Black Death Transmission: The Rodent-Flea-Human Infection Cycle

Plague (Yersinia pestis) transmission follows a precise enzootic-epizootic-epidemic cascade:

1. Rodent Reservoir (Enzootic Cycle)

• Wild rodents (ground squirrels, prairie dogs, gerbils) maintain low-level circulation.
• Resistant species survive; susceptible species die off → epizootic.

2. Flea Vector Dynamics

• Fleas (Xenopsylla cheopis, others) feed on bacteremic rodents.
• Y. pestis forms biofilm in proventriculus → blockage → "blocked flea" regurgitates bacteria during futile feeding attempts.
• Blocked fleas feed aggressively, increasing transmission efficiency.

3. Human Infection (Bubonic Plague)

• Flea bite → bacilli drain to regional lymph nodes → buboes (suppurative lymphadenitis).
• Incubation: 2–6 days. Fever, chills, extreme bubo tenderness.
• Without treatment: 50–60% mortality; bacteremia → septicemic plague.

4. Septicemic Plague

• Primary (direct bacteremia) or secondary (bubonic progression).
• DIC, purpura, multi-organ failure. High mortality even with treatment.

5. Pneumonic Plague (Person-to-Person)

• Secondary to septicemic (hematogenous lung seeding) or primary (inhalation).
• Respiratory droplets → primary pneumonic in contacts.
• Incubation 1–3 days; fulminant pneumonia, hemoptysis. 100% fatal if untreated >24h.

Modern Control

Rodent surveillance, flea control, rapid diagnostics, streptomycin/doxycycline/fluoroquinolones, contact prophylaxis. Bioweapon potential (Category A) drives preparedness.