Early-Phase Transmission of<i>Yersinia pestis</i>by Unblocked<i>Xenopsylla cheopis</i>(Siphonaptera: Pulicidae) Is as Efficient as Transmission by Blocked Fleas

Abstract

For almost a century, the oriental rat ea, Xenopsylla cheopis (Rothschild) (Siphonaptera: Pulicidae), was thought to be the most efcient vector of the plague bacterium Yersinia pestis (Yersin). Approximately 2 wk after consuming an infectious bloodmeal, a blockage often forms in the eas proventriculus, which forces the ea to increase its biting frequency and consequently increases the likelihood of transmission. However, if eas remain blocked and continue to feed, they usually die within 5 d of blocking, resulting in a short infectious window. Despite observations of X. cheopis transmitting Y. pestis shortly after pathogen acquisition, early-phase transmission (e.g., transmission 1 4 d postinfection [ p.i.]) by unblocked eas was viewed as anomalous and thought to occur only by mass action. We used an articial feeding system to infect colony-reared X. cheopis with a fully virulent strain of Y. pestis, and we evaluated transmission efciency 1 4 d p.i. We demonstrate 1) that a single infected and unblocked X. cheopis can infect a susceptible host as early as 1 d p.i., 2) the number of eas per host required for unblocked eas to drive a plague epizootic by early-phase transmission is within the ea infestation range observed in nature, and 3) early-phase transmission by unblocked eas in the current study was at least as efcient as transmission by blocked eas in a previously published study using the same colony of eas and same bacterial strain. Furthermore, transmission efciency seemed to remain constant until block formation, resulting in an infectious period considerably longer than previously thought.