The primary clinical manifestations of plague include bubonic (ie, enlarged lymph nodes), septicemic, and pneumonic. Cats and humans manifest clinical signs and pathologic changes similarly.16,32-34 If plague is left untreated and death does not occur, disease often progresses from bubonic to septicemic to secondary pneumonic plague; however, the bubonic form does not always manifest.16,33,35,36 These stages may occur in dogs as well, although dogs are less likely than cats to develop illness.22,37 Bubonic plague results from a flea bite32 or, in cats and possibly dogs, from consuming infected prey.16 Septicemia can lead to secondary pneumonic plague. Cats with secondary pneumonic plague can shed Y pestis in respiratory droplets via breathing, coughing, or sneezing, which can lead to primary pneumonic plague in humans. Pneumonic plague is the most serious stage, as it is the only form that can spread directly from human to human.32
The incubation period for Y pestis in cats has been estimated to be 1 to 4 days.38 Clinical signs include fever, anorexia, lethargy, lymphadenitis, and lymphadenopathy (buboes), with 75% of patients exhibiting submandibular lymphadenopathy.29,33 Lymph nodes may be abscessed or show evidence of hemorrhage or necrosis.33,35 Abscesses in many other areas of the body have also been reported.33 Signs of more advanced disease include vomiting, diarrhea, discharge from the mouth or nose, coughing, sneezing, ataxia, dehydration, weak pulse, cold extremities, pale or brick-red mucous membranes, prolonged capillary refill time, disseminated intravascular coagulation, dyspnea, pneumonia, and coma.16,33,34 Approximately 10% of feline cases are pneumonic.5,29,33 If left untreated, mortality rates can approach 41%.33
Infected dogs are often subclinically infected or have only mild, self-limiting febrile illness, but severe disease and death have been reported.25,39 Clinical signs in dogs include fever, lethargy, anorexia, submandibular lymphadenitis, lymphadenopathy, abscesses, oral cavity lesions, purulent intramandibular lesions, coughing, bloody sputum, ataxia, vomiting, diarrhea, dyspnea, and harsh lung sounds.13,39 The mortality rate in dogs has been estimated to be approximately 3%.39