Plague – Veterinary Considerations

Russ Daly, DVM, Extension Veterinarian

 

Although most human cases of plague (Yersinia pestis infection) have resulted from bites from infected fleas, some have been caused by contact with infected animals, including domestic cats.  Veterinarians therefore play a role in early recognition of the disease and protecting other animals and people from contracting plague.

 

Pathogenesis/Ecology

  • Yersinia pestis is maintained in the environment by rodents and their associated fleas. 
    • Rodents present in South Dakota responsible for maintaining enzootic transmission are commonly:
      • Prairie dogs
      • Ground squirrels
      • Wood rats
      • Mice
    • Dog and cat fleas are not efficient vectors of plague.  Prairie dog fleas and ground squirrel fleas are.
  • Cats and dogs (potentially pocket pets, also) are most commonly exposed by ingesting infected rodents or rabbits or by bites from an infected flea.
  • Risk factors for animals:
    • Hunting and eating infected rodents and rabbits
    • Exposure to carcasses of infected animals
    • Flea infestation
  • Humans are infected by:
    • Bites from infected fleas
    • Contact with secretions or tissues from infected animals
    • Respiratory droplets from cats or people infected with the pneumonic form of plague.

 

 

Clinical signs

  • Plague may exhibit any of three different manifestations:

1.       Bubonic plague

·         Transmitted through oral ingestion, flea bites, or contact with infected tissues

·         Organisms travel through lymphatics to regional lymph nodes. 

·         Signs include:

o        Fever

o        Lethargy

o        Anorexia

o        Enlarged lymph node (bubo)

§         Usually a single lymph node or local cluster, not usually symmetric enlargement

§         May or may not be abcessed or draining

§         May be extremely painful

·         Incubation period is short: 1-4 days.

·         Based on experimental infections, oral exposure is more likely to cause enlarged lymph nodes in head and neck area, while SQ exposure may only result in a SQ abscess near the site of entry.

·         Bubonic plague may progress to secondary septicemic plague when it disseminates past the lymph nodes.

2.       Septicemic plague

·         When plague infection is present in the absence of buboes.

·         Clinical signs include fever, lethargy, anorexia, signs of sepsis but no enlarged lymph nodes.

3.       Pneumonic plague

·         Primary pneumonic plague (respiratory infection from inhalation of infected aerosols) has not been documented in cats

·         Secondary pneumonic plague occurs as a result of hematogenous spread of the organism to the lungs.

·         Results in increased risk of spread of infection from respiratory droplets.

 

  • Dogs are relatively resistant to clinical illness from plague. Cattle, horses, and sheep are not known to develop plague.

 

 

Diagnosis

  • Differentials are cat bite abcesses, tularemia
    • Gram stain of exudates from lymph nodes will have a nearly homogenous population of gram negative organisms, whereas cat bite abscesses will show a mixed population
  • Antemortem samples:
    • Preferably should be taken before treatment
    • Place on ice and ship to lab overnight
    • Preferred samples:
      • Lymph node aspirates
        • If no fluid is aspirated, 1 ml of sterile saline may be injected into the lymph node and aspirated back into the syringe.
      • Swabs of draining lesions
      • Swabs of cat’s oral cavity (if oral lesions present)
      • Entire carcass or liver, spleen, lung, affected lymph nodes
    • Serology has also been used; paired samples are usually required.

 

 

Treatment

  • Gentamicin is drug of choice, especially for seriously ill cases
  • Doxycycline is appropriate for uncomplicated cases.
    • Other alternatives include: tetracyclines, chloramphenicol, or, as a last resort, sulfas.  Penicillins are not considered effective.
  • Treatment duration should last 10-21 days.  Rapid improvement should be noted within 3 days of treatment.
  • Cats should be hospitalized and not sent home.  Human plague cases have occurred when owners administered oral treatments to their cats at home. 
    • Cats are believed to be non-infectious after 72 hours of treatment if there is clinical improvement.

 

 

Protection of animal caretakers.

  • Because cats with the pneumonic form of plague can transmit infection via aerosols, initial care of suspect patients should include:
    • Treatment of the animal for external parasites (fleas)
    • Physical separation and isolation of the animal from other patients
    • Wearing of a surgical mask and eye protection when treating patient
    • Wearing of a surgical mask within 2 meters of the patient
    • Observing standard hospital precautions such as:
      • Washing hands before and after donning gloves
      • Wearing gloves when handling the patient
      • Use of gowns, surgical masks, and eye protection
      • Proper handling of linens & instruments associated with the patient’s treatment and hospitalization

 

Reference:  Orloski K, Lathrop S.  Plague: a veterinary perspective.  J Am Vet Med Assoc 2003;222:444-448.

 

Russ Daly, DVM

Extension Veterinarian

Assistant Professor

Veterinary Science Department

South Dakota State University

Box 2175

Brookings, SD  57007

Phone: 605-688-6589

Fax:  605-688-6003