Epidemiology

Yersinia pestis causes plague, a systemic zoonosis that primarily affects small rodents in rural areas of Africa (where 96% of all human cases worldwide occur), Asia, and the Americas. As the rodent population succumbs to disease, fleas (the arthropod vector) search for a new host and can transmit the bacteria to humans.

• In addition to fleabites, direct contact with infected tissues or airborne droplets can cause human infections. Given the possibility of airborne transmission, Y. pestis is a potential agent of bioterrorism.
• A mean of seven cases per year occur in the United States, most of them near the “Four Corners” (the junction point of New Mexico, Arizona, Colorado, and Utah) and further west in California, southern Oregon, and western Nevada.

Clinical Manifestations

Worldwide, bubonic plague accounts for 80-95% of all plague cases, with primary septicemic plague occurring in 10-20% of cases and primary pulmonary plague in only a small minority of cases.

• Bubonic plague: After an incubation period of 2-6 days, the onset of bubonic plague is sudden and is characterized by fever (>38°C), malaise, myalgia, dizziness, and increasing pain due to progressive lymphadenitis in the regional lymph nodes near the fleabite or other inoculation site.
  • The tender, swollen lymph node (bubo) has a boggy consistency with an underlying hard core when palpated.
  • With treatment, fever resolves within 2-5 days, although buboes may remain enlarged for >1 week and can become fluctuant; without treatment, infection can disseminate and cause serious illness (e.g., secondary pneumonic plague, meningitis).
• Primary septicemic plague: Pts present with gram-negative septicemia not preceded by lymphadenopathy. Persons >40 years old are at greater risk, although this form of the disease can occur in all age groups. Diabetes and hemochromatosis are additional risk factors.
• Pneumonic plague: After a short incubation period averaging a few hours to 3 days, pts experience a sudden onset of fever, nonspecific signs and symptoms (e.g., headache, myalgias, vomiting), and respiratory manifestations (e.g., cough, chest pain, sputum production with hemoptysis).
  • Pneumonitis that is initially segmental can progress to lobar pneumonia and then to bilateral lung involvement.
  • The mortality rate is nearly 100% without treatment and is still >50% with effective treatment.

Diagnosis

The WHO recommends an initial presumptive diagnosis followed by confirmation in a reference laboratory.

• The appropriate specimens for diagnosis of bubonic, pneumonic, and septicemic plague are bubo aspirate (after injection of 1 mL of normal saline), bronchoalveolar lavage fluid or sputum, and blood, respectively. Gram's, Wayson, or Wright-Giemsa staining of these samples may reveal bipolar gram-negative rods.
• Given the potential risk to laboratory workers, culture of Y. pestis should be performed only at reference laboratories, which use direct immunofluorescence, PCR, and/or specific bacteriophage lysis as confirmatory tests for identification. The optimal growth temperature is 25-29°C.
• In the absence of other positive diagnostic testing, a serologic diagnosis can be made.