Yersinia pestis

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Yersinia pestis
A scanning electron microscope micrograph depicting a mass of Yersinia pestis bacteria in the foregut of an infected flea.
Scientific classification
Domain: Bacteria
Kingdom: Eubacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Genus: Yersinia
Species: Y. pestis
Binomial name
Yersinia pestis
(Lehmann & Neumann, 1896)
van Loghem 1944

Yersinia pestis (formerly Pasteurella pestis) is a Gram-negative rod-shaped coccobacillus, a facultative anaerobic bacterium that can infect humans and other animals.[1]

Human Y. pestis infection takes three main forms: pneumonic, septicemic, and bubonic plagues.[1] All three forms were responsible for a number of high-mortality epidemics throughout human history, including the Justinianic plague of the sixth century and the Black Death that accounted for the death of at least one-third of the European population between 1347 and 1353.[2][3][4] It has now been shown that these plagues probably originated in rodent populations in China.[5][6]

Y. pestis was discovered in 1894 by Alexandre Yersin, a Swiss/French physician and bacteriologist from the Pasteur Institute, during an epidemic of plague in Hong Kong.[7] Yersin was a member of the Pasteur school of thought. Kitasato Shibasaburō, a German-trained Japanese bacteriologist who practiced Koch's methodology, was also engaged at the time in finding the causative agent of plague.[8] However, it was Yersin who actually linked plague with Yersinia pestis. Originally named Pasteurella pestis, the organism was renamed in 1967.

Every year, thousands of cases of plague are still reported to the World Health Organization, although, with proper treatment, the prognosis for victims is now much better. A five- to six-fold increase in cases occurred in Asia during the time of the Vietnam war, possibly due to the disruption of ecosystems and closer proximity between people and animals. Plague also has a detrimental effect on non-human mammals. In the United States of America, animals such as the black-tailed prairie dog and the endangered black-footed ferret are under threat from the disease.

Historical outbreaks

Plague of Justinian

Main article: Plague of Justinian

During the mid 6th century, the pandemic known as the Plague of Justinian wiped out roughly one third of the Byzantine Empire's population, creating major military and financial difficulties. Modern historians named this plague incident after the Eastern Roman Emperor Justinian I, who held power in the Byzantine capital of Constantinople at the time of the initial outbreak. The primary years of the plague were 541–542 AD, although plague returned throughout the Mediterranean basin in successive generations, until about 750.

The waves of disease had a major effect on the future course of European history. The plague's social and cultural impact during the Justinian period is comparable to that of the Black Death.

The most commonly accepted cause of the pandemic has been bubonic plague.[9] A genetic study suggests that the Plague of Justinian (and others from antiquity) arose from either now-extinct strains of Y. pestis, genetically distinct from the strain that broke out in the 14th century pandemic, or from pathogens entirely unrelated to bubonic plague.[10][11]

Role in Black Death

Main article: Theories of the Black Death

In 2000, Didier Raoult and others reported finding Y. pestis DNA by performing a "suicide PCR" on tooth pulp tissue from a fourteenth-century plague cemetery in Montpellier.[12]

A study by an international team of researchers published in October 2010 confirmed that Y. pestis was the cause of the Black Death and later epidemics on the entire European continent over a period of 400 years. The team used ancient DNA and proteins recovered from the bodies of plague victims buried in Hereford in England, in Saint-Laurent-de-la-Cabrerisse in France, and Bergen op Zoom in the Netherlands to identify the pathogen.[13] They found two previously unknown, older strains of Y. pestis that had spread from China by two different routes, rather than the modern Orientalis and Medievalis.[14]

Three biovars of Y. pestis were originally thought to correspond to one of the historical pandemics of bubonic plague.[15] Biovar Antiqua is thought to correspond to the Plague of Justinian; it is not known whether this biovar also corresponds to earlier or smaller epidemics of bubonic plague, or whether these were even truly bubonic plague.[16] Biovar Mediaevalis was formerly thought to correspond to the Black Death, while Biovar Orientalis was thought to correspond to the Third Pandemic and the majority of modern outbreaks of plague. However, calculations of Y pestis's evolutionary age, found using the number of synonymous single nucleotide polymorphisms (SNPs) in conjunction with molecular clock rates, date the emergence of the biovars prior to any of the historical epidemics due to the length of time needed to accumulate such mutations.[17] Additional evidence against this hypothesis includes the fact that Mediaevalis is likely too young to have produced the Black Death due to its recent divergence from Orientalis.[18]

Use in biological warfare

Y. pestis is potentially one of the first examples of biological warfare in history, when in 1347 plague victims were catapulted by the Mongols over the city walls of Caffa, a town currently known as Feodosiya located in present day Ukraine. It is possible that infected inhabitants may have fled to Italy, thus spreading the Black Death to Europe, though this is likely only one of a few routes that could have brought the Plague from the East.[19]

Y. pestis has been used as a biological weapon in World War II, when on October 4, 1940 a Japanese airplane flying over Chushien, Chekiang Province, China released rice and wheat plus rat fleas carrying Y. pestis. A second plane load was released 3 weeks later. These actions led to a local plague that killed 121 people.[20] Leon A. Fox from the U.S. Army Medical Corps had suggested a similar approach in 1933 proposing to drop infested rats from planes.[20]

General characteristics

Y. pestis is a, non-motile, rod-shaped, facultative anaerobe with bipolar staining (giving it a safety pin appearance).[21] Similar to other Yersinia members, it tests negative for urease, lactose fermentation, and indole.[22] The closest relative is the gastrointestinal pathogen Yersinia pseudotuberculosis, and more distantly Yersinia enterocolitica.

Genome

The complete genomic sequence is available for two of the three sub-species of Y. pestis: strain KIM (of biovar Medievalis),[23] and strain CO92 (of biovar Orientalis, obtained from a clinical isolate in the United States).[24] As of 2006, the genomic sequence of a strain of biovar Antiqua has been recently completed.[25] Similar to the other pathogenic strains, there are signs of loss of function mutations. The chromosome of strain KIM is 4,600,755 base pairs long; the chromosome of strain CO92 is 4,653,728 base pairs long. Like its cousins Y. pseudotuberculosis and Y. enterocolitica, Y. pestis is host to the plasmid pCD1. In addition, it also hosts two other plasmids, pPCP1 (also called pPla or pPst) and pMT1 (also called pFra) that are not carried by the other Yersinia species. pFra codes for a phospholipase D that is important for the ability of Y. pestis to be transmitted by fleas.[26] pPla codes for a protease, Pla, that activates plasminogen in human hosts and is a very important virulence factor for pneumonic plague.[27] Together, these plasmids, and a pathogenicity island called HPI, encode several proteins that cause the pathogenesis, for which Y. pestis is famous. Among other things, these virulence factors are required for bacterial adhesion and injection of proteins into the host cell, invasion of bacteria in the host cell (via a Type III secretion system), and acquisition and binding of iron that is harvested from red blood cells (via siderophores). Y. pestis is thought to be descendant from Y. pseudotuberculosis, differing only in the presence of specific virulence plasmids.

A comprehensive and comparative proteomics analysis of Y. pestis strain KIM was performed in 2006.[28] The analysis focused on the transition to a growth condition mimicking growth in host cells.

Pathogenics and immunity

Oriental rat flea (Xenopsylla cheopis) infected with the Yersinia pestis bacterium which appears as a dark mass in the gut. The foregut (proventriculus) of this flea is blocked by a Y. pestis biofilm; when the flea attempts to feed on an uninfected host, Y. pestis is regurgitated into the wound, causing infection.

In the urban and sylvatic (forest) cycles of Y. pestis, most of the spreading occurs between rodents and fleas. In the sylvatic cycle, the rodent is wild, but, in the urban cycle, the rodent is domestic. In addition, Y. pestis can spread from the urban environment and back. Infected animal carcasses can transmit pestis to humans through blood and tissue. If the disease has progressed to the pneumonic form, humans can spread the bacterium to others by coughing and possibly sneezing.

In reservoir hosts

The reservoir commonly associated with Y. pestis is several species of rodents. In the steppes, the reservoir species is believed to be principally the marmot. In the south western United States, several species of rodents are thought to maintain Y. pestis. However, the expected disease dynamics have not been found in any rodent. It is known that rodent populations will have a variable resistance, which could lead to a carrier status.[29] There is evidence that fleas from other mammals have a role in human plague outbreaks.[30]

This lack of knowledge of the dynamics of plague in mammal species is also true among susceptible rodents such as the black-tailed prairie dog (Cynomys ludovicianus), in which plague can cause colony collapse, resulting in a massive effect on prairie food webs.[31] However, the transmission dynamics within prairie dogs does not follow the dynamics of blocked fleas; carcasses, unblocked fleas, or another vector could possibly be important instead.[32]

In other regions of the world, the reservoir of the infection is not clearly identified, which complicates prevention and early warning programs. One such example was seen in a 2003 outbreak in Algeria.[33]

Vector

The transmission of Y. pestis by fleas is well characterized.[34] Initial acquisition of Y. pestis by the vector occurs during feeding on an infected animal. Several proteins then contribute to the maintenance of the bacteria in the flea digestive tract, among them the hemin storage (Hms) system and Yersinia murine toxin (Ymt).

Although Yersinia murine toxin is highly toxic to rodents and was once thought to be produced to ensure reinfection of new hosts, it has been demonstrated that Ymt is important for the survival of Y. pestis in fleas.[26]

The Hms system plays an important role in the transmission of Y. pestis back to a mammalian host.[35] While in the insect vector, proteins encoded by Hms genetic loci induce biofilm formation in the proventriculus, a valve connecting the midgut to the esophagus.[36] Aggregation in the biofilm inhibits feeding, as a mass of clotted blood and bacteria forms (referred to as "Bacot's block" [37]). Transmission of Y. pestis occurs during the futile attempts of the flea to feed. Ingested blood is pumped into the esophagus, where it dislodges bacteria lodged in the proventriculus and is regurgitated back into the host circulatory system.

In humans and other susceptible hosts

Pathogenesis due to Y. pestis infection of mammalian hosts is due to several factors including an ability of these bacteria to suppress and avoid normal immune system responses such as phagocytosis and antibody production. Flea bites allow for the bacteria to pass the skin barrier. Y. pestis expresses the yadBC gene, which is similar to adhesins in other Yersinia species, allowing for adherence and invasion of epithelial cells.[38] Y. pestis expresses a plasminogen activator that is an important virulence factor for pneumonic plague and that might degrade on blood clots in order to facilitate systematic invasion.[27] Many of the bacteria's virulence factors are anti-phagocytic in nature. Two important anti-phagocytic antigens, named F1 (Fraction 1) and V or LcrV, are both important for virulence.[21] These antigens are produced by the bacterium at normal human body temperature. Furthermore, Y. pestis survives and produces F1 and V antigens while it is residing within white blood cells such as monocytes, but not in neutrophils. Natural or induced immunity is achieved by the production of specific opsonic antibodies against F1 and V antigens; antibodies against F1 and V induce phagocytosis by neutrophils.[39]

In addition, the Type III secretion system (T3SS) allows Y. pestis to inject proteins into macrophages and other immune cells. These T3SS-injected proteins are called Yops (Yersinia Outer Proteins) and include Yop B/D, which form pores in the host cell membrane and have been linked to cytolysis. The YopO, YopH, YopM, YopT, YopJ, and YopE are injected into the cytoplasm of host cells via T3SS into the pore created in part by YopB and YopD.[40] The injected Yop proteins limit phagocytosis and cell signaling pathways important in the innate immune system, as discussed below. In addition, some Y. pestis strains are capable of interfering with immune signaling (e.g., by preventing the release of some cytokines).

Yersinia pestis proliferates inside lymph nodes where it is able to avoid destruction by cells of the immune system such as macrophages. The ability of Yersinia pestis to inhibit phagocytosis allows it to grow in lymph nodes and cause lymphadenopathy. YopH is a protein tyrosine phosphatase that contributes to the ability of Yersinia pestis to evade immune system cells.[41] In macrophages, YopH has been shown to dephosphorylate p130Cas, Fyb (Fyn binding protein) SKAP-HOM and Pyk, a tyrosine kinase homologous to FAK. YopH also binds the p85 subunit of phosphoinositide 3-kinase, the Gab1, the Gab2 adapter proteins, and the Vav guanine nucleotide exchange factor.

YopE functions as a GTPase activating protein for members of the Rho family of GTPases such as RAC1. YopT is a cysteine protease that inhibits RhoA by removing the isoprenyl group, which is important for localizing the protein to the cell membrane. It has been proposed that YopE and YopT may function to limit YopB/D-induced cytolysis.[42] This might limit the function of YopB/D to create the pores used for Yop insertion into host cells and prevent YopB/D-induced rupture of host cells and release of cell contents that would attract and stimulate immune system responses.

YopJ is an acetyltransferase that binds to a conserved α-helix of MAPK kinases.[43] YopJ acetylates MAPK kinases at serines and threonines that are normally phosphorylated during activation of the MAP kinase cascade.[44][45] YopJ is activated in eukaryotic cells by interaction with target cell Phytic acid (IP6).[46] This disruption of host cell protein kinase activity causes apoptosis of macrophages, and it has been proposed that this is important for the establishment of infection and for evasion of the host immune response. YopO is a protein kinase also known as Yersinia protein kinase A (YpkA). YopO is a potent inducer of human macrophage apoptosis.[47]

Immunity

Main article: Plague vaccine

A formalin-inactivated vaccine once was available in the United States for adults at high risk of contracting the plague until removal from the market by the U.S. Food and Drug Administration. It was of limited effectiveness and could cause severe inflammation. Experiments with genetic engineering of a vaccine based on F1 and V antigens are underway and show promise. However, bacteria lacking antigen F1 are still virulent, and the V antigens are sufficiently variable, such that vaccines composed of these antigens may not be fully protective.[48] United States Army Medical Research Institute of Infectious Diseases (USAMRIID) have found that an experimental F1/V antigen-based vaccine protects cynomolgus macaques but fails to protect African green monkeys.[49] A systematic review by the Cochrane Collaboration found no studies of sufficient quality to make any statement on the efficacy of the vaccine.[50]

Clinical aspects

Symptoms and disease progression

  • Bubonic plague
    • Incubation period of 2–6 days, when the bacteria is actively replicating.
    • General malaise
    • Fever
    • Headache and chills occur suddenly at the end of the incubation period
    • Swelling of lymph nodes resulting in buboes, the classic sign of bubonic plague. Most flea bites will occur on the legs, so the inguinal nodes are most frequently affected ("boubon" is Greek for "groin.")
    • Death can occur in less than 2 weeks

If this occurs with the classic buboes, this is considered primary, while secondary occurs after symptoms of bubonic or pneumonic infection. Since the bacteria are blood-borne, several organs can be affected, including the spleen and brain. The diffuse infection can cause an immunologic cascade to occur, leading to disseminated intravascular coagulation (DIC), which in turn results in bleeding and necrotic skin and tissue. Such a disseminated infection increases mortality to 22%.

With the exception of the buboes, the initial symptoms of plague are very similar to many other diseases, making diagnosis difficult.[52]

ICD-9 codes for the diseases caused by Y. pestis:

  • 020.0 Bubonic plague
  • 020.2 Septicemic plague
  • 020.3 Primary pneumonic plague
  • 020.4 Secondary pneumonic plague
  • 020.5 Unspecified pneumonic plague

Clinical determination

Gram's stains can confirm the presence of gram-negative rods, and in some cases the identification of the double-curved shape. An anti-F1 serology test can differentiate between different species of Yersinia, and Polymerase chain reaction (PCR) can be used to identify Y. pestis.

The protein H of the tail fiber of the bacteriophage Yersinia phage L-413C permits the differenciation between Y. pestis and Y. pseudotuberculosis", the gastro-intestinal corrolary (Kane et al.).[53]

Treatment

The traditional first line treatment for Y. pestis has been streptomycin,[54][55] chloramphenicol, tetracycline,[56] and fluoroquinolones. There is also good evidence to support the use of doxycycline or gentamicin.[57] Resistant strains have been isolated; treatment should be guided by antibiotic sensitivities where available. Antibiotic treatment alone is insufficient for some patients, who may also require circulatory, ventilator, or renal support.

In an emergency department setting, Harrison's Principles of Internal Medicine outlines the following treatment course.[58] Antibiotics within the first 24 hours are very beneficial, with intravenous being preferred in pulmonary or advanced cases. Streptomycin or gentamicin are the first-line drugs, with chloramphenicol for critically ill patients, or rarely for suspected neuro-involvement.

Recent events

In September 2009, the death of Malcolm Casadaban, a molecular genetics professor at the University of Chicago, was linked to his work on a weakened laboratory strain of Y. pestis.[59] Hemochromatosis was hypothesised to be a predisposing factor in Dr Casadaban's demise from this attenuated strain used for research.[60]

In 2012, researchers in Germany collected samples of Yersinia pestis from gravesites with a view to reconstructing the DNA of the bacterium.[61]

Notes

  1. ↑ 1.0 1.1 Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 484–488. ISBN 0-8385-8529-9. 
  2. ↑ Austin Alchon, Suzanne (2003). A pest in the land: new world epidemics in a global perspective. University of New Mexico Press. p. 21. ISBN 0-8263-2871-7. 
  3. ↑ http://www.sciencedaily.com/releases/2013/05/130510075449.htm
  4. ↑ http://www.cbc.ca/news/canada/hamilton/news/black-death-mysteries-unlocked-by-mcmaster-scientists-1.2512930?cmp=fbtl
  5. ↑ G. Morelli, Y. Song, C.J. Mazzoni, M. Eppinger, P. Roumagnac, D.M. Wagner et al. (2010). "Phylogenetic diversity and historical patterns of pandemic spread of Yersinia pestis". Nature Genetics 42 (12): 1140–3. doi:10.1038/ng.705. PMC 2999892. PMID 21037571. 
  6. ↑ Nicholas Wade (October 31, 2010). "Europe’s Plagues Came From China, Study Finds". New York Times. Retrieved 2010-11-01. 
  7. ↑ Bockemühl J (1994). "100 years after the discovery of the plague-causing agent--importance and veneration of Alexandre Yersin in Vietnam today". Immun Infekt 22 (2): 72–5. PMID 7959865. 
  8. ↑ Howard-Jones N (1973). "Was Kitasato Shibasaburō the discoverer of the plague bacillus?". Perspect Biol Med 16 (2): 292–307. PMID 4570035. 
  9. ↑ "Europe’s Plagues Came From China, Study Finds". The New York Times. October 31, 2010. Retrieved 2010-11-01. 
  10. ↑ McGrath, Matt (12 October 2011). "Black Death Genetic Code 'Built'". BBC World Service. Retrieved 12 October 2011. 
  11. ↑ Bos, Kirsten; Schuenemann, Verena J.; Golding, G. Brian; Burbano, Hernán A.; Waglechner, Nicholas; Coombes, Brian K.; McPhee, Joseph B.; Dewitte, Sharon N.; Meyer, Matthias; Schmedes, Sarah; Wood, James; Earn, David J. D.; Herring, D. Ann; Bauer, Peter; Poinar, Hendrik N.; Krause, Johannes (12 October 2011). "A draft genome of Yersinia pestis from victims of the Black Death". Nature 478 (7370): 506–510. Bibcode:2011Natur.478..506B. doi:10.1038/nature10549. PMC 3690193. PMID 21993626. 
  12. ↑ Drancourt M, Aboudharam G, Signolidagger M, Dutourdagger O, Raoult D. (2002). "Detection of 400-year-old Yersinia pestis DNA in human dental pulp: An ory of plague". Microbes Infect. 4 (1): 105–9. doi:10.1016/S1286-4579(01)01515-5. PMID 11825781. 
  13. ↑ Haensch, Stephanie; Bianucci, Raffaella; Signoli, Michel; Rajerison, Minoarisoa; Schultz, Michael; Kacki, Sacha; Vermunt, Marco; Weston, Darlene A.; Hurst, Derek; Achtman, M; Carniel, E; Bramanti, B (2010). "Distinct Clones of Yersinia pestis Caused the Black Death". In Besansky, Nora J. PLoS Pathogens (PLoS Pathogens) 6 (10): e1001134. doi:10.1371/journal.ppat.1001134. PMC 2951374. PMID 20949072. 
  14. ↑ "Black Death Blamed on Bacteria". Associated Free Press (Discovery). October 8, 2010. Retrieved October 11, 2010. 
  15. ↑ Zhou D, Tong Z, Song Y, Han Y, Pei D, Pang X, Zhai J, Li M, Cui B, Qi Z, Jin L, Dai R, Du Z, Wang J, Guo Z, Wang J, Huang P, Yang R (2004). "Genetics of Metabolic Variations between Yersinia pestis Biovars and the Proposal of a New Biovar, microtus". J Bacteriol 186 (15): 5147–52. doi:10.1128/JB.186.15.5147-5152.2004. PMC 451627. PMID 15262951. 
  16. ↑ Guiyoule A, Grimont F, Iteman I, Grimont P, Lefèvre M, Carniel E (1994). "Plague pandemics investigated by ribotyping of Yersinia pestis strains". J Clin Microbiol 32 (3): 634–41. PMC 263099. PMID 8195371. 
  17. ↑ Parkhill, J., B. W. Wren, N. R. Thomson, R. W. Titball, M. T. G. Holden, M. B. Prentice, M. Sebaihia et al. (2001). "Genome sequence of Yersinia pestis, the causative agent of plague". Nature 413 (6855): 523–527. doi:10.1038/35097083. PMID 11586360.  More than one of |displayauthors= and |display-authors= specified (help)
  18. ↑ Achtman M, Morelli G, Zhu P, Wirth T, Diehl I, Kusecek B et al. (2004). "Microevolution and history of the plague bacillus, Yersinia pestis". Proc Natl Acad Sci U S A 101 (51): 17837–42. Bibcode:2004PNAS..10117837A. doi:10.1073/pnas.0408026101. PMC 535704. PMID 15598742. 
  19. ↑ Wheelis, Mark (September 2002). "Biological Warfare at the 1346 Siege of Caffa". Emerging Infectious Diseseases 8 (9): 971–75. doi:10.3201/eid0809.010536. PMC 2732530. PMID 12194776. 
  20. ↑ 20.0 20.1 Drisdelle R. Parasites. Tales of Humanity's Most Unwelcome Guests. Univ. of California Publishers, 2010. p. 162f. ISBN 978-0-520-25938-6. 
  21. ↑ 21.0 21.1 Collins FM (1996). Pasteurella, Yersinia, and Francisella. In: Baron's Medical Microbiology (Baron S et al, eds.) (4th ed.). Univ. of Texas Medical Branch. ISBN 0-9631172-1-1. 
  22. ↑ Stackebrandt, Erko; Dworkin, Martin; Falkow, Stanley; Rosenberg, Eugene; Karl-Heinz Schleifer (2005). The Prokaryotes: A Handbook on the Biology of Bacteria:Volume 6: Proteobacteria: Gamma Subclass. Berlin: Springer. ISBN 0-387-25499-4. 
  23. ↑ Deng W et al. (2002). "Genome Sequence of Yersinia pestis KIM". Journal of Bacteriology 184 (16): 4601–4611. doi:10.1128/JB.184.16.4601-4611.2002. PMC 135232. PMID 12142430. 
  24. ↑ Parkhill J et al. (2001). "Genome sequence of Yersinia pestis, the causative agent of plague". Nature 413 (6855): 523–527. doi:10.1038/35097083. PMID 11586360.  More than one of |displayauthors= and |display-authors= specified (help)
  25. ↑ Chain PS, Hu P, Malfatti SA et al. (2006). "Complete Genome Sequence of Yersinia pestis Strains Antiqua and Nepal516: Evidence of Gene Reduction in an Emerging Pathogen". J. Bacteriol. 188 (12): 4453–63. doi:10.1128/JB.00124-06. PMC 1482938. PMID 16740952. 
  26. ↑ 26.0 26.1 Hinnebusch BJ, Rudolph AE, Cherepanov P, Dixon JE, Schwan TG, Forsberg A (2002). "Role of Yersinia murine toxin in survival of Yersinia pestis in the midgut of the flea vector". Science 296 (5568): 733–5. Bibcode:2002Sci...296..733H. doi:10.1126/science.1069972. PMID 11976454. 
  27. ↑ 27.0 27.1 Lathem WW, Price PA, Miller VL, Goldman WE (2007). "A plasminogen-activating protease specifically controls the development of primary pneumonic plague". Science 315 (5811): 509–13. Bibcode:2007Sci...315..509L. doi:10.1126/science.1137195. PMID 17255510. 
  28. ↑ Hixson K et al. (2006). "Biomarker candidate identification in Yersinia pestis using organism-wide semiquantitative proteomics". Journal of Proteome Research 5 (11): 3008–3017. doi:10.1021/pr060179y. PMID 16684765. 
  29. ↑ MEYER KF (1957). "The natural history of plague and psittacosis: The R. E. Dyer Lecture". Public Health Rep 72 (8): 705–19. doi:10.2307/4589874. PMC 2031327. PMID 13453634. 
  30. ↑ von Reyn CF, Weber NS, Tempest B et al. (1977). "Epidemiologic and clinical features of an outbreak of bubonic plague in New Mexico". J. Infect. Dis. 136 (4): 489–94. doi:10.1093/infdis/136.4.489. PMID 908848. 
  31. ↑ Pauli JN, Buskirk SW, Williams ES, Edwards WH (2006). "A plague epizootic in the black-tailed prairie dog (Cynomys ludovicianus)". J. Wildl. Dis. 42 (1): 74–80. doi:10.7589/0090-3558-42.1.74. PMID 16699150. 
  32. ↑ Webb CT, Brooks CP, Gage KL, Antolin MF (2006). "Classic flea-borne transmission does not drive plague epizootics in prairie dogs". Proc. Natl. Acad. Sci. U.S.A. 103 (16): 6236–41. Bibcode:2006PNAS..103.6236W. doi:10.1073/pnas.0510090103. PMC 1434514. PMID 16603630. 
  33. ↑ Bertherat E, Bekhoucha S, Chougrani S et al. (2007). "Plague Reappearance in Algeria after 50 Years, 2003". Emerging Infect. Dis. 13 (10): 1459–1462. doi:10.3201/eid1310.070284. PMC 2851531. PMID 18257987. 
  34. ↑ Zhou D, Han Y, Yang R (2006). "Molecular and physiological insights into plague transmission, virulence and etiology". Microbes Infect. 8 (1): 273–84. doi:10.1016/j.micinf.2005.06.006. PMID 16182593. 
  35. ↑ B.J. Hinnebusch, R.D. Perry and T.G. Schwan (1996). "Role of the Yersinia pestis hemin storage (hms) locus in the transmission of plague by fleas". Science. 273 (5237): 367–70. Bibcode:1996Sci...273..367H. doi:10.1126/science.273.5273.367. PMID 8662526. 
  36. ↑ Erickson, D. L., N. R. Waterfield, V. Vadyvaloo, D. Long, E. R. Fischer, R. ffrench-Constant, and B. J. Hinnebusch (2007). "Acute oral toxicity of yersinia pseudotuberculosis to fleas: Implications for the evolution of vector-borne transmission of plague". Cellular Microbiology 9 (11): 2658–2666. doi:10.1111/j.1462-5822.2007.00986.x. PMID 17587333. 
  37. ↑ Pepper, C., M. Nascarella, E. Marsland, J. Montford, L. Wood, S. Cox, C. Bradford, T. Burns, and S. Presley. 2004. Threatened or endangered? Keystone species or public health threat? The black-tailed prairie dog, the Endangered Species Act, and the imminent threat of bubonic plague. Journal of Land, Resources, and Environmental Law 24: 355-391.
  38. ↑ Forman S, Wulff CR, Myers-Morales T, Cowan C, Perry RD, Straley SC (2008). "yadBC of Yersinia pestis, a New Virulence Determinant for Bubonic Plague". Infect. Immun. 76 (2): 578–87. doi:10.1128/IAI.00219-07. PMC 2223446. PMID 18025093. 
  39. ↑ Salyers AA, Whitt DD (2002). Bacterial Pathogenesis: A Molecular Approach (2nd ed.). ASM Press. pp. 207-12. 
  40. ↑ Viboud GI, Bliska JB (2005). "Yersinia outer proteins: role in modulation of host cell signaling responses and pathogenesis". Annu. Rev. Microbiol. 59: 69–89. doi:10.1146/annurev.micro.59.030804.121320. PMID 15847602. 
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