Paper I
Question
Name the causative agent and transmitting vector for plague
Answer
Plague is caused by Yersinia pestis, a Gram-negative coccobacillus (showing characteristic bipolar/“safety-pin” staining with Wayson or Giemsa stain).
Transmitting vector: the rat flea, Xenopsylla cheopis — transmits the organism between infected rodents (the natural reservoir, e.g., rats) and to humans through its bite; the flea’s proventriculus becomes blocked by a mass of multiplying bacilli (“blocked flea”), causing it to regurgitate infected blood into the bite wound when it attempts to feed, efficiently inoculating the organism into the new host. (Person-to-person transmission of the pneumonic form occurs directly via respiratory droplets, without requiring the flea vector.)