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.

Molecular and Genetic Mechanisms That Mediate Transmission of Yersinia pestis by Fleas

The ability to cause plague in mammals represents only half of the life history of Yersinia pestis. It is also able to colonize and produce a transmissible infection in the digestive tract of the flea, its insect host. Parallel to studies of the molecular mechanisms by which Y. pestis is able to overcome the immune response of its mammalian hosts, disseminate, and produce septicemia, studies of Y. pestis-flea interactions have led to the identification and characterization of important factors that lead to transmission by flea bite. Y. pestis adapts to the unique conditions in the flea gut by altering its metabolic physiology in ways that promote biofilm development, a common strategy by which bacteria cope with a nutrient-limited environment. Biofilm localization to the flea foregut disrupts normal fluid dynamics of blood feeding, resulting in regurgitative transmission. Many of the important genes, regulatory pathways, and molecules required for this process have been identified and are reviewed here.

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.

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.

Yersinia pestis can be transmitted by fleas within a few days after taking a blood meal from a highly bacteremic host, termed early-phase or mass transmission; and again after it forms a dense biofilm in the foregut of its vector that can eventually block blood feeding. The relative importance of the two transmission modes in the ecology of plague is a matter of current debate, but estimates of transmission rate, efficiency, and other parameters are limited. We compared transmission and disease progression dynamics in mice bitten by groups of fleas three days after their infectious blood meal (early-phase or mass transmission mode) and in mice bitten by individual blocked fleas. In general, a higher percentage of transmissions by blocked fleas led to terminal disease, whereas early-phase transmissions more often led to survival and an immune response, which are nonproductive infections in the sense that the bacteremia required to continue the Y. pestis life cycle did not develop and these animals would be removed from the pool of susceptibles in the host population. The data will be useful in mathematical models of plague dynamics in wild rodent populations.

Control measures of a 400-year-old plague epidemic: an example of past efficiency at controlling disease and similarities with current epidemics

The plague caused by the bacterium Yersinia pestis, provides one of the best historical examples of pandemic infection. It can therefore be considered the first "globalized" disease, thanks also to the crowds that favoured the rebalancing of infectious agents between Europe and the Middle East. In this paper we analyse all the official documents of the time, highlighting the most effective prevention measures implemented in the city of Ferrara during the Italian plague. Historical mortality data for the 1630 Italian plague in northern Italy are first analysed. In contrast to the high rates recorded throughout the area from Milan to Florence, the mortality rate in Ferrara remained normal over the period. From the city's documents it emerged that the authorities, from the 16th century onwards, had already understood that the spread of the contagion could also occur through domestic animals, although rats are never mentioned. The strength of Ferrara's response to the "plague emergency" stems from an efficient and emergency-ready health control system, financed and supported by the "permanent surveillance team of the city and the Pontifical Legation of Ferrara - Azienda Sanitaria Pubblica" even in times of great economic difficulty for the State. Among the various measures that the city of Ferrara adopted to deal with the plague the following should be mentioned: guards at the city gates, lazarettos, safety of doctors, self-isolation and treatment of every suspicious case as if it were a real case of plague, measures to support the poorer classes of the population, veterinary and hygiene standards for the city and for housing, management of Catholic religious functions and the precepts of the Legation of Ferrara, which was under papal control, closure of churches to avoid mass gatherings, and limitations of all kinds of social and economic relations within and outside the population. The broad regimen, laid down in the 16th century, contains extremely modern health rules which are very much in line with those recommended by the WHO and the health authorities of each individual state in the current COVID-19 pandemic, even starting with hand-washing. The fight against epidemics of the past, especially the history of the plague in the 17th century, anticipates very important and valid concepts, and represents a wake-up call for the recent epidemics of emerging pathogens.

The changing triad of plague in Uganda: invasive black rats (Rattus rattus), indigenous small mammals, and their fleas

Rattus rattus was first reported from the West Nile Region of Uganda in 1961, an event that preceded the appearance of the first documented human plague outbreak in 1970. We investigated how invasive R. rattus and native small mammal populations, as well as their fleas, have changed in recent decades. Over an 18-month period, a total of 2,959 small mammals were captured, sampled, and examined for fleas, resulting in the identification of 20 small mammal taxa that were hosts to 5,109 fleas (nine species). Over three-fourths (75.8%) of captured mammals belonged to four taxa: R. rattus, which predominated inside huts, and Arvicanthis niloticus, Mastomys sp., and Crocidura sp., which were more common outside huts. These mammals were hosts for 85.8% of fleas collected, including the efficient plague vectors Xenopsylla cheopis and X. brasiliensis, as well as likely enzootic vectors, Dinopsyllus lypusus and Ctenophthalmus bacopus. Flea loads on small mammals were higher in certain environments in villages with a recent history of plague compared to those that lacked such a history. The significance of these results is discussed in relation to historical data, the initial spread of plague in the WNR and the continuing threat posed by the disease.

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.

Estimating the basic reproduction number for the 2015 bubonic plague outbreak in Nyimba district of Eastern Zambia

BACKGROUND

Plague is a re-emerging flea-borne infectious disease of global importance and in recent years, Zambia has periodically experienced increased incidence of outbreaks of this disease. However, there are currently no studies in the country that provide a quantitative assessment of the ability of the disease to spread during these outbreaks. This limits our understanding of the epidemiology of the disease especially for planning and implementing quantifiable and cost-effective control measures. To fill this gap, the basic reproduction number, R0, for bubonic plague was estimated in this study, using data from the 2015 Nyimba district outbreak, in the Eastern province of Zambia. R0 is the average number of secondary infections arising from a single infectious individual during their infectious period in an entirely susceptible population.

METHODOLOGY/PRINCIPAL FINDINGS

Secondary epidemic data for the most recent 2015 Nyimba district bubonic plague outbreak in Zambia was analyzed. R0 was estimated as a function of the average epidemic doubling time based on the initial exponential growth rate of the outbreak and the average infectious period for bubonic plague. R0 was estimated to range between 1.5599 [95% CI: 1.382-1.7378] and 1.9332 [95% CI: 1.6366-2.2297], with average of 1.7465 [95% CI: 1.5093-1.9838]. Further, an SIR deterministic mathematical model was derived for this infection and this estimated R0 to be between 1.4 to 1.5, which was within the range estimated above.

CONCLUSIONS/SIGNIFICANCE

This estimated R0 for bubonic plague is an indication that each bubonic plague case can typically give rise to almost two new cases during these outbreaks. This R0 estimate can now be used to quantitatively analyze and plan measurable interventions against future plague outbreaks in Zambia.

Vector-Borne Pathogens in Ectoparasites Collected from High-Elevation Pika Populations

The American pika, Ochotona princeps, is projected to decline throughout North America as climate change reduces its range, and pikas have already disappeared from several locations. In addition to climate, disease spillover from lower elevation mammalian species might affect pikas. We sampled pika fleas in Colorado and Montana across elevations ranging from 2896 to 3612 m and screened them for the presence of DNA from rodent-associated bacterial pathogens (Bartonella species and Yersinia pestis) to test the hypothesis that flea exchange between pikas and rodents may lead to occurrence of rodent-associated pathogens in pika ectoparasites. We collected 275 fleas from 74 individual pikas at 5 sites in Colorado and one site in Montana. We found that 5.5% of 275 pika fleas in this study tested positive for rodent-associated Bartonella DNA but that variation in Bartonella infection prevalence in fleas among sites was not driven by elevation. Specifically, we detected DNA sequences from two loci (gltA and rpoB) that are most similar to Bartonella grahamii isolates collected from rodents in Canada. We did not detect Y. pestis DNA in our survey. Our results demonstrate evidence of rodent-associated flea-borne bacteria in pika fleas. These findings are also consistent with the hypothesis that rodent-associated pathogens could be acquired by pikas. Flea-borne pathogen spillover from rodents to pikas has the potential to exacerbate the more direct effects of climate that have been suggested to drive pika declines.

Pentaplex real-time PCR for differential detection of Yersinia pestis and Y. pseudotuberculosis and application for testing fleas collected during plague epizootics

Upon acquiring two unique plasmids (pMT1 and pPCP1) and genome rearrangement during the evolution from Yersinia pseudotuberculosis, the plague causative agent Y. pestis is closely related to Y. pseudotuberculosis genetically but became highly virulent. We developed a pentaplex real-time PCR assay that not only detects both Yersinia species but also differentiates Y. pestis strains regarding their plasmid profiles. The five targets used were Y. pestis-specific ypo2088, caf1, and pst located on the chromosome, plasmids pMT1 and pPCP1, respectively; Y. pseudotuberculosis-specific chromosomal gene opgG; and 18S ribosomal RNA gene as an internal control for flea DNA. All targets showed 100% specificity and high sensitivity with limits of detection ranging from 1 fg to 100 fg, with Y. pestis-specific pst as the most sensitive target. Using the assay, Y. pestis strains were differentiated 100% by their known plasmid profiles. Testing Y. pestis and Y. pseudotuberculosis-spiked flea DNA showed there is no interference from flea DNA on the amplification of targeted genes. Finally, we applied the assay for testing 102 fleas collected from prairie dog burrows where prairie dog die-off was reported months before flea collection. All flea DNA was amplified by 18S rRNA; no Y. pseudotuberculosis was detected; one flea was positive for all Y. pestis-specific targets, confirming local Y. pestis transmission. Our results indicated the assay is sensitive and specific for the detection and differentiation of Y. pestis and Y. pseudotuberculosis. The assay can be used in field investigations for the rapid identification of the plague causative agent.

Human plague: An old scourge that needs new answers

Yersinia pestis, the bacterial causative agent of plague, remains an important threat to human health. Plague is a rodent-borne disease that has historically shown an outstanding ability to colonize and persist across different species, habitats, and environments while provoking sporadic cases, outbreaks, and deadly global epidemics among humans. Between September and November 2017, an outbreak of urban pneumonic plague was declared in Madagascar, which refocused the attention of the scientific community on this ancient human scourge. Given recent trends and plague's resilience to control in the wild, its high fatality rate in humans without early treatment, and its capacity to disrupt social and healthcare systems, human plague should be considered as a neglected threat. A workshop was held in Paris in July 2018 to review current knowledge about plague and to identify the scientific research priorities to eradicate plague as a human threat. It was concluded that an urgent commitment is needed to develop and fund a strong research agenda aiming to fill the current knowledge gaps structured around 4 main axes: (i) an improved understanding of the ecological interactions among the reservoir, vector, pathogen, and environment; (ii) human and societal responses; (iii) improved diagnostic tools and case management; and (iv) vaccine development. These axes should be cross-cutting, translational, and focused on delivering context-specific strategies. Results of this research should feed a global control and prevention strategy within a "One Health" approach.

Presence of the Oriental Rat Flea (Siphonaptera: Pulicidae) Infesting an Endemic Mammal and Confirmed Plague Circulation in a Forest Area of Madagascar

The Oriental rat flea, Xenopsylla cheopis (Rothschild 1903), is a cosmopolitan flea usually found infesting domestic rats. This flea is a well-known major human plague vector in Madagascar. As part of field sampling, fleas and small mammals were collected in the village of South Andranofeno and the natural reserve of Sohisika, two sites of the district of Ankazobe, located in the Central Highlands of Madagascar. Rats inside houses and forest small mammals were trapped using Besancon Technical Services and pitfall traps, respectively. Their fleas were collected and preserved for laboratory works. Collected fleas from the village and forest belonged to five species, which were X. cheopis, Synopsyllus fonquerniei (Wagner and Roubaud 1932) (Siphonaptera: Pulicidae), Echidnophaga gallinacea (Westwood 1875) (Siphonaptera: Pulicidae), Ctenocephalides felisstrongylus (Jordan 1925) (Siphonaptera: Pulicidae), Pulex irritans (Linnaeus 1758) (Siphonaptera: Pulicidae). After sampling in the forest zone, one specimen of X. cheopis was unexpectedly collected while infesting an endemic tenrec Setifer setosus (Schreber 1777) (Afrosoricida: Tenrecidae). Polymerase chain reaction (PCR) diagnosis on all collected fleas allowed detecting plague bacterium Yersinia pestis (Lehmann and Neumann 1896) (Enterobacterales: Yersiniaceae) on nine specimens of the endemic flea S. fonquerniei collected inside forest. The presence of the oriental rat flea in forest highlights the connection between human and wild environments due to animal movements and the fact that the rat flea can infest various hosts. As only one specimen of X. cheopis was collected on S. setosus, we hypothesize that flea was carried from the village to forest. Yersinia pestis infection of forest fleas outlines plague circulation in this sylvatic area.

Modeling the Justinianic Plague: Comparing hypothesized transmission routes

The Justinianic Plague, the first part of the earliest of the three plague pandemics, has minimal historical documentation. Based on the limited primary sources, historians have argued both for and against the "maximalist narrative" of plague, i.e. that the Justinianic Plague had universally devastating effects throughout the Mediterranean region during the sixth century CE. Using primary sources of one of the pandemic’s best documented outbreaks that took place in Constantinople during 542 CE, as well as modern findings on plague etiology and epidemiology, we developed a series of dynamic, compartmental models of disease to explore which, if any, transmission routes of plague are feasible. Using expected parameter values, we find that the bubonic and bubonic-pneumonic transmission routes exceed maximalist mortality estimates and are of shorter detectable duration than described by the primary sources. When accounting for parameter uncertainty, several of the bubonic plague model configurations yielded interquartile estimates consistent with the upper end of maximalist estimates of mortality; however, these models had shorter detectable outbreaks than suggested by the primary sources. The pneumonic transmission routes suggest that by itself, pneumonic plague would not cause significant mortality in the city. However, our global sensitivity analysis shows that predicted disease dynamics vary widely for all hypothesized transmission routes, suggesting that regardless of its effects in Constantinople, the Justinianic Plague would have likely had differential effects across urban areas around the Mediterranean. Our work highlights the uncertainty surrounding the details in the primary sources on the Justinianic Plague and calls into question the likelihood that the Justinianic Plague affected all localities in the same way.

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.

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.

Co-infection Assay to Determine Yersinia pestis Competitive Fitness in Fleas

Co-infection refers to the simultaneous infection of a host by multiple pathogenic organisms. Experimental co-infection studies using a mutant and its isogenic wild type have proven to be profoundly sensitive to analysis of pathogen factor mutation-associated fitness effects in in vivo models of infectious disease. Here we discuss the use of such co-infection experiments in studying the interaction between Yersinia pestis and its flea vector to more sensitively determine the critical bacterial determinants for Y. pestis survival, adaptation, and transmission from fleas. This chapter comprises two main sections, the first detailing how to infect fleas with mutant and wild type Y. pestis strains, and secondly how to process infected fleas and specifically quantify distinct Y. pestis strain burdens per flea. The Y. pestis competitive fitness co-infection model in fleas is insightful in evaluating the consequence of a mutation which may not be obvious in single-strain flea infections where there is less selective pressure.

An Uneasy Pleasure: Representing the Dangers of Skin-to-skin Contact in Eighteenth-century London 'The William Bynum Prize Essay'

This article considers the social function of contagious disease as moderator of class relationships in England during the first half of the eighteenth century and takes into account the ways in which the 'communicability' of the plague, great pox (syphilis) and smallpox (variola) was used by authors to crystallise social interaction and tension along class lines. The essay begins by examining the representation of the plague, syphilis and smallpox in the medical tradition, before shifting its attention to the practice of maritime quarantine, as laid out by Richard Mead in his Short Discourse Concerning Pestilential Contagion (1720). By foregrounding medical writing on contagion through skin contact, I suggest that pornographic texts such as John Cleland's The Memoirs of a Woman of Pleasure (Fanny Hill) (1748) had an interventionist function. Cleland is often charged with sanitising the true horrors of sex work in this period. This article proposes that if we take the time to appreciate the way infectious cutaneous diseases were believed to operate and spread we can recognise the moments in which he not only alludes to disease but invokes it for structural and thematic purposes. In proposing this, I am challenging the dominant interpretation that the problematic realities of eighteenth-century prostitution, especially disease, are subordinated to the narrative's greater interest in erotic pleasure.

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.

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.

Intravital Confocal Microscopy of Dermal Innate Immune Responses to Flea-Transmitted Yersinia pestis

The technique known as intravital microscopy (IVM), when used in conjunction with transgenic mice expressing fluorescent proteins in various cell populations, is a powerful tool with the potential to provide new insights into host-pathogen interactions in infectious disease pathogenesis in vivo. Yersinia pestis, the causative agent of plague, is typically deposited in a host's skin during feeding of an infected flea. IVM has been used to characterize the innate immune response to Y. pestis in the skin and identify differences between the responses to needle-inoculated and flea-transmitted bacteria that would have been difficult, if not impossible, to detect by other means. Here we describe techniques used to image the neutrophil response to flea-transmitted Y. pestis in the dermis of live mice using conventional confocal microscopy.

Exposure to Yersinia pestis increases resistance to plague in black rats and modulates transmission in Madagascar

OBJECTIVES

In Madagascar, plague (Yersinia pestis infection) is endemic in the central highlands, maintained by the couple Rattus rattus/flea. The rat is assumed to die shortly after infection inducing migration of the fleas. However we previously reported that black rats from endemic areas can survive the infection whereas those from non-endemic areas remained susceptible. We investigate the hypothesis that lineages of rats can acquire resistance to plague and that previous contacts with the bacteria will affect their survival, allowing maintenance of infected fleas. For this purpose, laboratory-born rats were obtained from wild black rats originating either from plague-endemic or plague-free zones, and were challenged with Y. pestis. Survival rate and antibody immune responses were analyzed.

RESULTS

Inoculation of low doses of Y. pestis greatly increase survival of rats to subsequent challenge with a lethal dose. During challenge, cytokine profiles support activation of specific immune response associated with the bacteria control. In addition, F1 rats from endemic areas exhibited higher survival rates than those from non-endemic ones, suggesting a selection of a resistant lineage. In Madagascar, these results support the role of black rat as long term reservoir of infected fleas supporting maintenance of plague transmission.

Descriptions of two new species of flea of the genus Plocopsylla in northwestern Argentina

Two new species of flea of the genus Plocopsylla, subgenus Plocopsylla, (Siphonaptera: Stephanocircidae) collected from sigmodontine rodents in northwestern Argentina are described and a key to identification of species of the genus Plocopsylla, subgenus Plocopsylla, in Argentina is presented. Plocopsylla (P.) inti is cited for the first time in Argentina, extending its distribution ∼ 970 km further south than previously documented. New locality data and flea-host associations are recorded. The contributions of this study are relevant because they increase knowledge of the diversity of flea fauna in northwestern Argentina bordering a plague endemic area and will be useful in the implementation of plague control management plans in the future.

The Deer Mouse (Peromyscus maniculatus) as an Enzootic Reservoir of Plague in California

It has long been theorized that deer mice (Peromyscus maniculatus) are a primary reservoir of Yersinia pestis in California. However, recent research from other parts of the western USA has implicated deer mice as spillover hosts during epizootic plague transmission. This retrospective study analyzed deer mouse data collected for plague surveillance by public health agencies in California from 1971 to 2016 to help elucidate the role of deer mice in plague transmission. The fleas most commonly found on deer mice were poor vectors of Y. pestis and occurred in insufficient numbers to maintain transmission of the pathogen, while fleas whose natural hosts are deer mice were rarely observed and even more rarely found infected with Y. pestis on other rodent hosts. Seroprevalence of Y. pestis antibodies in deer mice was significantly lower than that of several chipmunk and squirrel species. These analyses suggest that it is unlikely that deer mice play an important role in maintaining plague transmission in California. While they may not be primary reservoirs, results supported the premise that deer mice are occasionally exposed to and infected by Y. pestis and instead may be spillover hosts.

Entangled histories of plague ecology in Russia and the USSR

During the mid-twentieth century, Soviet scientists developed the "natural focus" theory-practice framework to explain outbreaks of diseases (such as bubonic plague) endemic to wild animals and transmitted to humans. Focusing on parasitologist-physician Evgeny N. Pavlovsky and other field scientists' work in the Soviet borderlands, this article explores how the natural focus framework's concepts and practices were entangled in political as well as material ecologies of knowledge and practice. We argue that the very definition of endemic plague incorporated both hands-on materialist experience (including the identification of microbes/pathogens, insects/vectors, and mammals/reservoirs) and ideological concepts that supported Soviet colonization ("improving" hinterlands, "controlling natural focuses of disease," and "sanitizing" landscapes). Theorizing and fighting plague assisted with the goals of controlling and improving landscapes and peoples in southern Russia and Central Asia. The history of the natural focus framework illustrates how Soviet disease ecology co-developed with the needs of local and central political powers in the Soviet borderlands.

Ten years of surveillance of the Yulong plague focus in China and the molecular typing and source tracing of the isolates

Plague, caused by Yersinia pestis, was classified as a reemerging infectious disease by the World Health Organization. The five human pneumonic plague cases in Yulong County in 2005 gave rise to the discovery of a Yulong plague focus in Yunnan province, China. Thereafter, continuous wild rodent plague (sylvatic plague) was identified as the main plague reservoir of this focus. In this study, the epizootics in Yulong focus were described, and three molecular typing methods, including the different region (DFR) analysis, clustered regularly interspaced short palindromic repeats (CRISPRs), and the multiple-locus variable number of tandem repeats (VNTR) analysis (MLVA) (14+12), were used for the molecular typing and source tracing of Y. pestis isolates in the Yulong plague focus. Simultaneously, several isolates from the vicinity of Yunnan were used as controls. The results showed that during the 10-year period from 2006 to 2016, an animal plague epidemic occurred in 6 of those years, and 5 villages underwent an animal plague epidemic within a 30-km² area of the Yulong plague focus. Searching for dead mice was the most effective monitoring method in this plague focus. No positive sample has been found in 6937 captured live rodents thus far, suggesting that the virulence of strains in the Yulong plague focus is stronger and the survival time of mice is shorter after infection. Strains from Lijiang, Sichuan and Tibet were of the same complex based on a typing analysis of DFR and CRISPR. The genetic relationship of Y. pestis illustrated by MLVA “14+12” demonstrates that Tibet and Sichuan strains evolved from the strains 1.IN2 (Qinghai, 1970 and Tibet, 1976), and Lijiang strains are closer to Batang strains (Batang County in Sichuan province, 2011, Himalaya marmot plague foci) in terms of genetic or phylogenic relationships. In conclusion, we have a deeper understanding of this new plague focus throughout this study, which provides a basis for effective prevention and control.

Plague is a type of zoonosis that is highly lethal to humans. The surveillance of animal hosts is critical for the prevention and control of plague. The Yulong plague focus is a newly discovered plague focus in China in recent years. The plague outbreak had attracted widespread attention because 5 people were infected in 2005, 2 of whom died. We have monitored the plague focus for a decade, and isolated strains and DNAs of Yersinia pestis were studied. The structure, origin and evolutionary trend of the Yulong plague focus were clarified, which provides a scientific basis for the effective prevention and control of human plague. This article also provides a set of paradigms for the systematic study of new plague foci, which is a perfect combination of traditional monitoring methods and modern research methods.

Pneumonic Plague Transmission, Moramanga, Madagascar, 2015

During a pneumonic plague outbreak in Moramanga, Madagascar, we identified 4 confirmed, 1 presumptive, and 9 suspected plague case-patients. Human-to-human transmission among close contacts was high (reproductive number 1.44) and the case fatality rate was 71%. Phylogenetic analysis showed that the Yersinia pestis isolates belonged to group q3, different from the previous outbreak.

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.

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.

Yersinia pestis, the bacterial agent of plague, is transmitted by fleas that feed on blood from rodents that carry this disease. The conclusions from studies comparing how efficiently fleas transmit plague after becoming infected have been inconsistent, possibly because a variety of rodent blood sources have been used. To investigate this, we infected three different flea species with Y. pestis using four different types of rodent blood and compared how well they could transmit three days later. The two rodent flea species that transmitted efficiently tended to reflux bacteria and blood into their esophagus when rat or guinea pig blood was used for the infections, but not when mouse or gerbil blood was used. This reflux phenomenon appears to be related to the solubility of the hemoglobin molecule of different rodent species. In contrast, cat fleas, inefficient transmitters, never refluxed their infected blood meal into the esophagus. Rodent fleas that were infected using reflux-inducing rat blood transmitted more Y. pestis than those that fed on infected mouse blood. These findings improve our understanding of how fleas transmit Y. pestis soon after becoming infected and suggest a reason why certain rodents figure more prominently in plague ecology than others.

Investigation of and Response to 2 Plague Cases, Yosemite National Park, California, USA, 2015

Rapid interagency investigation and public health response probably reduced risk for transmission to other Yosemite visitors and staff.

In August 2015, plague was diagnosed for 2 persons who had visited Yosemite National Park in California, USA. One case was septicemic and the other bubonic. Subsequent environmental investigation identified probable locations of exposure for each patient and evidence of epizootic plague in other areas of the park. Transmission of Yersinia pestis was detected by testing rodent serum, fleas, and rodent carcasses. The environmental investigation and whole-genome multilocus sequence typing of Y. pestis isolates from the patients and environmental samples indicated that the patients had been exposed in different locations and that at least 2 distinct strains of Y. pestis were circulating among vector–host populations in the area. Public education efforts and insecticide applications in select areas to control rodent fleas probably reduced the risk for plague transmission to park visitors and staff.

The Asian house shrew Suncus murinus as a reservoir and source of human outbreaks of plague in Madagascar

Identifying key reservoirs for zoonoses is crucial for understanding variation in incidence. Plague re-emerged in Mahajanga, Madagascar in the 1990s but there has been no confirmed case since 1999. Here we combine ecological and genetic data, from during and after the epidemics, with experimental infections to examine the role of the shrew Suncus murinus in the plague epidemiological cycle. The predominance of S. murinus captures during the epidemics, their carriage of the flea vector and their infection with Yersinia pestis suggest they played an important role in the maintenance and transmission of plague. S. murinus exhibit a high but variable resistance to experimental Y. pestis infections, providing evidence of its ability to act as a maintenance host. Genetic analyses of the strains isolated from various hosts were consistent with two partially-linked transmission cycles, with plague persisting within the S. murinus population, occasionally spilling over into the rat and human populations. The recent isolation from a rat in Mahajanga of a Y. pestis strain genetically close to shrew strains obtained during the epidemics reinforces this hypothesis and suggests circulation of plague continues. The observed decline in S. murinus and Xenopsylla cheopis since the epidemics appears to have decreased the frequency of spillover events to the more susceptible rats, which act as a source of infection for humans. Although this may explain the lack of confirmed human cases in recent years, the current circulation of plague within the city highlights the continuing health threat.

The reemergence of plague is related to the persistence and dynamics of its reservoirs and vectors. During the human plague outbreaks that re-occurred in Mahajanga harbor, Madagascar in the 1990s, the shrew Suncus murinus was found infected with Yersinia pestis. Combining field surveys, experimental infections and genetic analysis, we examined the role of Asian shrew Suncus murinus in plague transmission and maintenance comparatively with others potential hosts in Mahajanga. The genomes of nineteen Y. pestis isolates recovered from humans, shrews, rats and fleas in this focus were sequenced and compared. We observed the predominance of S. murinus captured during the epidemics and their carriage of the flea vector. Shrews exhibited high resistance to Y. pestis experimental infections. Genetic analyses of the strains isolated from various hosts were consistent with two partially-linked transmission cycles, with plague persisting within the S. murinus population, occasionally spilling over into the rat and human populations. The isolation of a Y. pestis strain from a rat in Mahajanga in 2014 genetically close to shrew strains reinforces this hypothesis. The current circulation of plague within the city highlights the continuing health threat.

Swabbing Prairie Dog Burrows for Fleas That Transmit Yersinia pestis: Influences on Efficiency

Scientists and health-care professionals sometimes use a swabbing technique to collect fleas from rodent burrows, and later test the fleas for Yersinia pestis, the causative agent of plague. Detection of Y. pestis is enhanced when large pools of fleas are available. The following study investigated factors that might affect the rate at which fleas are collected from burrows in colonies of black-tailed prairie dogs (Cynomys ludovicianus). Data were collected from 13 colonies in New Mexico during 0600-1000 hours, June-August 2010-2011. Fleas were scarce on swabs inserted into burrows that were not actively used by prairie dogs; fleas are presumably suppressed in burrows that are void of hosts and might have begun to collapse due to a lack of maintenance. Fleas were scarce on swabs inserted into burrows with little sunlight entering the tunnel; many species of fleas use changes in light intensity to locate objects, but if light is limited, it might be difficult to locate a swab. Fleas were scarce on swabs inserted to shallow depths underground, especially during hot mornings, and during the hottest portions of mornings; when conditions are hot above ground, ectothermic fleas might migrate into the deep components of burrows, or become less willing to jump onto hosts, making it difficult to collect the fleas with swabs. If the swabbing technique is used to survey for Y. pestis on colonies of black-tailed prairie dogs, investigators might use the results of this study to modify their methods and increase the number of fleas collected.

Genetic evidence for subspecies differentiation of the Himalayan marmot, Marmota himalayana, in the Qinghai-Tibet Plateau

The primary host of plague in the Qinghai-Tibet Plateau (QTP), China, is Marmota himalayana, which plays an essential role in the maintenance, transmission, and prevalence of plague. To achieve a more clear insight into the differentiation of M. himalayana, complete cytochrome b (cyt b) gene and 11 microsatellite loci were analyzed for a total of 423 individuals from 43 localities in the northeast of the QTP. Phylogenetic analyses with maximum likelihood and Bayesian inference methods showed that all derived haplotypes diverged into two primary well-supported monophyletic lineages, I and II, which corresponded to the referential sequences of two recognized subspecies, M. h. himalayana and M. h. robusta, respectively. The divergence between the two lineages was estimated to be at about 1.03 million years ago, nearly synchronously with the divergence between M. baibacina and M. kastschenkoi and much earlier than that between M. vancouverensis and M. caligata. Genetic structure analyses based on the microsatellite dataset detected significant admixture between the two lineages in the mixed region, which verified the intraspecies level of the differentiation between the two lineages. Our results for the first time demonstrated the coexistence of M. h. himalayana and M. h. robusta, and also, determined the distribution range of the two subspecies in the northeast of QTP. We provided fundamental information for more effective plague control in the QTP.

Potential Roles of Pigs, Small Ruminants, Rodents, and Their Flea Vectors in Plague Epidemiology in Sinda District, Eastern Zambia

A cross-sectional study was conducted in the Eastern part of Zambia that previously reported a plague outbreak. The aim of the study was to evaluate the potential role of pigs, goats, and sheep as sero-surveillance hosts for monitoring plague, and to investigate the flea vectors and potential reservoir hosts to establish the current status of plague endemicity in the district. Serum samples were collected from 96 rodents, 10 shrews, 245 domestic pigs, 232 goats, and 31 sheep, whereas 106 organs were eviscerated from rodents and shrews. As for fleas, 1,064 Echidnophaga larina Jordan & Rothschild, 7 Xenopsylla cheopis (Rothschild), and 382 Echidnophaga gallinacea (Westwood) were collected from these animals in 34 villages. Enzyme-Linked Immunosorbent Assay (ELISA) and Polymerase Chain Reaction (PCR) tests were performed on serum, and organs and fleas to determine IgG antibodies against Fraction 1 antigen and pla gene of Yersinia pestis, respectively. ELISA results showed that 2.83% (95% CI = 0.59-8.05) rodents, 9.0% (95% CI = 5.71-13.28) domestic pigs, 4.7% (95% CI = 2.39-8.33) goats, and 3.2% (95% CI = 0.08-16.70) sheep were positive for IgG antibodies against Fra1 antigen of Y. pestis. On PCR, 8.4% (95% CI = 3.96-15.51) of the rodents were detected with Y. pestis pla gene, whereas all fleas were found negative. The common fleas identified were E. larina from pigs, whereas X. cheopis were the only fleas collected from rodents. The presence of sero-positive animals as well as the occurrence of X. cheopis on local rodents suggests that Y. pestis remains a risk in the district.

Effectiveness of Fipronil as a Systemic Control Agent Against Xenopsylla cheopis (Siphonaptera: Pulicidae) in Madagascar

Fipronil was evaluated as a systemic control agent for the rat flea Xenopsylla cheopis (Rothschild), the main vector of Yersinia pestis (Yersin), the causative agent of plague, in Madagascar. The effectiveness of fipronil as a systemic control agent against X. cheopis was assessed by determining the toxicity values of the "Lethal Dose 50" (LD50). Two techniques were used to evaluate the systemic action of the insecticide on the vector: 1) an artificial feeding device filled with blood-fipronil mixture from which X. cheopis was fed and 2) rodent hosts, Rattus norvegicus (Berkenhout) and Rattus rattus (L.), which fed on fipronil-treated bait. As a standardized control method, the susceptibility of X. cheopis to fipronil was evaluated by exposure to impregnated paper within World Health Organization (WHO) insecticide test protocol to compare its effect to the systemic activity of the studied insecticide. Results showed that when administered in a systemic way, fipronil appears to be more effective: the toxicity level was evaluated to be ninefold higher compared with the WHO test. Compared with other methods, which require indiscriminate dusting of rodent burrows and human dwellings, fipronil applied in a systemic way enables the direct targeting of the plague vector. Thus, this method appears to be a superior alternative to fipronil-dusting for the control of the main plague vector in Madagascar. However, subsequent tests in the field are necessary to confirm the suitability of fipronil administration in a systemic way on large scales.

Comparative Ability of Oropsylla montana and Xenopsylla cheopis Fleas to Transmit Yersinia pestis by Two Different Mechanisms

Background

Transmission of Yersinia pestis by flea bite can occur by two mechanisms. After taking a blood meal from a bacteremic mammal, fleas have the potential to transmit the very next time they feed. This early-phase transmission resembles mechanical transmission in some respects, but the mechanism is unknown. Thereafter, transmission occurs after Yersinia pestis forms a biofilm in the proventricular valve in the flea foregut. The biofilm can impede and sometimes completely block the ingestion of blood, resulting in regurgitative transmission of bacteria into the bite site. In this study, we compared the relative efficiency of the two modes of transmission for Xenopsylla cheopis, a flea known to become completely blocked at a high rate, and Oropsylla montana, a flea that has been considered to rarely develop proventricular blockage.

Methodology/Principal findings

Fleas that took an infectious blood meal containing Y. pestis were maintained and monitored for four weeks for infection and proventricular blockage. The number of Y. pestis transmitted by groups of fleas by the two modes of transmission was also determined. O. montana readily developed complete proventricular blockage, and large numbers of Y. pestis were transmitted by that mechanism both by it and by X. cheopis, a flea known to block at a high rate. In contrast, few bacteria were transmitted in the early phase by either species.

Conclusions

A model system incorporating standardized experimental conditions and viability controls was developed to more reliably compare the infection, proventricular blockage and transmission dynamics of different flea vectors, and was used to resolve a long-standing uncertainty concerning the vector competence of O. montana. Both X. cheopis and O. montana are fully capable of transmitting Y. pestis by the proventricular biofilm-dependent mechanism.

The ecology of plague is complex and its epidemiology is enigmatic. Many different flea species are able to transmit Yersinia pestis, the plague bacillus, and they can transmit in two different ways. Early-phase transmission can occur during the first week after a flea has fed on a diseased animal. Thereafter, transmission occurs only as bacterial growth in the flea foregut interferes with and eventually blocks blood feeding. Comparisons of the relative ability of different flea vectors to transmit have been problematic, and contradictory results have been reported for the ability of the ground squirrel flea Oropsylla montana to transmit beyond the early phase. Our results show that O. montana readily develops foregut blockage, and transmission by that mechanism was as good as or better than observed for Xenopsylla cheopis, a flea known to block at a high rate. In contrast, very few bacteria were transmitted in the early phase by either of these fleas compared to later times after infection, suggesting that early-phase transmission is pertinent only to highly susceptible animals. Improved characterization of the transmission patterns of different flea vectors will aid in modeling plague incidence in its various natural settings.

Zoonoses As Ecological Entities: A Case Review of Plague

As a zoonosis, Plague is also an ecological entity, a complex system of ecological interactions between the pathogen, the hosts, and the spatiotemporal variations of its ecosystems. Five reservoir system models have been proposed: (i) assemblages of small mammals with different levels of susceptibility and roles in the maintenance and amplification of the cycle; (ii) species-specific chronic infection models; (ii) flea vectors as the true reservoirs; (iii) Telluric Plague, and (iv) a metapopulation arrangement for species with a discrete spatial organization, following a source-sink dynamic of extinction and recolonization with naïve potential hosts. The diversity of the community that harbors the reservoir system affects the transmission cycle by predation, competition, and dilution effect. Plague has notable environmental constraints, depending on altitude (500+ meters), warm and dry climates, and conditions for high productivity events for expansion of the transmission cycle. Human impacts are altering Plague dynamics by altering landscape and the faunal composition of the foci and adjacent areas, usually increasing the presence and number of human cases and outbreaks. Climatic change is also affecting the range of its occurrence. In the current transitional state of zoonosis as a whole, Plague is at risk of becoming a public health problem in poor countries where ecosystem erosion, anthropic invasion of new areas, and climate change increase the contact of the population with reservoir systems, giving new urgency for ecologic research that further details its maintenance in the wild, the spillover events, and how it links to human cases.

Distribution Characteristics of Spermophilus dauricus in Manchuria City in China in 2015 through '3S' Technology

Plague is a virulent infectious disease in China. In this study, '3S' technology was used to perform spatial autocorrelation analysis and spatial interpolation analysis for Spermophilus dauricus (S. Dauricus, a species of ground squirrel) captured in Manchuria City in 2015. The results were visually inspected. During the two-month (May to July) plague surveillance in 2015, 198 S. dauricus individuals were captured in the study area in Manchuria City (48 monitoring areas) by using a day-by-day catching method. Spatial autocorrelation was conducted using the ArcGIS software, and the following significantly different results were obtained: Moran's I=0.228472, Z-score=2.889126, and P<0.05. Thus, a spatial aggregation was observed. In 2015, the distribution of S. dauricus diminished from west to east and from north to south of Manchuria. Geo Detector software was used to analyze the habitat factors affecting the spatial distribution of S. dauricus. This highly clustered species mainly exists in suburban communities, construction sites, and areas surrounding factories. In future studies, plague surveillances should be performed in areas around Manchuria and Zhalainuoer.

Evaluation of Yersinia pestis Transmission Pathways for Sylvatic Plague in Prairie Dog Populations in the Western U.S

Sylvatic plague, caused by the bacterium Yersinia pestis, is periodically responsible for large die-offs in rodent populations that can spillover and cause human mortalities. In the western US, prairie dog populations experience nearly 100% mortality during plague outbreaks, suggesting that multiple transmission pathways combine to amplify plague dynamics. Several alternate pathways in addition to flea vectors have been proposed, such as transmission via direct contact with bodily fluids or inhalation of infectious droplets, consumption of carcasses, and environmental sources of plague bacteria, such as contaminated soil. However, evidence supporting the ability of these proposed alternate pathways to trigger large-scale epizootics remains elusive. Here we present a short review of potential plague transmission pathways and use an ordinary differential equation model to assess the contribution of each pathway to resulting plague dynamics in black-tailed prairie dogs (Cynomys ludovicianus) and their fleas (Oropsylla hirsuta). Using our model, we found little evidence to suggest that soil contamination was capable of producing plague epizootics in prairie dogs. However, in the absence of flea transmission, direct transmission, i.e., contact with bodily fluids or inhalation of infectious droplets, could produce enzootic dynamics, and transmission via contact with or consumption of carcasses could produce epizootics. This suggests that these pathways warrant further investigation.

Infection Prevalence, Bacterial Loads, and Transmission Efficiency in Oropsylla montana (Siphonaptera: Ceratophyllidae) One Day After Exposure to Varying Concentrations of Yersinia pestis in Blood

Unblocked fleas can transmit Yersinia pestis, the bacterium that causes plague, shortly (≤4 d) after taking an infectious bloodmeal. Investigators have measured so-called early-phase transmission (EPT) efficiency in various fleas following infection with highly bacteremic blood (≥10⁸cfu/ml). To date, no one has determined the lower limit of bacteremia required for fleas to acquire and transmit infection by EPT, though knowing this threshold is central to determining the length of time a host may be infectious to feeding fleas. Here, we evaluate the ability of Oropsylla montana (Baker) to acquire and transmit Y. pestis after feeding on blood containing 10³ to 10⁹cfu/ml. We evaluated the resulting infection prevalence, bacterial loads, and transmission efficiency within the early-phase time period at 1 d postinfection. Fleas acquired infection from bacteremic blood across a wide range of concentrations, but transmission was observed only when fleas ingested highly bacteremic blood.

'Tipping the Balance': Karl Friedrich Meyer, Latent Infections, and the Birth of Modern Ideas of Disease Ecology

The Swiss-born medical researcher Karl Friedrich Meyer (1884-1974) is best known as a 'microbe hunter' who pioneered investigations into diseases at the intersection of animal and human health in California in the 1920s and 1930s. In particular, historians have singled out Meyer's 1931 Ludwig Hektoen Lecture in which he described the animal kingdom as a 'reservoir of disease' as a forerunner of 'one medicine' approaches to emerging zoonoses. In so doing, however, historians risk overlooking Meyer's other intellectual contributions. Developed in a series of papers from the mid-1930s onwards, these were ordered around the concept of latent infections and sought to link microbial behavior to broader bio-ecological, environmental, and social factors that impact hostpathogen interactions. In this respect Meyer-like the comparative pathologist Theobald Smith and the immunologist Frank Macfarlane Burnet-can be seen as a pioneer of modern ideas of disease ecology. However, while Burnet's and Smith's contributions to this scientific field have been widely acknowledged, Meyer's have been largely ignored. Drawing on Meyer's published writings and private correspondence, this paper aims to correct that lacuna while contributing to a reorientation of the historiography of bacteriological epidemiology. In particular I trace Meyer's intellectual exchanges with Smith, Burnet and the animal ecologist Charles Elton, over brucellosis, psittacosis and plague-exchanges that not only showed how environmental and ecological conditions could 'tip the balance' in favor of parasites but which transformed Meyer thinking about resistance to infection and disease.

Predicting small mammal and flea abundance using landform and soil properties in a plague endemic area in Lushoto District, Tanzania

Small mammals particularly rodents, are considered the primary natural hosts of plague. Literature suggests that plague persistence in natural foci has a root cause in soils. The objective of this study was to investigate the relationship between on the one hand landforms and associated soil properties, and on the other hand small mammals and fleas in West Usambara Mountains in Tanzania, a plague endemic area. Standard field survey methods coupled with Geographical Information System (GIS) technique were used to examine landform and soils characteristics. Soil samples were analysed in the laboratory for physico-chemical properties. Small mammals were trapped on pre-established landform positions and identified to genus/species level. Fleas were removed from the trapped small mammals and counted. Exploration of landform and soil data was done using ArcGIS Toolbox functions and descriptive statistical analysis. The relationships between landforms, soils, small mammals and fleas were established by generalised linear regression model (GLM) operated in R statistics software. Results show that landforms and soils influence the abundance of small mammals and fleas and their spatial distribution. The abundance of small mammals and fleas increased with increase in elevation. Small mammal species richness also increases with elevation. A landform-soil model shows that available phosphorus, slope aspect and elevation were statistically significant predictors explaining richness and abundance of small mammals. Fleas' abundance and spatial distribution were influenced by hill-shade, available phosphorus and base saturation. The study suggests that landforms and soils have a strong influence on the richness and evenness of small mammals and their fleas' abundance hence could be used to explain plague dynamics in the area.

Feeding Behavior Modulates Biofilm-Mediated Transmission of Yersinia pestis by the Cat Flea, Ctenocephalides felis

BACKGROUND

The cat flea, Ctenocephalides felis, is prevalent worldwide, will parasitize animal reservoirs of plague, and is associated with human habitations in known plague foci. Despite its pervasiveness, limited information is available about the cat flea's competence as a vector for Yersinia pestis. It is generally considered to be a poor vector, based on studies examining early-phase transmission during the first week after infection, but transmission potential by the biofilm-dependent proventricular-blocking mechanism has never been systematically evaluated. In this study, we assessed the vector competence of cat fleas by both mechanisms. Because the feeding behavior of cat fleas differs markedly from important rat flea vectors, we also examined the influence of feeding behavior on transmission dynamics.

METHODOLOGY/PRINCIPAL FINDINGS

Groups of cat fleas were infected with Y. pestis and subsequently provided access to sterile blood meals twice-weekly, 5 times per week, or daily for 4 weeks and monitored for infection, the development of proventricular biofilm and blockage, mortality, and the ability to transmit. In cat fleas allowed prolonged, daily access to blood meals, mimicking their natural feeding behavior, Y. pestis did not efficiently colonize the digestive tract and could only be transmitted during the first week after infection. In contrast, cat fleas that were fed intermittently, mimicking the feeding behavior of the efficient vector Xenopsylla cheopis, could become blocked and regularly transmitted Y. pestis for 3-4 weeks by the biofilm-mediated mechanism, but early-phase transmission was not detected.

CONCLUSIONS

The normal feeding behavior of C. felis, more than an intrinsic resistance to infection or blockage by Y. pestis, limits its vector competence. Rapid turnover of midgut contents results in bacterial clearance and disruption of biofilm accumulation in the proventriculus. Anatomical features of the cat flea foregut may also restrict transmission by both early-phase and proventricular biofilm-dependent mechanisms.

Current Perspectives on Plague Vector Control in Madagascar: Susceptibility Status of Xenopsylla cheopis to 12 Insecticides

Plague is a rodent disease transmissible to humans by infected flea bites, and Madagascar is one of the countries with the highest plague incidence in the world. This study reports the susceptibility of the main plague vector Xenopsylla cheopis to 12 different insecticides belonging to 4 insecticide families (carbamates, organophosphates, pyrethroids and organochlorines). Eight populations from different geographical regions of Madagascar previously resistant to deltamethrin were tested with a World Health Organization standard bioassay. Insecticide susceptibility varied amongst populations, but all of them were resistant to six insecticides belonging to pyrethroid and carbamate insecticides (alphacypermethrin, lambdacyhalothrin, etofenprox, deltamethrin, bendiocarb and propoxur). Only one insecticide (dieldrin) was an efficient pulicide for all flea populations. Cross resistances were suspected. This study proposes at least three alternative insecticides (malathion, fenitrothion and cyfluthrin) to replace deltamethrin during plague epidemic responses, but the most efficient insecticide may be different for each population studied. We highlight the importance of continuous insecticide susceptibility surveillance in the areas of high plague risk in Madagascar.