Yersinia pestis and Rattus rattus


“Few days following the death of the rats,

Men pass away like falling walls!”

Shih Tao-nan in China, writing a poem in 1792


1(Yersinia pestis seen in a fluorescent antibody test, 2000x,


Yersinia pestis is a gram negative bacterium, discovered and named in 1894 by a Swiss/French physician and bacteriologist Alexandre Yersin.  It is the etiologic agent in the plague which is an arthropodborne or zoonotic disease, ie it requires a reservoir (mammalian) and a vector (invertebrate).  The most common host or reservoir is the wild rodent population and the most common vector is the rat flea, Xenopsylla cheopsis.  There are over 200 species of rodents worldwide and at least 80 species of fleas that have been found to carry Y. pestis (Hinnebusch, Fisher and Schwan 1). 



The bacterium may or may not result in overt disease in the rodent host.  It does however result in the eventual death of the vector, not however, by direct infection.  After the flea bites the rodent, the organisms replicate and occlude the digestive tract.  The flea therefore is not able to utilize any nutrients.  It becomes hungry and searches for a new host (human).  As the flea bites, it is unable to swallow the blood because it is “blocked” by the Y. pestis organisms.  The flea then regurgitates the organisms and this ingested blood into the bite, thus contaminating the wound.  The flea also passes contaminated feces onto the human host (Hinnebusch, Fisher and Schwan 1).







Male rat flea, Xenopsylla cheopis, engorged with blood.  This flea is the primary vector of the plague.






(Rattus rattus, or the black rat, the most common host/reservoir of the plague,                                                                                                                                                                                   Life cycle of Yersinia pestis. )                                                                                                                                               




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