<i>Y</i>. <i>pestis</i> is capable of producing proventricular blockage in <i>C</i>. <i>felis</i>.

<p><b>(A</b>) Female cat flea blocked with <i>Y</i>. <i>pestis</i> expressing GFP, examined immediately after a feeding attempt. Fresh red blood is present in the esophagus only (arrow), with none in the midgut, which contains dark, digested blood from previous blood meals. (<b>B</b>) Bright field and (<b>C</b>) fluorescent microscopy images of the dissected digestive tract of the same blocked flea, showing the dense GFP+ bacterial mass that fills and blocks the proventriculus (PV) and also extends into the esophagus (E) and midgut (MG). Fresh red blood that was blocked from entering the midgut is still observable in the esophagus. (<b>D-F</b>) The same image series for an uninfected cat flea, which has fresh red blood in the midgut and an absence of a dark bacterial mass in the proventriculus. Scale bar = 100 μm.