Signs, Symptoms, and Transmission of Plague
Plague is primarily transmitted to humans through the bite of infected fleas, direct contact with infected tissues or fluids, or inhalation of infectious respiratory droplets, with distinct clinical presentations including bubonic, pneumonic, and septicemic forms, each with characteristic symptoms and high fatality rates if untreated. 1
Modes of Transmission
- Flea vector transmission: The most common mode of transmission is through the bite of infected fleas, particularly the Oriental rat flea (Xenopsylla cheopis) and other flea species 1, 2
- Direct contact: Infection can occur through direct contact with infectious body fluids or tissues while handling infected animals 1
- Inhalation: Pneumonic plague can be acquired by inhaling infectious respiratory droplets from humans or animals with plague pneumonia 1
- Person-to-person: Only pneumonic plague is transmissible from person to person via large respiratory droplets, requiring close contact (within 6 feet) 1
Clinical Presentations and Symptoms
1. Bubonic Plague (80-90% of cases)
- Incubation period: 2-6 days 1, 2
- Key symptoms:
- Mortality: 50-60% if untreated; 13% with appropriate antimicrobial treatment 1
- Important note: Some patients may develop "occult buboes" in deep lymph nodes that are difficult to detect on examination, leading to diagnostic delays 3
2. Pneumonic Plague
- Incubation period: 1-3 days 1, 2
- Key symptoms:
- Mortality: Nearly 100% if untreated; treatment must begin within 18 hours of symptom onset for survival 1
3. Septicemic Plague
- Types: Primary (without lymphadenopathy) or secondary to bubonic plague 1
- Key symptoms:
- Fever
- Gastrointestinal symptoms (abdominal pain, nausea, vomiting, diarrhea)
- No appreciable lymphadenopathy in primary form 1
- Mortality: Higher than bubonic plague; approximately 50% mortality reported in US cases from 1947-1977 1
- Complications: Septic shock, consumptive coagulopathy, meningitis, coma 1
4. Less Common Presentations
Plague meningitis:
Plague pharyngitis:
- Occurs from contamination of oropharynx with infected materials
- Symptoms: Pharyngeal inflammation and cervical lymphadenopathy 1
Epidemiology and Risk Factors
- Plague exists in natural enzootic cycles involving wild rodents and their fleas in certain regions of Asia, Africa, the Americas, and southeastern Europe 1
- Risk increases during epizootics that cause high mortality in rodent populations, forcing infected fleas to seek alternative hosts, including humans 1, 2
- In the United States, most cases occur in southwestern states, particularly in coniferous forest habitats 2
- Seasonal pattern with most human cases occurring during summer months when flea activity is highest 2
Prevention and Early Recognition
- Avoid rodent-infested areas during recreational activities in endemic regions 2
- Apply insect repellents when in endemic areas 2
- Healthcare providers should consider plague in patients with compatible symptoms who have traveled to plague-endemic areas 4
- Early diagnosis and prompt treatment are critical, especially for pneumonic plague 5
- Plague is a reportable disease requiring immediate notification of public health authorities 2
Key Diagnostic Considerations
- Plague should be suspected in endemic areas when patients present with adenitis or pneumonia with bloody expectorate 6
- The typical bubo is painful, red, and non-fluctuating 6
- Diagnosis can be confirmed by identifying gram-negative bacilli in lymph node aspirate or blood cultures 6
- Delays in diagnosis are common with occult buboes and non-bubonic forms of plague 3
Early recognition of plague symptoms and prompt initiation of appropriate antibiotic therapy are essential to reduce mortality from this potentially fatal disease.