Transmission efficiency of the plague pathogen (Y. pestis) by the flea, Xenopsylla skrjabini, to mice and great gerbils

Abstract Background Plague, a zoonotic disease caused by Yersinia pestis, is characterized by its ability to persist in the plague natural foci. Junggar Basin plague focus was recently identified in China, with Rhombomys opimus (great gerbils) and Xenopsylla skrjabini as the main reservoir and vector for plague. No transmission efficiency data of X. skrjabini for Y. pestis is available till now. Methods In this study, we estimated the median infectious dose (ID50) and the blockage rates of X. skrjabini with Y. pestis, by using artificial feeders. We then evaluated the flea transmission ability of Y. pestis to the mice and great gerbils via artificial bloodmeal feeding. Finally, we investigated the transmission of Y. pestis to mice with fleas fed by infected great gerbils. Results ID50 of Y. pestis to X. skrjabini was estimated as 2.04 × 105 CFU (95% CI, 1.45 × 105 – 3.18 × 105 CFU), around 40 times higher than that of X. cheopis. Although fleas fed by higher bacteremia bloodmeal had higher infection rates for Y. pestis, they lived significantly shorter than their counterparts. X. skrjabini could get fully blocked as early as day 3 post of infection (7.1%, 3/42 fleas), and the overall blockage rate of X. cheopis was estimated as 14.9% (82/550 fleas) during the 14 days of investigation. For the fleas infected by artificial feeders, they seemed to transmit plague more efficiently to great gerbils than mice. Our single flea transmission experiments also revealed that, the transmission capacity of naturally infected fleas (fed by infected great gerbils) was significantly higher than that of artificially infected ones (fed by artificial feeders). Conclusion Our results indicated that ID50 of Y. pestis to X. skrjabini was higher than other fleas like X. cheopis, and its transmission efficiency to mice might be lower than other flea vectors in the artificial feeding modes. We also found different transmission potentials in the artificially infected fleas and the naturally infected ones. Further studies are needed to figure out the role of X. skrjabini in the plague epidemiological cycles in Junggar Basin plague focus

Dynamics of the F1-antibody in great gerbils challenged by different doses of <i>Y. pestis.</i>

a<p>:The antibody titer was recorded as the dilution time, by setting 1∶8 as “1”, 1∶16 as “2”, 1∶32 as “3”, and 1∶4096 as “10”. The antibody was determined by IHA and each sample was repeated three times.</p>b<p>:Only one of the 4 animals produced F1-Ab and there was no measurable F1-Ab for the other three animals.</p>c<p>:Only one animal was available for F1-Ab determination because of the unexpected death of the other three animals.</p

Gross anatomic changes in the liver and spleen of great gerbils (A) and guinea pigs (B).

<p>Approximately 7.4×10¹⁰ CFU of <i>Y. </i><i>pestis</i> strain 2505 was injected subcutaneously into the groin of great gerbils, whereas around 5.0×10⁴ CFU was injected subcutaneously into the groin of guinea pigs. The animals were dissected immediately after death on day 3 p.i. The abscesses are clearly seen on the surface of both the liver and spleen of guinea pigs, but no abscesses were observed on the corresponding organs of great gerbils.</p

Dynamics of bacterial load (BL) in both the liver and spleen of great gerbils challenged with 2.0×10¹⁰ CFU of <i>Y. pestis</i>.

*<p>: At day 14 p.i., <i>Y. pestis</i> was isolated from the liver of a live animal with a bacterial load of 4.31 CFU/g.</p>**<p>: At day 15 p.i., <i>Y. pestis</i> was isolated from the spleen of a live animal with a bacterial load of 176 CFU/g.</p><p>ζ: Six great gerbils died of non-specific causes.</p

Post infection changes in the average body weight (A) and average anal temperature (B).

<p>Approximately 2.0×10⁹ CFU of <i>Y. pestis</i> strain 2505 was injected subcutaneously into the groin of 18 great gerbils on day 0, and then observed for changes in body weight and anal temperature. The average body weight and anal temperature of the 18 animals were employed to represent the changing trends at different time points (days) post infection.</p