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Plague for Veterinarians
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Plague for Veterinarians

What is plague?


Plague is an infectious disease that affects rodents, certain other animals, and humans. It is caused by the Yersinia pestis bacteria. These bacteria are found in many areas of the world, including the United States.

How do people become infected with plague?

People most commonly acquire plague when they are bitten by a flea that is infected with the plague bacteria. People can also become infected from direct contact with infected tissues or fluids while handling an animal that is sick with or that has died from plague. Finally, people can become infected from inhaling respiratory droplets after close contact with cats and humans with pneumonic plague.


What is the basic transmission cycle of plague?

Fleas become infected by feeding on rodents, such as chipmunks, prairie dogs, ground squirrels, mice, and other mammals that are infected with the bacterium Yersinia pestis. Fleas transmit the plague bacteria to humans and other mammals during a subsequent feeding. The plague bacteria survive briefly (a few days) in the blood of rodents and for longer periods in the fleas. An illustration of plague ecology in the United States is available.

What is the incubation period for plague?


A person usually becomes ill with bubonic plague 2 to 6 days after being infected. Someone exposed to Yersinia pestis through the air would become ill within 1 to 3 days. When bubonic plague is left untreated, plague bacteria can invade the bloodstream. When plague bacteria multiply in the bloodstream, they spread rapidly throughout the body and cause a severe and often fatal condition called septicemic plague. Untreated bubonic plague can also progress into an infection of the lungs, causing pneumonic plague. If plague patients are not given specific antibiotic therapy, all forms of plague can progress rapidly to death.


How is plague treated?


Plague can be successfully treated with antibiotics. Once a patient is diagnosed with suspected plague they should be hospitalized and, in the case of pneumonic plague, medically isolated. Laboratory tests should be done, including blood cultures for plague bacteria and microscopic examination of lymph node, blood, and sputum samples. Antibiotic treatment should begin as soon as possible after laboratory specimens are taken. To prevent a high risk of death in patients with pneumonic plague, antibiotics should be given as soon as possible, preferably within 24 hours of the first symptoms


Who is at risk for getting plague in the United States?


Human plague occurs in areas where the bacteria are present in wild rodent populations. The risks are generally highest in rural and semi-rural areas, including homes that provide food and shelter for various ground squirrels, chipmunks and wood rats, or other areas where you may encounter rodents.


Cats are highly susceptible to plague and are a common source of Yersinia pestis infection in humans (owners and veterinarians). Dogs infected with plague are less likely to develop clinical illness than cats.

Most cases of plague in cats present with a sub-mandibular lymphadenitis. Abscessed lymph nodes may be clinically indistinguishable from abscesses due to other causes, like bite wounds. Cats with pneumonic plague can pose a significant plague risk to owners, veterinarians and others who handle or come into close contact with these animals due to possible aerosolization of bacteria. Transmission to humans can also occur directly, by bites, scratches, and direct contact with infectious exudates or indirectly, as the pets may carry infected fleas to their owners.


Diagnostic testing

  • Y. pestis may be identified microscopically by examination of Gram, Wright, Giemsa, or Wayson’s stained smears of peripheral blood, sputum, or lymph node specimen. Visualization of bipolar-staining, ovoid, Gram-negative organisms with a “safety pin” appearance permits a rapid presumptive diagnosis of plague.
  • Appropriate biologic samples for diagnostic testing include: lymph node aspirate, lymph node, liver, spleen, lung, bone marrow, and whole blood.
  • Antibiotic therapy should be started promptly, but pre-treatment samples are ideal for diagnostic testing.
  • Tissue samples should be placed in a clean container and chilled (not frozen).
  • Contact local or state health officials to determine the best place to send samples for diagnostic testing.


Treatment for dogs and cats

  • Streptomycin is the treatment of choice, but is difficult to obtain. Alternatives include:
    • Gentamicin (2-3 mg/kg tid, IM or SQ)
    • Doxycycline (5 mg/kg bid, PO)
    • Tetracycline (22 mg/kg tid, PO)
    • Chloramphenicol (50 mg/kg bid, PO)
  • Parenteral antibiotics may be switched to oral therapy upon clinical improvement.


Avoiding occupational risks

  • Veterinary staff is at risk of plague if there is contact with infectious exudates, respiratory droplets, oral secretions, tissues or fleas.
  • Any material used in examination of plague-suspect cats should be disinfected, autoclaved, or incinerated.
  • Masks and gloves should be worn when examining and treating cats suspected of having plague.
  • Veterinarians should use appropriate personal protective equipment (PPE) before beginning a necropsy on a plague-suspect animal. PPE should include gloves, an N95 respirator or the equivalent, and protective eye equipment.
  • If any veterinary staff is exposed to infectious material, they should watch their health closely for 2 weeks following the exposure and discuss post-exposure prophylaxis or fever watch with a health care provider and public health officials.


Prevention and control

It is important that public health officials be notified promptly when plague is suspected in a cat.

  • Public health officials will assist with follow-up investigations and implementation of preventive measures at sites where cats might have been exposed to minimize future risk.
  • Follow-up will help identify persons who might have been exposed to the infected animal so that appropriate preventive measures, including prophylactic antibiotic therapy if indicated, can be implemented.
  • Owners of plague–positive animals frequently ask veterinarians about their own risk of contracting plague. They should be advised:
    • That their risk of plague depends on the type of contact with the infected animal (casual contact versus inhalation of infectious coughed material).
    • To see their health care provider and watch their health closely for 2 weeks following the last possible contact with the infected animal.
    • That post-exposure antibiotic therapy may be warranted, depending on the type and duration of the contact. Health department personnel should be placed in touch with owners of plague-positive animals to ensure no animal–to–human transmission occurs.

General plague prevention messages can be disseminated by veterinarians to animal owners. These include:

  • All ill animals, especially cats, should be seen by a veterinarian.
  • 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.
  • Eliminate sources of food and nesting places for rodents around homes, work places, and recreation areas; remove brush, rock piles, junk, cluttered firewood, and potential food supplies, such as pet and wild animal food. Make your home rodent-proof.
  • Pet owners should be encouraged to not pick up or touch dead animals.


Additional Information


Source: Center for Disease Control and Prevention

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