Studies of Vector Competency and Efficiency of North American Fleas for <I>Yersinia pestis</I>: State of the Field and Future Research Needs

The etiological agent of plague, Yersinia pestis, is most commonly transmitted by the bite of infectious fleas. To date, at least 28 flea species occurring in North America have been experimentally confirmed as vectors of Y. pestis. Transmission efficiency differs among species and also between different studies of a single species. These differences may, however, in large part reflect nonstandardized experimental conditions used during the first half of the 20th century when such studies were conducted in response to the rapid spread of Y. pestis across the western United States after its introduction at the beginning of this century. The majority of these early transmission studies focused on the blocked flea mechanism of transmission, which typically does not occur until > 2-3 wk after the flea becomes infected. Recent studies have challenged the paradigm that Y. pestis is usually spread by blocked fleas by demonstrating that numerous flea species, including the oriental rat flea Xenopsylla cheopis, which was the focus of the early classical studies on blocked flea transmission, are capable of"early-phase" transmission during the first few days after becoming infected and before a complete blockage can form. The aims of this review are to 1) summarize Y. pestis vector competency and efficiency studies for fleas occurring in North America, 2) discuss the implications of the results of these studies for our understanding of the dynamics of plague epizootics, 3) demonstrate why older transmission studies need to be repeated using a standardized experimental system, and 4) outline future directions for studies of fleas as vectors of Y. pestis.