Plague is caused by Yersinia pestis. Symptoms are either severe pneumonia or massive lymphadenopathy with high fever, often progressing to septicemia. Diagnosis is epidemiologic and clinical, confirmed by culture and serologic testing. Treatment is with streptomycin or doxycycline.
Yersinia (formerly Pasteurella) pestis is a short bacillus that often shows bipolar staining (especially with Giemsa stain) and may resemble a safety pin.
Plague occurs primarily in wild rodents (eg, rats, mice, squirrels, prairie dogs) and is transmitted from rodent to human by the bite of an infected rat flea vector. Human-to-human transmission occurs by inhaling droplet nuclei from patients with pulmonary infection (primary pneumonic plague), which is highly contagious. In endemic areas in the US, several cases may have been caused by household pets, especially cats. Transmission from cats can be by bite or, if the cat has pneumonic plague, by inhalation of infected droplets.
Massive human epidemics (eg, the Black Death of the Middle Ages) have occurred. More recently, plague has occurred sporadically or in limited outbreaks. In the US, > 90% of human plague occurs in the Southwest, especially New Mexico, Arizona, California, and Colorado. Yersinia is considered a possible agent of bioterrorism.
Symptoms and Signs
In bubonic plague, the most common form, the incubation period is usually 2 to 5 days but varies from a few hours to 12 days. Onset of fever of 39.5 to 41° C is abrupt, often with chills. The pulse may be rapid and thready; hypotension may occur. Enlarged lymph nodes (buboes) appear with or shortly after the fever. The femoral or inguinal lymph nodes are most commonly involved, followed by axillary, cervical, or multiple nodes. Typically, the nodes are extremely tender and firm, surrounded by considerable edema. They may suppurate in the 2nd wk. The overlying skin is smooth and reddened but often not warm. A primary cutaneous lesion, varying from a small vesicle with slight local lymphangitis to an eschar, occasionally appears at the bite. The patient may be restless, delirious, confused, and uncoordinated. The liver and spleen may be enlarged. Bubonic plague may be complicated by pneumonic plague.
Primary pneumonic plague has a 2- to 3-day incubation period, followed by abrupt onset of high fever, chills, tachycardia, chest pain, and headache, often severe. Cough, not prominent initially, develops within 24 h. Sputum is mucoid at first, rapidly develops blood specks, and then becomes uniformly pink or bright red (resembling raspberry syrup) and foamy. Tachypnea and dyspnea are present, but pleuritic chest pain is not. Signs of consolidation are rare, and rales may be absent.
Septicemic plague may occur with the bubonic form or without the bubonic form (called primary septicemic plague) as an acute, fulminant illness. Abdominal pain, presumably due to mesenteric lymphadenopathy, occurs in 40% of patients. Disseminated intravascular coagulopathy, gangrene of the extremities (hence, the name Black Death), and multiorgan failure eventually develop. Pharyngeal plague and plague meningitis are less common forms.
Pestis minor, a more benign form of bubonic plague, usually occurs only in endemic areas. Lymphadenitis, fever, headache, and prostration subside within a week.
The mortality rate for untreated patients with bubonic plague is about 60%; most deaths result from septicemia in 3 to 5 days. Most untreated patients with pneumonic plague die within 48 h of symptom onset. Septicemic plague may be fatal before bubonic or pulmonary manifestations predominate.
Diagnosis is made by stain and culture of the organism, typically by needle aspiration of a bubo (surgical drainage may disseminate the organism); blood and sputum cultures should also be obtained. Other tests include immunofluorescent staining and serology; a titer of > 1:16 or a 4-fold rise between acute and convalescent titers is positive. PCR testing, if available, is diagnostic. Prior vaccination does not exclude plague; clinical illness may occur in vaccinated people.
Patients with pulmonary symptoms or signs should have a chest x-ray, which shows a rapidly progressing pneumonia in pneumonic plague. The WBC count is usually 10,000 to 20,000/μL with numerous immature neutrophils.
Immediate treatment reduces mortality to < 5%. In septicemic or pneumonic plague, treatment must begin within 24 h with streptomycin 15 mg/kg (up to 1 g) IM bid for 10 days or until 3 days after temperature has returned to normal. Doxycycline 100 mg IV or po q 12 h is an alternative. Gentamicin and chloramphenicol are also effective.
Chloramphenicol is preferred for patients with infection of tissue spaces into which other drugs pass poorly (eg, plague meningitis, endophthalmitis). Chloramphenicol should be given in a loading dose of 25 mg/kg IV, followed by 12.5 mg/kg IV or po q 6 h.
Routine isolation precautions are adequate for patients with bubonic plague. Those with primary or secondary pneumonic plague require strict respiratory isolation.
All pneumonic plague contacts should be under medical surveillance. Temperature should be taken q 4 h for 6 days. They and others in close contact with plague patients or with contaminated fluids or tissue should receive prophylaxis with doxycycline 100 mg po q 12 h (for children < 8 yr, trimethoprim/sulfamethoxazole [TMP/SMX] 20 mg/kg [of the SMX component] q 12 h). Travelers should be given prophylaxis with doxycycline 100 mg po q 12 h during exposure periods. Plague vaccine is available but is recommended mainly for laboratory workers and researchers because immunity requires about 1 mo to develop.
Rodents should be controlled and repellents used to minimize flea bites.
Other Yersinia Infections
Yersinia enterocolitica and Y. pseudotuberculosis are zoonoses acquired by ingestion of contaminated food or water; they occur worldwide.
Y. enterocolitica is a common cause of diarrheal disease and mesenteric adenitis that clinically mimics appendicitis. Y. pseudotuberculosis most commonly causes mesenteric adenitis and has been suspected in cases of interstitial nephritis, hemolytic-uremic syndrome, and a scarlet fever–like illness. Both species can cause pharyngitis, septicemia, focal infections in multiple organs, postinfectious erythema nodosum, and reactive arthritis. In patients with chronic liver disease or iron overload, mortality from septicemia may be as high as 50%, even with treatment.
The organisms can be identified in standard cultures from normally sterile sites. Selective culture methods are required for nonsterile specimens. Serologic assays are available but difficult and not standardized. Diagnosis, particularly of reactive arthritis, requires a high index of suspicion and close communication with the clinical laboratory.
Treatment of diarrhea is supportive because the disease is self-limited. Septic complications require β-lactamase–resistant antibiotics guided by susceptibility testing.
Prevention focuses on food handling and preparation, household pets, and epidemiology of suspected outbreaks.
Last full review/revision August 2009 by Burke A. Cunha, MD
Content last modified February 2012