have been exposed. If a person in the household is diagnosed with plague, pets should be investigated as possible sources and should be treated prophylactically in case they have been exposed.

Lower photo: Bubo in the leg of a person infected with bubonic plague (source: Centers for Disease Control and Prevention)

"Black Death" in South Dakota - Plagued Prairie Dogs

While people often consider plague (aka the 'black death') to be something from history books, it is in fact still alive and well is some areas of the world. In North America, it is most common in the southwestern US but can extend into other regions.

Plague is caused by the bacterium Yersinia pestis, which is carried primarily by wild rodents.  Infection with Yersina pestis can cause bubonic plague (swollen lymph glands), septicemia plague (bloodstream infection) or pneumonic plague (pneumonia/lung infection).  An average of 13 human cases are diagnosed in the US every year.  In today's modern times, the infection can be effectively treated with antibiotics, but if left untreated the mortality rate is still 50-90%. The Canadian Notifiable Disease Database has never received a report of plague in a human.

Plague has been reported in a variety of animal species, including cats and dogs. However, dogs seem to be relatively resistant to the infection compared to cats. Yersinia pestis gets from rodents to other animals and people mainly by flea bites. Fleas become infected by biting an infected animal, and can then pass on the infection by biting another animal or person.  It is also possible for plague to be transmitted by direct  contact with infected animals, but this is less common.

A recent report described an outbreak of plague in prairie dogs in western South Dakota.  There is  concern that the disease could also affect the endangered black-footed ferret in that area. Plague almost always kills prairie dogs, and black-footed ferrets almost exclusively eat prairie dogs.

When plague is present in wild animals in a region, there is always a risk of transmission to people and pets through contact with infected animals or bites from infected fleas.  Some basic measures to reduce the risk of plague exposure in areas where the disease exists in wild animals are:
  • Keep cats indoors
  • Talk to your veterinarian about a flea control program for your pets
  • Never touch wild animals, especially sick or dead ones
  • Don't keep wild animals as pets
  • Try to keep wild animals away from your pets
Photo: Scanning electron micrograph of an Oriental Rat Flea, Xenopsylla cheopis.
(Centers for Disease Control and Prevention (CDC) / Janice Carr)