Bubonic Plague Cause: Yersinia pestis Biology, Transmission, and History

The bubonic plague (historically known as the Black Death) is caused by Yersinia pestis, a gram-negative, non-motile, facultative anaerobic coccobacillus.

Transmission Cycle

1. Reservoir: Rodents including rats, ground squirrels, and prairie dogs.
2. Vector: Fleas (Xenopsylla cheopis). Bacteria block the flea's proventriculus, causing regurgitation during feeding.
3. Human infection: Flea bite transmits bubonic plague (lymphadenitis); respiratory droplets transmit pneumonic plague (person-to-person).

Pathogenesis

• Type III secretion system injects Yops (effectors) that inhibit phagocytosis, cytokine signaling, and apoptosis.
• LPS (endotoxin) drives septic shock in septicemic plague.
• F1 capsule provides antiphagocytic protection.

Clinical Forms

• Bubonic: Painful buboes (inguinal, axillary, cervical), fever, sepsis.
• Septicemic: Bacteremia, DIC, necrosis (acral gangrene—the "Black Death" namesake).
• Pneumonic: Rapid pneumonia, hemoptysis, airborne spread.

History and Current Status

• Central Asian origin; 14th century pandemic killed 30–50% of Europe's population.
• Endemic in rodent populations across the southwestern US, Africa, Asia, and South America.
• Treatable with streptomycin, gentamicin, doxycycline, or ciprofloxacin. Early treatment yields good prognosis.
• Vaccine not routinely used.