Plague: Cause, Symptoms, Diagnosis, and Treatment
Plague is caused by Yersinia pestis, a gram-negative bacterium transmitted primarily through infected flea bites, direct contact with infected tissues, or inhalation of respiratory droplets, and requires immediate antibiotic treatment to prevent mortality rates ranging from 13% to nearly 100% depending on clinical form. 1
Causative Agent and Transmission
Yersinia pestis is a nonmotile, gram-negative coccobacillus that persists in nature through rodent-flea cycles 1. The bacterium reaches humans through three main routes:
- Flea bites from the Oriental rat flea (Xenopsylla cheopis) and other flea species (most common)
- Direct contact with infected animal tissues or fluids
- Inhalation of infectious respiratory droplets from infected humans or animals 1
The route of transmission directly determines which clinical form develops, making exposure history critical for diagnosis 1.
Clinical Manifestations
Bubonic Plague (Most Common Form)
After a flea bite or skin contamination, bacteria travel via lymphatics to regional lymph nodes within 2-8 days 1. Key features include:
- Painful, tender lymph node swelling ("bubo") - the hallmark finding
- Fever, chills, malaise, and headache
- Untreated mortality: 66%
- Treated mortality: 13% 1
Critical pitfall: Without prompt treatment, hematogenous spread causes secondary septicemic, pneumonic, or meningeal plague 1.
Pneumonic Plague (Most Lethal Form)
Develops 1-3 days after inhaling Y. pestis from infected animals or humans 1. Progression is rapid:
- Fever and dyspnea initially
- Productive cough with purulent or bloody sputum in late stages
- Rapid progression to respiratory failure and sepsis
- Untreated mortality: nearly 100% 1
This form is almost always fatal without immediate antibiotic treatment 1.
Septicemic Plague
Characterized by bloodstream infection without appreciable lymphadenopathy or pneumonia 1. Presents with:
- Fever
- Gastrointestinal symptoms: abdominal pain, nausea, vomiting, diarrhea
- Higher fatality rate than bubonic plague due to diagnostic delays 1
Less Common Forms
Plague meningitis: Typically occurs 9-14 days after inadequate treatment of primary plague 1. Features include:
- Fever, nuchal rigidity, confusion
- CSF neutrophilic pleocytosis with Y. pestis visible on Gram stain
Pharyngeal plague: From consuming inadequately cooked infected meat 1. Presents with pharyngeal inflammation and cervical lymphadenopathy.
Diagnosis
Suspect plague in endemic areas when patients present with painful adenitis or pneumonia with bloody sputum 2. Diagnostic approach:
Microbiological Methods
- Gram stain of lymph node aspirate or biopsy: gram-negative bacilli
- Blood cultures: Y. pestis grows easily
- Sputum culture for pneumonic plague
- Identification via biochemical tests and MALDI-TOF mass spectrometry 2
Additional Testing
- Serological examination
- Rapid diagnostic tests
- PCR 3
Critical diagnostic pitfall: Pneumonic and septicemic plague without adenitis are frequently diagnosed late or retrospectively, contributing to higher mortality 2. When plague is suspected, start treatment immediately while awaiting test results 2.
Treatment
Begin antibiotics immediately upon suspicion—do not wait for laboratory confirmation 2. The evidence shows:
Recommended Antibiotics
While aminoglycosides (particularly gentamicin) are traditionally considered most effective, current recommendations include 2, 4:
- Gentamicin (traditional first-line)
- Doxycycline (currently recommended in France) 3
- Fluoroquinolones (ciprofloxacin)
- Chloramphenicol 5
Streptomycin remains the historical drug of choice but gentamicin or doxycycline should be initiated as soon as plague is suspected 2, 5.
Treatment Timing
Mortality dramatically increases with delays >24 hours in antibiotic and antishock treatment 4. Early diagnosis and prompt treatment reduce mortality to 5-50% for bubonic and septicemic plague 4.
Antibiotic Resistance Considerations
While rare, antibiotic-resistant Y. pestis strains exist through plasmid transfer and gene mutations 6. Streptomycin resistance has been documented via rpsL gene mutations 6. This reinforces the importance of using effective first-line agents and maintaining surveillance.
Prevention
- Active surveillance in endemic plague foci
- Ectoparasite (flea) control
- Avoid contact with sick or dead rodents
- Avoid consuming inadequately cooked meat from potentially infected animals 7
- Isolation measures for pneumonic plague cases to prevent human-to-human transmission 5
Common pitfall: Plague remains endemic in rural Africa, Madagascar, Peru, and the Democratic Republic of Congo 3. Maintain high clinical suspicion in patients from or traveling to these regions presenting with compatible symptoms.