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

Abstract

For almost a century, the oriental rat ßea, Xenopsylla cheopis (Rothschild) (Sipho-naptera: Pulicidae), was thought to be the most efÞcient vector of the plague bacterium Yersinia pestis (Yersin). Approximately 2 wk after consuming an infectious bloodmeal, a blockage often forms in the ßeaÕs proventriculus, which forces the ßea to increase its biting frequency and consequently increases the likelihood of transmission. However, if ßeas 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 1Ð 4 d postinfection [ p.i.]) by unblocked ßeas was viewed as anomalous and thought to occur only by mass action. We used an artiÞcial feeding system to infect colony-reared X. cheopis with a fully virulent strain of Y. pestis, and we evaluated transmission efÞciency 1Ð 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 ßeas per host required for unblocked ßeas to drive a plague epizootic by early-phase transmission is within the ßea infestation range observed in nature, and 3) early-phase transmission by unblocked ßeas in the current study was at least as efÞcient as transmission by blocked ßeas in a previously published study using the same colony of ßeas and same bacterial strain. Furthermore, transmission efÞciency seemed to remain constant until block formation, resulting in an infectious period considerably longer than previously thought.