Plague FAQ

May 1, 2013

Q:  What causes plague?
A:  Plague is an infectious disease of animals and humans caused by Yersinia pestis, a bacterium.

Q:  What species are typically at risk for developing plague infection?
A:  Wild rodents (eg, rats, ground squirrels, and prairie dogs) are the reservoir species for the disease. Other mammals, such as rabbits, dogs, coyotes, raccoons, skunks, goats, llamas, and especially cats, may also become infected. Domestic cats appear to have increased susceptibility, and nearly 60% of cats affected will eventually die of the disease.

Q:  How can humans acquire plague?
A:  Humans may develop bubonic, primary septicemic, or pneumonic plague after becoming infected with Y pestis from the bite of infected fleas; by handling tissues of infected animals, especially rodents and rabbits; by contact with airborne droplets from human patients or household pets (especially cats) with plague pharyngitis or pneumonia; or through careless manipulation of laboratory cultures.

Q: How can animals acquire plague?
A: Fleas are a major vector for transmission of the disease-causing organisms. In addition to fleas, animals may acquire plague from direct contact with infected animals or their tissues (eg, grooming a denmate or eating infected prey or carrion), eating feed contaminated with excretions from infected animals, inhalation of aerosolized Y pestis in respiratory droplets from infected animals, or even digging in soil contaminated with the bacteria. Raptors may assist in the dissemination of plague through transport of infected fleas or prey. Carnivores are effective transporters of infected fleas to other rodent populations as well as to humans and are most often the direct source of human infection. Infection of carnivores is most likely due to ingestion of infected animals rather than fleabites. Similarly, ingestion appears to be the source of the disease in goats, sheep, and camels.  

Q:  What are the signs of plague in humans?
A:   Clinical signs in humans usually develop within 2 to 6 days after infection and include sudden onset of headache, fever, chills, and weakness. An acutely swollen and painful lymph node (bubo) can appear approximately 24 hours after the start of clinical signs. Some individuals may develop septicemia without a bubo (primary septicemic plague), or septicemia may occur secondary to bubonic plague. Gangrene (hence the name black death), coagulopathies, and multiple organ failure may result from advanced plague septicemia. 

Secondary pneumonic plague develops in approximately 10% to 15% of patients with bubonic or primary septicemic plague by spread of Y pestis to the lungs. Clinical signs in patients with pneumonic plague include cough, chest pain, bronchopneumonia, and hemoptysis. Primary pneumonic plague caused by inhalation of Y pestis is rare but has been reported after handling of cats with pneumonic plague.

Q: What are the signs of plague in animals?
The disease manifestation in animals varies and is dependent on many factors, especially mode of transmission and susceptibility of the host. Some animals, such as cats, seem to be much more susceptible or at risk than others, such as cattle and horses. The term bubonic plague means that bubos are present with the disease; however, plague can occur without causing them. Plague can also occur in septicemic and pneumonic forms in animals. An ocular plague, characterized by keratoconjunctivitis, endophthalmitis, and panophthalmitis, has also been documented in deer. Signs in animals, especially cats, may include lethargy, signs of depression, anorexia, vomiting, diarrhea, dehydration, fever, enlarged or abscessed lymph nodes draining the site of exposure (eg, submandibular lymph nodes if the cat ate infective tissues), oral lesions (eg, necrotic tonsillitis and ulcers), muscle soreness, coughing, hemoptysis, and weight loss. If the disease progresses to septicemic plague, the animal may also have signs of shock, be dyspneic (likely from pulmonary edema), or develop disseminated intravascular coagulation. Animals may develop septicemic plague or pneumonic plague without developing a bubo.

Veterinarians need to consider plague in animals with a systemic infection that have a history of potential exposure in an endemic area. Veterinarians and their staff need be aware of the of the plague risks in their area and use appropriate infection control procedures and personal protective equipment with suspected cases. In addition, states may require that suspected or diagnosed cases of plague be reported to the state’s public health authority. Check with your state or territory’s public health authority and state veterinarian to learn what is required in your state.


Q:  How deadly is plague?
A:  The case fatality rate for untreated bubonic plague in humans is approximately 50% and in cats is nearly 60%. Untreated primary septicemic plague and pneumonic plague are invariably fatal. Modern treatment has markedly reduced the number and proportion of fatalities from bubonic plague. Pneumonic and septicemic plague also respond if recognized and treated early.

Q:  Where can plague be found?
A:  Foci of plague exist on all continents, with the exception of Australia and Antarctica. In North America, plague is found in animals and their associated fleas from the Pacific Coast to the Great Plains and from southwestern Canada to Mexico. According to the CDC, most human cases within the United States occur within 2 regions:

  • Northern New Mexico, northern Arizona, and southern Colorado.
  • California, southern Oregon, and far western Nevada.

Q:  How is plague diagnosed?
  Yersinia pestis may be identified microscopically by examination of Gram, Wright, Giemsa, or Wayson stained smears of peripheral blood, sputum, bubo fluid, or CSF. Finding bipolar-staining, ovoid, gram-negative organisms with a safety pin appearance permits a rapid presumptive diagnosis of plague. Yersinia pestis infection may also be diagnosed through bacteriologic culture, rapid immunoassays, immunofluorescence, ELISAs, and other tests.

Q:  How is plague treated?
A:   If diagnosed in time, bubonic, septicemic, and pneumonic plague are usually responsive to treatment with certain antimicrobials, while the bacterium responsible has innate resistance to other classes of antimicrobials. In suspected cases of plague, appropriate treatment by trained healthcare professionals should be sought out immediately. Diagnoses and treatment regimens require clinical judgment and may need to be adjusted for the patient (human or animal) based on age, medical history, and underlying health conditions

Q:  How can plague be prevented?
A:  Prevention of plague is multifaceted and is based on avoidance and control of the disease reservoir species and its vectors. Appropriate flea and rodent control are essential—consult your veterinarian for products that can be used on or around your pets or livestock. Means for avoidance depend on the situation and may include simple steps such ensuring feeds for livestock and pets are stored in rodent-proof containers or areas; not camping in or adjacent to a prairie dog colony; not feeding wild chipmunks or other wild rodents; not allowing pets to roam freely outdoors; rodent-proofing as much as possible residences, farms, workplaces, and recreational areas; and staying away from sick or dead rodents.

Q:  How might plague be used as a biological weapon?

A:   Spread of plague resulting from aerosolization of Y pestis is of most concern when considering the use of the causative agent of plague as a biological weapon because it can survive for up to an hour in the air, which could allow for increased dispersal and population impact. The CDC offers an online training module for veterinarians on plague relative to animal and public health issues associated with Y pestis bioterrorism.

This information has been prepared as a service by the American Veterinary Medical Association. Redistribution is acceptable, but the document's original content and format must be maintained, and its source must be prominently identified.