Y. pestis causes plague, which occurs in three forms: (a) bubonicplague, (b) pneumonic plague, and (c) septicemic plague.
Bubonic plague is the most common clinical form of the disease. The incubation period varies from 2 to 6 days. The condition is associated with a sudden onset of high fever, chills, and headache, and also body aches, extreme exhaustion, abdominal pain, and diarrhea. Presence of painful, swollen lymph glands (buboes), usually in the groin, axilla, or neck, is the characteristic manifestation. The buboes are unilateral, oval, and extremely painful. They measure 2–10 cm in size. Inguinal lymph nodes are most commonly involved. Enlargement of the buboes may cause rupture and discharge of foul-smelling pus. Hepatomegaly and splenomegaly may occur causing tender-ness. Mortality rate for untreated plague is 40–70%.
Abrupt onset of fever and chills, lymphadenopathy, chest pain, dyspnea, purulent sputum, or hemoptysis are the manifesta-tions of pneumonic plague. Buboes may or may not be present in pneumonic plague. Pneumonic plague is highly infectious and transmitted by aerosol droplets. The condition if untreated is invariably fatal.
Septicemic plague is usually the terminal event in the bubonic or pneumonic plague, but may sometimes occur primarily in elderly patients. The condition is associated with a rapid onset of symptoms, such as nausea, vomiting, abdominal pain, and diar-rhea. Diarrhea may be the predominant symptom. Patients have a toxic appearance due to an overwhelming infection with the plague bacillus and may present with tachycardia, tachypnea, and hypotension. Hypothermia is common. Generalized pur-pura leading to necrosis and gangrene of the distal extremities may be observed. Buboes are not demonstrated in septicemic plague. Septicemic plague is associated with a high mortality rate, and patients usually die due to a high level of bacteremia.