Quantities of Yersinia pestis in fleas (Siphonaptera : Pulicidae, Ceratophyllidae, and Hystrichopsyllidae) collected from areas of known or suspectedplague activity

We used a quantitative competitive polymerase chain reaction (PCR) (QC-PCR) to determine bacterial loads in 669 fleas collected in areas of confirmed and suspected plague epizootics. Fleas were collected out of rodent borrows (67.9%) and off of captured animals (24.1%) and rodent carcasses (8.1%). A n initial PCR screening assay indicated that 12.1% (81/669) of all fleas we re positive for. Yersinia pestis Fleas collected from burrows had significa ntly lower (chi(2) = 264.9, P < 0.0001) infection rates (6.8%) but signific antly higher (Student t-test, P < 0.0001) bacterial loads (mean = 10(5.6) Y . pestis per flea) than fleas collected off of rodent carcasses (infection I ate = 92.6%; mean bacterial load = 10(4.8) Y. pestis per nea). None of th e fleas collected off of captured animals were positive for Y. pestis by PC R although seven of the 176 captured animals were serologically positive fo r Y. pestis.