Early divergent strains of Yersinia pestis in Eurasia 5,000 years ago.

The bacteria Yersinia pestis is the etiological agent of plague and has caused human pandemics with millions of deaths in historic times. How and when it originated remains contentious. Here, we report the oldest direct evidence of Yersinia pestis identified by ancient DNA in human teeth from Asia and Europe dating from 2,800 to 5,000 years ago. By sequencing the genomes, we find that these ancient plague strains are basal to all known Yersinia pestis. We find the origins of the Yersinia pestis lineage to be at least two times older than previous estimates. We also identify a temporal sequence of genetic changes that lead to increased virulence and the emergence of the bubonic plague. Our results show that plague infection was endemic in the human populations of Eurasia at least 3,000 years before any historical recordings of pandemics.

Yersinia pestis can infect the Pawlowsky glands of human body lice and be transmitted by louse bite.

Yersinia pestis, the causative agent of plague, is a highly lethal vector-borne pathogen responsible for killing large portions of Europe's population during the Black Death of the Middle Ages. In the wild, Y. pestis cycles between fleas and rodents; occasionally spilling over into humans bitten by infectious fleas. For this reason, fleas and the rats harboring them have been considered the main epidemiological drivers of previous plague pandemics. Human ectoparasites, such as the body louse (Pediculus humanus humanus), have largely been discounted due to their reputation as inefficient vectors of plague bacilli. Using a membrane-feeder adapted strain of body lice, we show that the digestive tract of some body lice become chronically infected with Y. pestis at bacteremia as low as 1 × 105 CFU/ml, and these lice routinely defecate Y. pestis. At higher bacteremia (≥1 × 107 CFU/ml), a subset of the lice develop an infection within the Pawlowsky glands (PGs), a pair of putative accessory salivary glands in the louse head. Lice that developed PG infection transmitted Y. pestis more consistently than those with bacteria only in the digestive tract. These glands are thought to secrete lubricant onto the mouthparts, and we hypothesize that when infected, their secretions contaminate the mouthparts prior to feeding, resulting in bite-based transmission of Y. pestis. The body louse's high level of susceptibility to infection by gram-negative bacteria and their potential to transmit plague bacilli by multiple mechanisms supports the hypothesis that they may have played a role in previous human plague pandemics and local outbreaks.

Acquisition of yersinia murine toxin enabled Yersinia pestis to expand the range of mammalian hosts that sustain flea-borne plague.

Yersinia murine toxin (Ymt) is a phospholipase D encoded on a plasmid acquired by Yersinia pestis after its recent divergence from a Yersinia pseudotuberculosis progenitor. Despite its name, Ymt is not required for virulence but acts to enhance bacterial survival in the flea digestive tract. Certain Y. pestis strains circulating in the Bronze Age lacked Ymt, suggesting that they were not transmitted by fleas. However, we show that the importance of Ymt varies with host blood source. In accordance with the original description, Ymt greatly enhanced Y. pestis survival in fleas infected with bacteremic mouse, human, or black rat blood. In contrast, Ymt was much less important when fleas were infected using brown rat blood. A Y. pestis Ymt- mutant infected fleas nearly as well as the Ymt+ parent strain after feeding on bacteremic brown rat blood, and the mutant was transmitted efficiently by flea bite during the first weeks after infection. The protective function of Ymt correlated with red blood cell digestion kinetics in the flea gut. Thus, early Y. pestis strains that lacked Ymt could have been maintained in flea-brown rat transmission cycles, and perhaps in other hosts with similar blood characteristics. Acquisition of Ymt, however, served to greatly expand the range of hosts that could support flea-borne plague.

Acceleration of plague outbreaks in the second pandemic.

Historical records reveal the temporal patterns of a sequence of plague epidemics in London, United Kingdom, from the 14th to 17th centuries. Analysis of these records shows that later epidemics spread significantly faster ("accelerated"). Between the Black Death of 1348 and the later epidemics that culminated with the Great Plague of 1665, we estimate that the epidemic growth rate increased fourfold. Currently available data do not provide enough information to infer the mode of plague transmission in any given epidemic; nevertheless, order-of-magnitude estimates of epidemic parameters suggest that the observed slow growth rates in the 14th century are inconsistent with direct (pneumonic) transmission. We discuss the potential roles of demographic and ecological factors, such as climate change or human or rat population density, in driving the observed acceleration.

Comparison of the transmission efficiency and plague progression dynamics associated with two mechanisms by which fleas transmit Yersinia pestis.

Yersinia pestis can be transmitted by fleas during the first week after an infectious blood meal, termed early-phase or mass transmission, and again after Y. pestis forms a cohesive biofilm in the flea foregut that blocks normal blood feeding. We compared the transmission efficiency and the progression of infection after transmission by Oropsylla montana fleas at both stages. Fleas were allowed to feed on mice three days after an infectious blood meal to evaluate early-phase transmission, or after they had developed complete proventricular blockage. Transmission was variable and rather inefficient by both modes, and the odds of early-phase transmission was positively associated with the number of infected fleas that fed. Disease progression in individual mice bitten by fleas infected with a bioluminescent strain of Y. pestis was tracked. An early prominent focus of infection at the intradermal flea bite site and dissemination to the draining lymph node(s) soon thereafter were common features, but unlike what has been observed in intradermal injection models, this did not invariably lead to further systemic spread and terminal disease. Several of these mice resolved the infection without progression to terminal sepsis and developed an immune response to Y. pestis, particularly those that received an intermediate number of early-phase flea bites. Furthermore, two distinct types of terminal disease were noted: the stereotypical rapid onset terminal disease within four days, or a prolonged onset preceded by an extended, fluctuating infection of the lymph nodes before eventual systemic dissemination. For both modes of transmission, bubonic plague rather than primary septicemic plague was the predominant disease outcome. The results will help to inform mathematical models of flea-borne plague dynamics used to predict the relative contribution of the two transmission modes to epizootic outbreaks that erupt periodically from the normal enzootic background state.

A refined model of how Yersinia pestis produces a transmissible infection in its flea vector.

In flea-borne plague, blockage of the flea's foregut by Yersinia pestis hastens transmission to the mammalian host. Based on microscopy observations, we first suggest that flea blockage results from primary infection of the foregut and not from midgut colonization. In this model, flea infection is characterized by the recurrent production of a mass that fills the lumen of the proventriculus and encompasses a large number of Y. pestis. This recurrence phase ends when the proventricular cast is hard enough to block blood ingestion. We further showed that ymt (known to be essential for flea infection) is crucial for cast production, whereas the hmsHFRS operon (known to be essential for the formation of the biofilm that blocks the gut) is needed for cast consolidation. By screening a library of mutants (each lacking a locus previously known to be upregulated in the flea gut) for biofilm formation, we found that rpiA is important for flea blockage but not for colonization of the midgut. This locus may initially be required to resist toxic compounds within the proventricular cast. However, once the bacterium has adapted to the flea, rpiA helps to form the biofilm that consolidates the proventricular cast. Lastly, we used genetic techniques to demonstrate that ribose-5-phosphate isomerase activity (due to the recent gain of a second copy of rpiA (y2892)) accentuated blockage but not midgut colonization. It is noteworthy that rpiA is an ancestral gene, hmsHFRS and rpiA2 were acquired by the recent ancestor of Y. pestis, and ymt was acquired by Y. pestis itself. Our present results (i) highlight the physiopathological and molecular mechanisms leading to flea blockage, (ii) show that the role of a gene like rpiA changes in space and in time during an infection, and (iii) emphasize that evolution is a gradual process punctuated by sudden jumps.

"Fleaing" the Plague: Adaptations of Yersinia pestis to Its Insect Vector That Lead to Transmission.

Interest in arthropod-borne pathogens focuses primarily on how they cause disease in humans. How they produce a transmissible infection in their arthropod host is just as critical to their life cycle, however. Yersinia pestis adopts a unique life stage in the digestive tract of its flea vector, characterized by rapid formation of a bacterial biofilm that is enveloped in a complex extracellular polymeric substance. Localization and adherence of the biofilm to the flea foregut is essential for transmission. Here, we review the molecular and genetic mechanisms of these processes and present a comparative evaluation and updated model of two related transmission mechanisms.

Historical and genomic data reveal the influencing factors on global transmission velocity of plague during the Third Pandemic.

Quantitative knowledge about which natural and anthropogenic factors influence the global spread of plague remains sparse. We estimated the worldwide spreading velocity of plague during the Third Pandemic, using more than 200 years of extensive human plague case records and genomic data, and analyzed the association of spatiotemporal environmental factors with spreading velocity. Here, we show that two lineages, 2.MED and 1.ORI3, spread significantly faster than others, possibly reflecting differences among strains in transmission mechanisms and virulence. Plague spread fastest in regions with low population density and high proportion of pasture- or forestland, findings that should be taken into account for effective plague monitoring and control. Temperature exhibited a nonlinear, U-shaped association with spread speed, with a minimum around 20 °C, while precipitation showed a positive association. Our results suggest that global warming may accelerate plague spread in warm, tropical regions and that the projected increased precipitation in the Northern Hemisphere may increase plague spread in relevant regions.

Yersinia pestis: the Natural History of Plague.

The Gram-negative bacterium Yersinia pestis is responsible for deadly plague, a zoonotic disease established in stable foci in the Americas, Africa, and Eurasia. Its persistence in the environment relies on the subtle balance between Y. pestis-contaminated soils, burrowing and nonburrowing mammals exhibiting variable degrees of plague susceptibility, and their associated fleas. Transmission from one host to another relies mainly on infected flea bites, inducing typical painful, enlarged lymph nodes referred to as buboes, followed by septicemic dissemination of the pathogen. In contrast, droplet inhalation after close contact with infected mammals induces primary pneumonic plague. Finally, the rarely reported consumption of contaminated raw meat causes pharyngeal and gastrointestinal plague. Point-of-care diagnosis, early antibiotic treatment, and confinement measures contribute to outbreak control despite residual mortality. Mandatory primary prevention relies on the active surveillance of established plague foci and ectoparasite control. Plague is acknowledged to have infected human populations for at least 5,000 years in Eurasia. Y. pestis genomes recovered from affected archaeological sites have suggested clonal evolution from a common ancestor shared with the closely related enteric pathogen Yersinia pseudotuberculosis and have indicated that ymt gene acquisition during the Bronze Age conferred Y. pestis with ectoparasite transmissibility while maintaining its enteric transmissibility. Three historic pandemics, starting in 541 AD and continuing until today, have been described. At present, the third pandemic has become largely quiescent, with hundreds of human cases being reported mainly in a few impoverished African countries, where zoonotic plague is mostly transmitted to people by rodent-associated flea bites.

Human ectoparasites and the spread of plague in Europe during the Second Pandemic.

Plague, caused by the bacterium Yersinia pestis, can spread through human populations by multiple transmission pathways. Today, most human plague cases are bubonic, caused by spillover of infected fleas from rodent epizootics, or pneumonic, caused by inhalation of infectious droplets. However, little is known about the historical spread of plague in Europe during the Second Pandemic (14-19th centuries), including the Black Death, which led to high mortality and recurrent epidemics for hundreds of years. Several studies have suggested that human ectoparasite vectors, such as human fleas (Pulex irritans) or body lice (Pediculus humanus humanus), caused the rapidly spreading epidemics. Here, we describe a compartmental model for plague transmission by a human ectoparasite vector. Using Bayesian inference, we found that this model fits mortality curves from nine outbreaks in Europe better than models for pneumonic or rodent transmission. Our results support that human ectoparasites were primary vectors for plague during the Second Pandemic, including the Black Death (1346-1353), ultimately challenging the assumption that plague in Europe was predominantly spread by rats.

Nutrient depletion may trigger the Yersinia pestis OmpR-EnvZ regulatory system to promote flea-borne plague transmission.

The flea's lumen gut is a poorly documented environment where the agent of flea-borne plague, Yersinia pestis, must replicate to produce a transmissible infection. Here, we report that both the acidic pH and osmolarity of the lumen's contents display simple harmonic oscillations with different periods. Since an acidic pH and osmolarity are two of three known stimuli of the OmpR-EnvZ two-component system in bacteria, we investigated the role and function of this Y. pestis system in fleas. By monitoring the in vivo expression pattern of three OmpR-EnvZ-regulated genes, we concluded that the flea gut environment triggers OmpR-EnvZ. This activation was not, however, correlated with changes in pH and osmolarity but matched the pattern of nutrient depletion (the third known stimulus for OmpR-EnvZ). Lastly, we found that the OmpR-EnvZ and the OmpF porin are needed to produce the biofilm that ultimately obstructs the flea's gut and thus hastens the flea-borne transmission of plague. Taken as a whole, our data suggest that the flea gut is a complex, fluctuating environment in which Y. pestis senses nutrient depletion via OmpR-EnvZ. Once activated, the latter triggers a molecular program (including at least OmpF) that produces the biofilm required for efficient plague transmission.

Infectious blood source alters early foregut infection and regurgitative transmission of Yersinia pestis by rodent fleas.

Fleas can transmit Yersinia pestis by two mechanisms, early-phase transmission (EPT) and biofilm-dependent transmission (BDT). Transmission efficiency varies among flea species and the results from different studies have not always been consistent. One complicating variable is the species of rodent blood used for the infectious blood meal. To gain insight into the mechanism of EPT and the effect that host blood has on it, fleas were fed bacteremic mouse, rat, guinea pig, or gerbil blood; and the location and characteristics of the infection in the digestive tract and transmissibility of Y. pestis were assessed 1 to 3 days after infection. Surprisingly, 10-28% of two rodent flea species fed bacteremic rat or guinea pig blood refluxed a portion of the infected blood meal into the esophagus within 24 h of feeding. We term this phenomenon post-infection esophageal reflux (PIER). In contrast, PIER was rarely observed in rodent fleas fed bacteremic mouse or gerbil blood. PIER correlated with the accumulation of a dense mixed aggregate of Y. pestis, red blood cell stroma, and oxyhemoglobin crystals that filled the proventriculus. At their next feeding, fleas with PIER were 3-25 times more likely to appear partially blocked, with fresh blood retained within the esophagus, than were fleas without PIER. Three days after feeding on bacteremic rat blood, groups of Oropsylla montana transmitted significantly more CFU than did groups infected using mouse blood, and this enhanced transmission was biofilm-dependent. Our data support a model in which EPT results from regurgitation of Y. pestis from a partially obstructed flea foregut and that EPT and BDT can sometimes temporally overlap. The relative insolubility of the hemoglobin of rats and Sciurids and the slower digestion of their blood appears to promote regurgitative transmission, which may be one reason why these rodents are particularly prominent in plague ecology.

Intervention To Stop Transmission of Imported Pneumonic Plague - Uganda, 2019.

Plague, an acute zoonosis caused by Yersinia pestis, is endemic in the West Nile region of northwestern Uganda and neighboring northeastern Democratic Republic of the Congo (DRC) (1-4). The illness manifests in multiple clinical forms, including bubonic and pneumonic plague. Pneumonic plague is rare, rapidly fatal, and transmissible from person to person via respiratory droplets. On March 4, 2019, a patient with suspected pneumonic plague was hospitalized in West Nile, Uganda, 4 days after caring for her sister, who had come to Uganda from DRC and died shortly thereafter, and 2 days after area officials received a message from a clinic in DRC warning of possible plague. The West Nile-based Uganda Virus Research Institute (UVRI) plague program, together with local health officials, commenced a multipronged response to suspected person-to-person transmission of pneumonic plague, including contact tracing, prophylaxis, and education. Plague was laboratory-confirmed, and no additional transmission occurred in Uganda. This event transpired in the context of heightened awareness of cross-border disease spread caused by ongoing Ebola virus disease transmission in DRC, approximately 400 km to the south. Building expertise in areas of plague endemicity can provide the rapid detection and effective response needed to mitigate epidemic spread and minimize mortality. Cross-border agreements can improve ability to respond effectively.

Animal Exposure and Human Plague, United States, 1970-2017.

Since 1970, >50% of patients with plague in the United States had interactions with animals that might have led to infection. Among patients with pneumonic plague, nearly all had animal exposure. Improved understanding of the varied ways in which animal contact might increase risk for infection could enhance prevention messages.

The Third Plague Pandemic in Europe.

Plague has a long history on the European continent, with evidence of the disease dating back to the Stone Age. Plague epidemics in Europe during the First and Second Pandemics, including the Black Death, are infamous for their widespread mortality and lasting social and economic impact. Yet, Europe still experienced plague outbreaks during the Third Pandemic, which began in China and spread globally at the end of the nineteenth century. The digitization of international records of notifiable diseases, including plague, has enabled us to retrace the introductions of the disease to Europe from the earliest reported cases in 1899, to its disappearance in the 1940s. Using supplemental literature, we summarize the potential sources of plague in Europe and the transmission of the disease, including the role of rats. Finally, we discuss the international efforts aimed at prevention and intervention measures, namely improved hygiene and sanitation, that ultimately led to the disappearance of plague in Europe.

Transcriptional regulation of Yersinia pestis biofilm formation.

Yersinia pestis, the causative agent of plague, is transmitted primarily by infected fleas in nature. Y. pestis can produce biofilms that block flea's proventriculus and promote flea-borne transmission. Transcriptional regulation of Y. pestis biofilm formation plays an important role in the response to complex changes in environments, including temperature, pH, oxidative stress, and restrictive nutrition conditions, and contributes to Y. pestis growth, reproduction, transmission, and pathogenesis. A set of transcriptional regulators involved in Y. pestis biofilm production simultaneously controls a variety of biological functions and physiological pathways. Interactions between these regulators contribute to the development of Y. pestis gene regulatory networks, which are helpful for a quick response to complex environmental changes and better survival. The roles of crucial factors and regulators involved in response to complex environmental signals and Y. pestis biofilm formation as well as the precise gene regulatory networks are discussed in this review, which will give a better understanding of the complicated mechanisms of transcriptional regulation in Y. pestis biofilm formation.

Plague: Recognition, Treatment, and Prevention.

Plague is caused by Yersinia pestis and is not commonly encountered in clinics, although natural plague foci are widely distributed around the world. Y. pestis has been listed as a category A bioterrorism agent. A neglected diagnosis will cause severe consequences. Therefore, this minireview briefly introduces the current understanding on Y. pestis and then focuses on practical aspects of plague, including clinical manifestations, diagnosis, treatment, and prevention, to alert clinicians about this notorious disease.

Molecular epidemiological investigations of plague in Eastern Province of Zambia.

BACKGROUND: Plague is a flea-borne zoonotic and invasive disease caused by a gram negative coccobacillus bacterium called Yersinia pestis. Plague has caused three devastating pandemics globally namely: the Justinian, Black Death and Oriental plague. The disease in the Eastern Province of Zambia has been reported in Nyimba and Sinda Districts in the past 15 years. The aim of this study was to investigate the molecular epidemiology of plague in the two affected districts. Polymerase Chain Reaction (PCR), targeting Plasminogen activator gene (pla gene) of Y. pestis, was performed on suspected human bubo aspirates (n = 7), rodents (n = 216), shrews (n = 27) and fleas (n = 1494). Of these, one positive sample from each source or host was subjected to sequencing followed by phylogenetic analysis. RESULTS: The plasminogen activator gene (pla gene) of Y. pestis was detected in 42.8% bubo aspirates, 6.9% rodents, 3.7% shrew and 0.8% fleas. The fleas were from pigs (n = 4), goats (n = 5) and rodents (n = 3). The sequencing and phylogenetic analysis suggested that the pla gene of Y. pestis in Nyimba and Sinda was similar and the isolates demonstrated a high degree of evolutionary relationship with Antiqua strains from the Republic of Congo and Kenya. CONCLUSION: It can be concluded that pla gene of Y. pestis was present in various hosts in the two districts and the strains circulating in each district were similar and resembles those in the Republic of Congo and Kenya.

Pre-industrial plague transmission is mediated by the synergistic effect of temperature and aridity index.

BACKGROUND: Although the linkage between climate change and plague transmission has been proposed in previous studies, the dominant approach has been to address the linkage with traditional statistical methods, while the possible non-linearity, non-stationarity and low frequency domain of the linkage has not been fully considered. We seek to address the above issue by investigating plague transmission in pre-industrial Europe (AD1347-1760) at both continental and country levels. METHODS: We apply Granger Causality Analysis to identify the casual relationship between climatic variables and plague outbreaks. We then apply Wavelet Analysis to explore the non-linear and non-stationary association between climate change and plague outbreaks. RESULTS: Our results show that 5-year lagged temperature and aridity index are the significant determinants of plague outbreaks in pre-industrial Europe. At the multi-decadal time scale, there are more frequent plague outbreaks in a cold and arid climate. The synergy of temperature and aridity index, rather than their individual effect, is more imperative in driving plague outbreaks, which is valid at both the continental and country levels. CONCLUSIONS: Plague outbreaks come after cold and dry spells. The multi-decadal climate variability is imperative in driving the cycles of plague outbreaks in pre-industrial Europe. The lagged and multi-decadal effect of climate change on plague outbreaks may be attributable to the complexity of ecological, social, or climate systems, through which climate exerts its influence on plague dynamics. These findings may contribute to improve our understanding of the epidemiology of plague and other rodent-borne or flea-borne infectious diseases in human history.