Definition and epidemiology
Plague is an acute febrile illness caused by the gram-negative coccobacillus Yersinia pestis. It is classified as a potential bioterrorism agent due to its high mortality and transmissibility in pneumonic form.
Three clinical forms exist:
- Bubonic plague (80-85% of cases): characterized by painful lymphadenopathy
- Septicemic plague (10-15%): bloodstream infection without bubo formation, or secondary to bubonic plague
- Pneumonic plague (rare, <5%): lung involvement, either primary (inhalation) or secondary to bacteremia
Plague remains endemic in parts of Africa (especially Madagascar and Democratic Republic of Congo), Asia (including China, Mongolia), and the Americas (southwestern United States, Peru). Worldwide, approximately 1,000-3,000 cases are reported annually to the WHO, though reporting is incomplete. In the United States, 1-17 cases occur yearly, primarily in rural New Mexico, Arizona, Colorado, and California.
Historically responsible for devastating pandemics including the Justinian Plague (6th century) and the Black Death (14th century, killing an estimated 30-60% of Europe's population), modern plague is treatable with antibiotics if diagnosed early. Case fatality rates range from 10-15% for treated bubonic plague to over 50% for untreated pneumonic plague.
- Plague is caused by Yersinia pestis, a gram-negative coccobacillus with characteristic bipolar "safety pin" staining.
- Bubonic plague presents with painful buboes (enlarged lymph nodes) in groin, axilla, or neck after flea bite.
- Pneumonic plague has the highest mortality and is the only form transmissible person-to-person via respiratory droplets.
- Streptomycin or gentamicin are first-line antibiotics; doxycycline and fluoroquinolones are effective alternatives.
- Pneumonic plague requires droplet precautions; treatment must begin within 24 hours of symptoms to prevent mortality.
- Septicemic plague can cause DIC with purpura and acral gangrene—the "Black Death" appearance.
- Cats can transmit pneumonic plague to humans; domestic cats are highly susceptible and an important reservoir.
- Plague is a nationally notifiable disease requiring immediate reporting to public health authorities.
Etiology and risk factors
Yersinia pestis is a gram-negative, facultative intracellular bacterium. The primary reservoir consists of wild rodents (ground squirrels, prairie dogs, rats, chipmunks). Transmission occurs through:
- Flea bites (most common): the Oriental rat flea (Xenopsylla cheopis) becomes infected feeding on bacteremic rodents
- Direct contact with infected tissue or fluids (hunters, veterinarians handling infected animals)
- Respiratory droplets (pneumonic plague only): person-to-person transmission
- Ingestion of infected meat (rare)
Risk factors
- Residence in or travel to endemic areas
- Occupational exposure: veterinarians, hunters, wildlife biologists, laboratory workers
- Outdoor activities in endemic regions during warmer months (flea activity peaks)
- Contact with sick or dead animals, especially rodents and their predators (cats, dogs)
- Camping, hiking, or hunting in rural endemic areas
- Poor hygiene and overcrowding (facilitates rodent-human contact)
- Immunosuppression (increases severity but not necessarily acquisition risk)
Domestic cats are particularly important in transmission; they are highly susceptible to plague and can transmit pneumonic plague to owners through respiratory droplets or scratches/bites.
Pathophysiology
After inoculation through a flea bite, Y. pestis is phagocytosed by neutrophils and macrophages at the entry site. The bacteria resist intracellular killing through several virulence factors:
- F1 capsular antigen: antiphagocytic
- Type III secretion system: injects Yop proteins that inhibit phagocytosis and cytokine production
- Plasminogen activator (Pla): promotes dissemination by degrading fibrin clots and extracellular matrix
Bacteria multiply within macrophages and travel via lymphatics to regional lymph nodes, where massive replication occurs, causing hemorrhagic lymphadenitis—the pathologic bubo. The node becomes necrotic, surrounded by edema and hemorrhage.
Without treatment, bacteria spread hematogenously, causing:
- Septicemia: high-grade bacteremia with endotoxemia, DIC, shock
- Secondary seeding of organs: lungs (secondary pneumonic plague), liver, spleen, meninges
- Vascular damage: capillary leakage, purpura, acral necrosis (historical "Black Death" appearance)
Primary pneumonic plague results from direct inhalation of respiratory droplets containing Y. pestis. The organisms multiply in alveolar spaces, causing necrotizing pneumonia with bloody, highly infectious sputum. This form progresses rapidly to respiratory failure and septic shock.
Septicemic plague can occur without preceding bubo formation (primary septicemic) or as progression of bubonic plague. It is characterized by overwhelming bacteremia, endotoxic shock, DIC, and multiorgan failure.
Clinical features
The incubation period is typically 2-8 days (range 1-14 days) after flea bite or exposure.
Bubonic plague
Onset is sudden with:
- High fever (38.5-41°C), chills, rigors
- Severe malaise, headache, myalgias
- Painful bubo: exquisitely tender, enlarged lymph node(s), typically in the groin (femoral/inguinal, 70%), axilla (20%), or neck (10%), corresponding to the bite site. The bubo is firm, non-fluctuant initially, 1-10 cm in diameter, and surrounded by edema. The overlying skin may be erythematous or normal.
- Gastrointestinal symptoms: nausea, vomiting, diarrhea, abdominal pain
Patients appear acutely ill and toxic. Without treatment, 40-60% progress to septicemic plague within days.
Septicemic plague
Presents with:
- High fever, chills, prostration
- Hypotension and shock
- DIC manifestations: purpura, ecchymoses, acral gangrene (digits, nose, ears—"Black Death" appearance)
- Abdominal pain, diarrhea (sometimes bloody)
- No bubo in primary septicemic form
- Mental status changes, delirium
Progresses rapidly to multiorgan failure. Mortality exceeds 50% even with treatment if presentation is delayed.
Pneumonic plague
Most rapidly fatal form. Presents with:
- Sudden onset of fever, chills, cough
- Productive cough with bloody, watery, or purulent sputum (hemoptysis common)
- Severe dyspnea, chest pain (pleuritic), tachypnea
- Rapid progression to respiratory failure (within 24-48 hours of symptom onset)
- Cyanosis, hypoxemia
- Gastrointestinal symptoms common
Pneumonic plague can occur as primary (inhalation) or secondary (hematogenous seeding). Without treatment within 18-24 hours of symptom onset, mortality approaches 100%.
Rare forms
Pharyngeal plague (cervical buboes, pharyngitis), meningeal plague (purulent meningitis, typically a complication of inadequately treated bubonic plague).
Diagnosis
Plague should be suspected in any patient with fever, painful lymphadenopathy, and epidemiologic risk factors (endemic area residence/travel, flea/animal exposure). Early diagnosis is critical, especially for pneumonic and septicemic forms.
Laboratory confirmation
Bacterial culture (gold standard):
- Lymph node aspirate (bubo): most diagnostic in bubonic plague
- Blood cultures: positive in septicemic and late bubonic plague (obtain before antibiotics)
- Sputum culture: in pneumonic plague
- Cerebrospinal fluid culture: in meningeal plague
Y. pestis grows on standard media (blood, MacConkey, chocolate agar) but identification may take 24-48 hours. Biosafety level 2 precautions are required initially; level 3 if pneumonic plague is suspected.
Rapid diagnostic tests:
- Direct fluorescent antibody (DFA) staining of aspirates or sputum: provides presumptive diagnosis within hours
- Rapid immunochromatographic dipstick (F1 antigen detection): available in endemic areas, results in 15 minutes
- PCR: highly sensitive and specific, increasingly available
Serology:
- Acute and convalescent antibody titers (passive hemagglutination, ELISA for anti-F1 antibodies)
- Fourfold rise or single titer ≥1:128 is diagnostic
- Useful for retrospective diagnosis, not for acute management
Supportive laboratory findings
- Leukocytosis (10,000-20,000/μL) with left shift, toxic granulation
- Thrombocytopenia (in septicemic plague with DIC)
- Elevated liver enzymes, bilirubin
- Elevated creatinine (renal impairment)
- Coagulopathy (prolonged PT/PTT, low fibrinogen, elevated D-dimer) in DIC
Imaging
- Chest X-ray in pneumonic plague: patchy bronchopneumonia, lobar consolidation, or diffuse infiltrates; pleural effusions common. Cavitation is rare.
- CT may show enlarged mediastinal/hilar lymph nodes
Differential diagnosis clues
Gram or Wayson stain of bubo aspirate may show characteristic gram-negative "safety pin" bipolar staining coccobacilli.
Notify public health authorities immediately when plague is suspected. In the US, plague is a nationally notifiable disease requiring immediate (within 4 hours) reporting to local/state health departments and CDC.
Differential diagnosis
Bubonic plague
- Tularemia: similar presentation with painful regional lymphadenopathy after animal/tick exposure; caused by Francisella tularensis; slower progression, ulceroglandular lesion at inoculation site often present
- Bacterial lymphadenitis (staphylococcal, streptococcal): less acute, less toxic appearance; primary skin lesion often evident
- Cat-scratch disease (Bartonella henselae): subacute lymphadenopathy, less systemic toxicity; papule at scratch site
- Lymphogranuloma venereum: inguinal buboes but sexually transmitted (Chlamydia trachomatis L1-L3)
- Filariasis: chronic inguinal lymphadenopathy in endemic areas; less acute
Septicemic plague
- Meningococcemia: purpuric rash, shock; similar DIC picture but no buboes
- Gram-negative sepsis (any cause): clinical overlap but epidemiologic context differs
- Rickettsial infections (Rocky Mountain spotted fever, typhus): fever, rash, exposure history; serologic diagnosis
- Ehrlichiosis/Anaplasmosis: tick-borne, intracellular organisms on blood smear
Pneumonic plague
- Community-acquired pneumonia: bacterial (pneumococcal, H. influenzae) or atypical pathogens; less fulminant course
- Anthrax (inhalation): widened mediastinum on CXR, hemorrhagic mediastinitis; potential bioterrorism agent
- Tularemia (pneumonic form): similar presentation; slower progression
- Hantavirus pulmonary syndrome: acute respiratory distress in endemic areas (southwestern US); thrombocytopenia prominent
- Q fever pneumonia: Coxiella burnetii; animal exposure; less acute
- Legionnaires' disease: atypical pneumonia with GI symptoms; hyponatremia common
Key distinguishing features: epidemiologic context (endemic area, flea/rodent exposure), tempo (plague progresses rapidly), presence of buboes, and characteristic bipolar staining organisms on microscopy.
Management
Plague is a medical emergency requiring immediate antibiotic therapy. Treatment should not be delayed for confirmatory testing when clinical suspicion is high.
Antibiotic therapy
First-line agents (according to WHO and CDC guidelines):
- Streptomycin (historically preferred): intramuscular administration
- Gentamicin: intravenous, now preferred in many settings due to better availability
Both are aminoglycosides with excellent bactericidal activity against Y. pestis.
Alternative agents (effective and increasingly used):
- Doxycycline: oral or intravenous; bacteriostatic but highly effective
- Fluoroquinolones (ciprofloxacin, levofloxacin): oral or IV; good tissue penetration
- Chloramphenicol: for meningeal plague (better CSF penetration); also for pregnant women in some guidelines
Duration: 10-14 days for bubonic/septicemic plague; continue until patient is afebrile for at least 3 days. Pneumonic plague may require slightly longer courses.
Note: Avoid beta-lactams as primary therapy; Y. pestis produces beta-lactamase and response is suboptimal.
Supportive care
- Aggressive fluid resuscitation for septic shock
- Vasopressors (norepinephrine) for refractory hypotension
- Mechanical ventilation for respiratory failure in pneumonic plague
- Management of DIC: fresh frozen plasma, platelets, cryoprecipitate as indicated
- Bubo management: do NOT incise and drain (increases dissemination risk); aspiration for diagnostic purposes only
Isolation and infection control
Bubonic/septicemic plague: standard precautions
Pneumonic plague: strict respiratory droplet precautions until 48 hours of effective antibiotic therapy and clinical improvement. Use surgical masks for healthcare workers (N95 if available). Place patient in negative-pressure room if possible.
Contacts of pneumonic plague patients require post-exposure prophylaxis.
Post-exposure prophylaxis
Indicated for:
- Close contacts (<2 meters) of pneumonic plague patients
- Laboratory workers with unprotected exposure
- Household members of bubonic plague patients (some guidelines)
Regimen: doxycycline or ciprofloxacin for 7 days, started within 7 days of exposure.
Public health measures
- Immediate reporting to health authorities
- Contact tracing for pneumonic cases
- Vector control: flea control measures in affected areas
- Environmental management: rodent population control (after flea control to avoid displacing infected fleas to humans)
- Travel advisories for outbreak areas
Special populations
Pregnant women: gentamicin is preferred; doxycycline avoided. Some experts recommend chloramphenicol despite theoretical risks.
Children: gentamicin preferred. Doxycycline may be used despite dental staining concerns given severity of plague.
Vaccination
A killed whole-cell vaccine was previously available but discontinued due to limited efficacy and side effects. No vaccine is currently licensed in the US. An experimental F1-V fusion protein vaccine is under development.
Complications and prognosis
Complications
Acute complications:
- Septic shock and multiorgan failure: most common cause of death
- Disseminated intravascular coagulation (DIC): purpura, hemorrhage, acral necrosis
- Acute respiratory distress syndrome (ARDS)
- Renal failure: acute tubular necrosis from shock
- Secondary pneumonic plague: hematogenous spread to lungs from bubonic plague
- Meningitis: complication of inadequately treated bubonic plague, rare; high mortality
- Gangrene: especially digits, nose, ears ("Black Death"); may require amputation
Late complications:
- Residual lymph node suppuration or scarring
- Neurologic sequelae from meningeal plague
- Chronic renal impairment
Prognosis
Bubonic plague:
- With early antibiotic treatment: mortality 10-15%
- Without treatment: mortality 40-60%
- Prognosis depends on time to antibiotic initiation; delayed treatment significantly increases mortality
Septicemic plague:
- Treated: mortality 20-40%
- Untreated: >50% mortality
- Worse prognosis with DIC, shock, delayed diagnosis
Pneumonic plague:
- Most lethal form
- Treated within 24 hours of symptom onset: mortality 30-50%
- Treatment delayed beyond 24 hours: mortality >50%
- Untreated: nearly 100% mortality within 2-4 days
Prognostic factors
Poor prognostic indicators:
- Delayed antibiotic therapy (>24 hours from symptom onset for pneumonic; >48 hours for bubonic)
- Septicemic or pneumonic forms
- Extremes of age
- Shock on presentation
- DIC, thrombocytopenia
- Leukopenia or marked leukocytosis (>20,000/μL)
- Renal or hepatic dysfunction
Good prognosis:
- Bubonic plague with early treatment
- Absence of sepsis on presentation
- Prompt appropriate antibiotic therapy
Complete recovery is expected in most patients who receive timely treatment. Survivors of bubonic plague typically have no long-term sequelae beyond potential lymph node scarring. However, survivors of septicemic or pneumonic plague with complications (DIC with gangrene, meningitis, ARDS) may have permanent disability.
Bioterrorism consideration: Pneumonic plague is classified as a Category A bioterrorism agent due to high mortality, person-to-person transmissibility, and potential for mass casualties. Any cluster of severe pneumonia cases with rapid progression should raise concern for intentional release.
Practice questions
1. A 35-year-old man presents to the emergency department with a 2-day history of fever to 40°C, chills, and severe left inguinal pain. He went camping in rural New Mexico one week ago. Examination reveals temperature 39.8°C, blood pressure 95/60 mmHg, heart rate 118/min. A 6-cm, exquisitely tender, firm mass is palpable in the left inguinal region with surrounding edema and mild erythema. No fluctuance is appreciated. The overlying skin is intact without lesions. Laboratory results show WBC 18,500/μL with 85% neutrophils and toxic granulation. Which of the following is the most appropriate next step in management?
2. A 28-year-old wildlife biologist develops fever to 39.5°C, productive cough with blood-tinged sputum, and dyspnea 3 days after performing a necropsy on a prairie dog found dead in Colorado. Physical examination shows temperature 39.7°C, blood pressure 88/52 mmHg, respiratory rate 32/min, oxygen saturation 88% on room air. Lung auscultation reveals bilateral crackles. Chest X-ray shows bilateral patchy infiltrates. Which of the following infection control measures is most critical?
3. A 45-year-old farmer presents with 4 days of fever, headache, and nausea. Yesterday he noticed dark purple patches on his hands and feet. Examination reveals temperature 40°C, blood pressure 78/45 mmHg, heart rate 135/min. Multiple purpuric lesions are present on the distal extremities with early dry gangrene of several fingertips. No lymphadenopathy is noted. Laboratory results: WBC 3,200/μL, platelets 42,000/μL, PT 18 seconds (normal 11-13), PTT 52 seconds (normal 25-35), fibrinogen 95 mg/dL (normal 200-400), D-dimer markedly elevated. Blood cultures are pending. He lives in rural Arizona and recalls finding dead rodents near his barn last week. What is the most likely diagnosis?
Frequently asked questions
What causes plague?
Plague is caused by the bacterium Yersinia pestis, transmitted primarily through bites from infected fleas that live on rodents such as rats, ground squirrels, and prairie dogs. Less commonly, transmission occurs through direct contact with infected animal tissues, inhalation of respiratory droplets from pneumonic plague patients, or rarely through ingestion of contaminated meat. The bacteria are maintained in nature through a rodent-flea-rodent cycle in endemic areas.
What are the first symptoms of plague?
The first symptoms typically appear 2-8 days after exposure and include sudden onset of high fever (often exceeding 39°C), severe chills, intense headache, and profound weakness. In bubonic plague, a painful, swollen lymph node (bubo) develops within 24 hours, usually in the groin, armpit, or neck. Patients often experience nausea, vomiting, and muscle aches. The rapid onset and severity of symptoms distinguish plague from most other infections.
How is plague diagnosed?
Plague is diagnosed by culturing Yersinia pestis from blood, lymph node aspirate (from a bubo), or sputum samples. Rapid diagnostic methods include direct fluorescent antibody (DFA) staining showing characteristic bipolar-staining bacteria and immunochromatographic dipstick tests detecting the F1 antigen, which provide results within hours. PCR testing is increasingly available and highly accurate. Blood tests typically show marked leukocytosis, and chest X-rays reveal infiltrates in pneumonic plague. Because plague progresses rapidly, treatment often begins based on clinical suspicion and exposure history before culture results are available.
Is plague curable?
Yes, plague is curable with prompt antibiotic treatment. Streptomycin or gentamicin are the preferred antibiotics, though doxycycline and fluoroquinolones are also effective. With early treatment, survival rates for bubonic plague exceed 85-90%. However, timing is critical—pneumonic plague must be treated within 24 hours of symptom onset to prevent high mortality. Untreated plague has mortality rates of 40-60% for bubonic forms and nearly 100% for pneumonic plague. Complete recovery without long-term effects is typical when treatment is started early.
Plague vs tularemia — what is the difference?
Both plague and tularemia cause fever and painful lymph node swelling after animal exposure, but plague progresses much more rapidly and severely. Plague, caused by Yersinia pestis, is transmitted by flea bites from rodents and causes larger, more painful buboes with more profound systemic toxicity. Tularemia, caused by Francisella tularensis, follows tick bites or handling infected rabbits and typically presents with an ulcer at the bite site (ulceroglandular form) with slower progression. Plague requires aminoglycosides or fluoroquinolones, while tularemia responds well to streptomycin, gentamicin, or doxycycline. Plague has higher mortality and person-to-person transmission potential (pneumonic form), whereas tularemia does not spread between humans.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: A20
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.