Table 1.
Outcomes of flea bite challenges.
Fig 1.
Distribution of rapid onset (within four days; IA) and prolonged onset (IB) terminal bubonic plague in mice following early-phase transmission (EPT) by fleas infected using mouse blood or rat blood, or following transmission by blocked fleas. Symbols represent individual mice.
Fig 2.
Examples of rapid onset terminal bubonic plague (A) and primary septicemic plague (B) within 4 days following flea bite. In bubonic plague, bioluminescent signal from the dermis at the flea bite sites appeared within 2 days after challenge (lateral images, top row) and increased in intensity over the following days, indicating bacterial multiplication. Dissemination to the draining lymph node was visible within 3 days (ventral images, bottom row) and systemic spread and terminal disease a day after that, within 4 days. Arrows indicate signal from lymph nodes; L = liver, S = spleen. In primary septicemic plague, no infection at the flea bite site or draining lymph node was seen; bioluminescent signal in peripheral areas appeared suddenly on day 3 together with signs of terminal disease. The scale bars indicate the intensity of bioluminescent signal (photons/sec/cm2/sr). Different scales were used due to the fluctuations in signal intensity throughout the observation period.
Fig 3.
Prolonged onset terminal bubonic plague in a mouse following flea bite.
A permanent focus of infection was detected by day 3 and dissemination to the draining lymph node on day 4–5. The lymph node bioluminescent signal receded and resurged twice before the infection eventually disseminated systemically to produce terminal sepsis on day 19 after transmission. The scale bars indicate the intensity of bioluminescent signal (photons/sec/cm2/sr). Different scales were used due to the fluctuations in signal intensity throughout the observation period.
Fig 4.
Recovery from prolonged bubonic plague infection without systemic dissemination.
(A) Three days after early-phase transmission intradermal infection was evident at two flea bite sites and in the draining inguinal lymph node in this mouse. The infection was resolved in the skin by day 19. Infection of the draining lymph node was evident on days 3–7, but regressed on day 9 before reappearing on day 12 before finally disappearing on day 17. (B) Recovery from bubonic plague infection in a mouse following blocked-flea transmission. The pattern is similar to (A) but involves the axillary lymph node as well as the draining lymph node, and a lessening of intensity of the intradermal infection on day 7–8. Dissemination to the draining inguinal lymph node was evident on day 4 and to the ipsilateral axillary lymph node on day 18, with the intensity of infection regressing and relapsing at both sites until day 24. This mouse appeared to be completely clear of infection on day 26 and was finally euthanized on day 55. The scale bars indicate the intensity of bioluminescent signal (photons/sec/cm2/sr). Different scales were used due to the fluctuations in signal intensity throughout the observation period.
Fig 5.
Distribution of outcomes following early-phase transmission challenges by fleas infected using mouse blood (blue bars) or rat blood (red bars); or following bites by blocked fleas (grey bars).
Data are from Table 1. Productive transmission (Outcome I) is defined as terminal sepsis and bacteremia required to continue the Y. pestis life cycle; the solid portion of the bars represents the percentage of rapid onset (Outcome IA) and the hatched portion prolonged onset (Outcome IB) terminal disease. Nonproductive transmission (Outcome II) is defined as an infection that is resolved before systemic dissemination and terminal sepsis, thus precluding continuation of the flea-borne transmission cycle of Y. pestis. These mice mount an immune response and are at least temporarily removed from the pool of susceptibles in the population. No evidence of infection (no transmission; Outcome III) was detected in other mice following flea bite challenge.
Fig 6.
Persistent intradermal infection at the flea bite site.
A mouse with terminal disease euthanized on day 10 photographed with (A) and without (B) bioluminescence detection. Skin sections taken at the site of infection were stained by H&E (C) and immunohistochemistry (IHC) using anti-Y. pestis antibody (D) and show ulcerative and necrotizing dermatitis extending into the panniculus and large numbers of bacteria (brown color on IHC). Higher magnification of areas demarcated by red boxes in (C) show a large dense microcolony of extracellular Y. pestis surrounded by neutrophils (E) and an area of viable and degenerate neutrophils and macrophages (necrosis) intermixed with Y. pestis (F). Scale bars = 0.5 mm (C, D); 50 μm (E, F).
Fig 7.
Association between the number of infected fleas that fed and early-phase transmission efficiency and disease outcome probabilities.
Left panel: The outcomes of all early-phase transmission challenges (S1 Table) plotted against the number of infected fleas that fed on a mouse. Outcome I = terminal disease; Outcome II = resolved, asymptomatic infection without sepsis, resulting in seroconversion and survival; Outcome III = no transmission. Right panel: Posterior probabilities of each outcome as a function of the number of infected flea bites estimated using a Bayesian ordinal cumulative logistic regression model fit to data from the left panel. Solid lines correspond to the posterior median probabilities of each of the disease outcomes and shaded areas correspond to 95% credible intervals. See text and supplemental S1 text for details.