Epidemiology of plague – dominant transmission route for bubonic disease: In human outbreaks of bubonic plague caused by Yersinia pestis, which route is classically responsible for transmission to people?

Introduction / Context:
Plague presents in bubonic, septicemic, and pneumonic forms. Identifying the major transmission route for bubonic plague is essential for vector control and public health responses in endemic settings and during epizootics among rodents.

Given Data / Assumptions:

• Agent: Yersinia pestis.
• Vectors: fleas parasitizing rodents, especially Xenopsylla cheopis.
• Clinical form in question: bubonic plague (painful buboes, regional lymphadenitis).

Concept / Approach:
Bubonic plague follows inoculation of Y. pestis into skin via bites of infected rodent fleas. The bacteria spread via lymphatics to draining nodes, producing buboes. Inhalation primarily causes primary pneumonic plague; ingestion is not a standard route for bubonic disease. Hence the vector bite is the classical route for bubonic cases.

Step-by-Step Solution:
Match clinical syndrome (bubonic) with known pathogenesis (lymphatic spread after skin inoculation). Identify the arthropod vector: rat flea. Exclude inhalation (pneumonic plague) and ingestion (nonclassical). Choose flea bite as the correct route.

Verification / Alternative check:
Historic and modern outbreaks correlate human bubonic cases with epizootics in rats and increased flea indices; vector control curbs transmission.

Why Other Options Are Wrong:
Inhalation relates to pneumonic plague; ingestion is not a typical route; “all of these” is overly inclusive for the bubonic form.

Common Pitfalls:
Conflating bubonic with pneumonic plague transmission dynamics.

Final Answer:
Rat flea (Xenopsylla cheopis) bite.