Interaction of plague microbe strains varying in plasmid composition with the fleas Xenopsylla cheopis (Roths. 1903)

: The interaction of two Yersinia pestis strains varying in plasmid composition with the fleas Xenopsylla cheopis was studied. The reference virulent strain I-2638 having four plasmids (pCad, pPst, pFra, and pTP 33) and its selected avirulent strain I-3480 that had lost the plasmids pCad and pPst formed a proventricular block in the fleas with equal frequency. There were no differences in the block-forming capacity of these strains among the infected females; however, the stock strain was more active in blocking the proventriculus in females in spring than was the mutant one. in summer. The fleas infected with a defect strain failed to transmit the pathogen. It follows that the presence of a proventricular block is not an indicator of how effectively the fleas transmit the causative agent of plague. While being in the insect, both strains of plague microbe did not alter their biological properties. In experiments, the death rate for insects infected with different strains was similar, but higher in spring than that in summer. The males naturally died more frequently than the females.