ESTIMATION OF VECTOR INFECTIVITY RATES FOR PLAGUE BY MEANS OF A STANDARD CURVE-BASED COMPETITIVE POLYMERASE-CHAIN-REACTION METHOD TO QUANTIFY YERSINIA-PESTIS IN FLEAS

The prevalence of infectivity within a vector population is a critical factor in arthropod-borne disease epidemiology but it is difficult to estimate. In the case of bubonic plague, infective flea vectors conta in large numbers of Yersinia pestis within a bacterial mass that block s the flea's foregut, and only such blocked fleas are important for bi ologic transmission. A bacterial quantitation method could therefore b e used to assess the prevalence of plague-infective (blocked) fleas in a population. We developed a standard, curve-based, competitive polym erase chain reaction (PCR) procedure to quantitate Y. pestis in indivi dual fleas. The quantitative PCR (Q-PCR) method equaled a colony count reference method in accuracy and precision when evaluated using mock samples and laboratory-infected fleas. The Q-PCR was more reliable tha n colony count, however, for field-collected fleas and for blocked fle as collected after their death. In a sample of fleas collected from a prairie dog colony in the aftermath of a plague epizootic, 48% were in fected but less than 2% contained numbers of Y. pestis indicative of b lockage. The method provides a means to monitor plague epizootics and associated risks of flea-borne transmission to humans, and is applicab le to the study of other vector-borne diseases.