Plague FAQ

Updated April 2017

Q:  What causes plague?
A:  Plague is an infectious, disease of animals and humans caused by the bacterium Yersinia pestis. Plague is usually transmitted by flea bites. It was introduced into the United States from Asia through shipping ports in the late 1900s, and spread from there. It can now be found in small wild mammal populations in the western U.S.

Q:  What species are typically at risk for plague infection?
A:   Essentially all mammals are susceptible to plague. In the US we most often see wild rodents, lagomorphs such as cottontails and jackrabbits, and domestic pets such as cats and dogs with the disease. Wild and domestic felids that contract plague seem to experience high mortality, especially when they do not receive prompt antibiotic treatment.

Q:  How do people get plague?
A:  Humans usually contract plague from the bites of infected fleas. Other, less common exposures include handling tissues of infected animals (hunters), eating undercooked game meat of plague infected animals, inhaling infectious respiratory droplets from people or pets with plague pneumonia, or handling  plague cultures in a laboratory setting.

Q: How do animals contract plague infection?
A:  Fleas become infected by feeding on infected animals such as chipmunks, prairie dogs, rabbits, ground squirrels, rock squirrels, tree squirrels, mice and woodrats infected with the bacteria. The infected fleas then transmit the plague bacteria to humans and other mammals during the blood feeding process. Carnivores typically get plague from ingesting infected animals or from flea bites. Other exposures may include contact with contaminated soil, and inhalation of aerosolized Y. pestis in respiratory droplets from infected animals.

Q:  What are the signs of plague in humans?
A:   People usually get symptoms of plague within 2 to 6 days after exposure. Human plague may be bubonic, septicemic, or pneumonic. Symptoms include sudden onset fever, chills, headache, nausea, vomiting, abdominal and/or back pain, and weakness.  Bubonic plague includes the development of characteristic buboes (acutely swollen and painful lymph nodes) that appear about 24 hours after symptoms start. Primary septicemic (blood-stream infection) plague does not have the characteristic buboes but may progress to coagulopathies and potentially multi-organ failure.  Septicemic plague may also result secondarily to bubonic plague. Gangrene (hence the name black death) may occur in untreated septicemic plague patients. 

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. Patients with pneumonic plague will have additional symptoms that include cough, chest pain, bronchopneumonia and hemoptysis. Primary pneumonic plague caused by inhalation of Y. pestis is rare but has been reported after handling animals with pneumonic plague or close contact with people with pneumonic plague.

Q: What are the signs of plague in animals?
Signs of plague in animals vary, are non-specific, and depend on many factors, especially mode of transmission and susceptibility of the host. Some animals, like cats, seem to be more susceptible than other animals. Bubonic plague means that buboes are present; however, plague can occur without buboes. Plague can also present in septicemic and pneumonic forms in animals. Ocular plague, characterized by keratoconjunctivitis, endophthalmitis, and panophthalmitis has been documented in deer.

Signs in animals, especially cats, can include lethargy, signs of depression, anorexia, vomiting, diarrhea, dehydration, fever, enlarged or abscessed lymph nodes draining the site of exposure (e.g., submandibular lymph nodes if the cat ate infective tissues), oral lesions (e.g., necrotic tonsillitis and ulcers), muscle soreness, coughing, hemoptysis and weight loss. If the disease progresses to septicemic plague, the animal may also develop shock, be dyspneic (likely from pulmonary edema), or develop disseminated intravascular coagulation.
Veterinarians should consider plague in animals with a systemic infection that have a history of roaming or hunting in a plague endemic area. If plague is suspected a flea control product should be applied to the animal as soon as possible. Veterinarians and their staff should be aware of plague risks in their area and use appropriate infection control procedures and personal protective equipment with suspect cases. Veterinarians should report suspect or diagnosed cases of plague to the state or local public health authority.    

Q:  How deadly is plague?
A:   The case fatality rate for untreated bubonic plague in humans is about 50% and in cats is nearly 60%. Modern treatment with antibiotics has reduced fatalities from bubonic plague. Pneumonic plague and septicemic plague also respond to antibiotic treatment if patients are treated quickly enough. Untreated primary septicemic plague and pneumonic plague are invariably fatal.

Q:  Where can plague be found?
A:  Plague is found on all continents except Australia and Antarctica. In North America, plague is found in animals and their fleas from the Pacific Coast to the Great Plains and from southwestern Canada to Mexico. Six states account for the majority of plague cases in the United States: New Mexico, Colorado, Arizona, California, Oregon and 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:   Plague can be treated with specific antibiotics from the aminoglycoside, fluoroquinolone and tetracycline families. If plague is suspected, appropriate treatment should be initiated immediately. Recommended dosage regimens are available that take into account the species affected, dosage can be adjusted based on age, medical history and underlying health conditions.

Q:  How can plague be prevented?
A:   Prevention of plague rests fundamentally with prevention of flea bites and restriction of pets from hunting and roaming. Appropriate flea control is essential—consult your veterinarian for products that can be used on your pets.  Reduce rodent presence in the domestic and peri-domestic environment by ensuring feeds for livestock and pets are stored in rodent-proof containers or areas, and rodent-proof residences, farms, workplaces and recreational areas. Other preventive measures include wearing insect repellent or applying flea control products to pets; not camping in, or adjacent to, rodent burrows, nests or colonies; not feeding wild chipmunks or other wild rodents; not allowing pets to roam freely outdoors and avoiding sick or dead rodents. 

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

A:   Plague bacteria can be weaponized for aerosol spread as a bioterrorism agent. Plague as a biological weapon can survive for up to an hour in the air, which allows for increased dispersal and population impact. The CDC offers an online training module for veterinarians on plague with respect to animal and public health issues associated with bioterrorism.


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