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Keywords:

  • Alexandre Yersin;
  • epidemics;
  • plague;
  • plague history;
  • Yersin

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

The causative bacterium of plague was described and cultured by Alexandre Yersin in Hong Kong in 1894, after which transmission of bacteria from rodents by flea bites was discovered by Jean-Paul Simond in 1898. Effective treatment with antiserum was initiated in 1896, but this therapy was supplanted by sulphonamides in the 1930s and by streptomycin starting in 1947. India suffered an estimated 6 million deaths in 1900–1909, and Vietnam, during its war in 1965–1975, accounted for approximately 80% of the world's cases; since then, African countries have dominated, with >90% of the world's cases in the 1990s and early 21st century. Serological diagnosis with fraction 1 antigen to detect anti-plague antibodies was developed in the 1950s. Vaccine development started in 1897 with killed whole bacterial cells, and this was followed by a live attenuated bacterial vaccine, leading to millions of persons receiving injections, but the benefits of these vaccines remain clouded by controversy. Plasmid-mediated virulence was established in 1981, and this was followed by specific DNA methods that have allowed detection of plague genes in skeletal specimens from European graves of the sixth to 17th centuries.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

The modern history of plague began in 1894, when Alexandre Yersin isolated the causative bacterium in culture and identified it under the microscope. This event allowed laboratory confirmation for accurate diagnoses. There followed many advances in treatment and diagnosis, as well as scientific understanding of the disease, which will be reviewed here. Table 1 gives a guide to some of these historic milestones [1-31].

Table 1. Modern milestones in the history of plague investigations
YearDiscoveryReferences
1894Isolation in culture and microscopic description of causative bacteria [1, 2]
1898Flea-borne transmission [3, 4]
1896Usefulness of antiserum for therapy [5]
1897First vaccine consisting of heat-killed bacteria developed and tested [6]
1900–1909Six million deaths in India [7]
1931Live attenuated bacterial vaccine EV developed and tested [8, 9]
1937Usefulness of sulphonamides for therapy [10, 11]
1939Formalin-killed bacterial vaccine developed and tested [12]
1940–1947Large concentrations of blood bacteria correlated with mortality [13]
1947Usefulness of streptomycin for therapy [13, 14]
1954Development of serological tests for diagnosis [15]
1965–1975Vietnam's ascendancy in incidence [16]
1973Usefulness of trimethoprim–sulphamethoxazole for therapy [17]
1975Measurement of blood endotoxin correlated with severity and large concentrations of blood bacteria [18]
1980Genetic relatedness of Yersinia pestis and Yersinia pseudotuberculosis [19]
1981Three virulence-associated plasmids in Y. pestis [20-22]
1982-presentAfrica's ascendancy in incidence [23]
1985–1999Usefulness of gentamicin for therapy [24]
1995Isolation of plasmid-mediated streptomycin-resistant Y. pestis from patients [25, 26]
2003Usefulness of fluoroquinolones for therapy [27]
2005–2011DNA of Y. pestis found in human graves from the Justinian and medieval eras [28-31]

Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

Credit for discovering the bacterial cause of plague is accorded to the French physician Alexandre Yersin (1863–1943), for his bacteriological investigations in June 1894 in Hong Kong during a deadly epidemic [32]. However, credit was not given to Yersin initially, and nor in the ensuing years by everyone, because he had a rival in Shibasaburo Kitasato (1851–1935), a Japanese bacteriologist sent by his government to investigate the cause of plague. Rarely in the history of medicine were conditions more ripe for fruitful investigation of a disease that is now known to have abundant bacteria in both blood and swollen lymph nodes (buboes) that are cultivatable and stainable on microscope slides, as well as readily obtainable in autopsy specimens. Both scientists were qualified and had portable laboratory equipment and supplies with which to solve the mystery of plague's cause. Yersin had worked with Emile Roux in Louis Pasteur's laboratories in Paris to characterize diphtheria toxin, and Kitasato had worked in Robert Koch's laboratories in Berlin, where he discovered the bacterial cause of tetanus.

Yersin was clearly the underdog. Kitasato arrived 3 days earlier than Yersin, and was accompanied by a team of six assistants, whereas Yersin had travelled alone from French Indochina, carrying in his baggage a microscope, sterilizer, and culture supplies [1]. Kitasato was more senior and famous, and obtained immediate access to autopsies of plague victims, whereas Yersin was denied such access for several days. Yersin was regarded as a small and shy man, spoke no English, and had an inherent disadvantage, perhaps, of being French in a British colony. Kitasato got a head start, observing bacteria in the blood and injecting animals with specimens before Yersin even arrived. The two rivals were introduced, but they exchanged little information in their common language of German. The story was related that, at a meeting of the two during an autopsy that Kitasato was conducting, Yersin was surprised to observe that Kitasato was examining blood rather than buboes [1]. Yersin relayed the suggestion via an intermediary that buboes should be examined too, with the result that Kitasato subsequently examined buboes. Both men found bacteria that they designated as the cause of the disease. Yersin correctly described his as Gram-negative, whereas Kitasato insisted that his organism was Gram-positive. Kitasato described his blood organism as diplococcal and his bubo organism as bacillary, causing confusion and suggesting to some that his cultures were contaminated by the pneumococcus. In the ensuing months and years, Kitasato asserted that his bacillus was different from that of Yersin [33].

Eventually, against the odds, Yersin took the prize of finding the cause of plague. His descriptions were accurate and consistent. Also, Yersin's reputation benefited from his links with colleagues in Paris. During his first autopsy in Hong Kong, he related his method of obtaining fluid from the bubo, seeing Gram-negative bacilli, injecting animals that he observed to die with bacteria in their tissues, and sealing a bubo specimen in a glass tube that was immediately mailed to Paris [2]. These specimens were received by Albert Calmette and Amedee Borrel, both destined to be famous for, respectively, a tuberculosis vaccine and the name of the cause of Lyme disease, who confirmed Yersin's findings and carried out research with the bacteria to produce a therapeutic antiserum. In 1895, the last year of Louis Pasteur's life, Yersin returned to Paris to collaborate with his associates in Pasteur's laboratories. Before the end of that year, he was back in Indochina, and in 1896 he went to Hong Kong again to treat plague victims with his new antiserum [1].

The name of the organism underwent several changes. It was Bacterium pestis until 1900, when it changed to Bacillus pestis. In 1923, it acquired a new designation as Pasteurella pestis, which it kept up to about 1970, when Yersin obtained posthumous honour through its final name, Yersinia pestis [32].

Role of James Lowson in the Discovery

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

A hundred years after the discovery of the plague bacillus, the daughter of James Lowson showed his diary to Yule [34], and Solomon [35] subsequently carried out historical research by obtaining further information from Lowson's granddaughter. Lowson was a Scottish physician sent by the British Colonial Medical Service to Hong Kong, where he served as acting Superintendent of the Civil Hospital, dealing with patient care, and as Port Medical Officer, dealing with quarantine needs. When plague broke out in Canton in May 1894, Lowson travelled there to witness fatal cases. When cases appeared in Hong Kong later in May, he advocated quarantine measures, established a hospital ship in the harbour for cases, and persuaded the Governor to initiate disinfection of houses in affected areas of the city. He was responsible for showing both Kitasato and Yersin the colony's facilities for patient care and diagnosis. He was preferentially hospitable to Kitasato's team, dining with them several times, and announcing to the Lancet by wire on 15 June, the same day on which Yersin arrived, that Kitasato had succeeded in finding the cause of plague. Although Lowson did not befriend Yersin, he spoke to him occasionally, recording in his diary on 22 June that the ‘Frenchman’ had found his bacillus. Lowson was aware of the rivalry between the two scientists, and also of the inaccurate descriptions by Kitasato of the organism being Gram-positive and, in blood, appearing as a diplococcus, but he continued to hold the opinion that Kitasato deserved credit for first finding the cause of plague. Solomon speculated that Lowson pushed Kitasato to hasty and careless publication in order to get the early report in the Lancet [35].

Yersin's Observation that Rats were Infected with the Plague Bacillus

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

While carrying out his studies of cadavers, Yersin noted many dead rats in Hong Kong. He examined the lymph glands of some dead rats, and found the same bacillus that he had described in human tissues [1, 2]. Thus, he was the first to make the important causal connection between rat mortality and human epidemics. He did not suspect fleas as vectors, but surmised that rats acquired their infections from contact with soil, from which he determined that his bacillus could be cultured [33, 35].

Flea-borne transmission

In 1898, Paul-Louis Simond (1858–1947) reported his experiments carried out in Karachi, India [2-4, 32]. When flea-infested infected rats were placed into cages separated by wire mesh from uninfected rats, the uninfected rats acquired fatal plague infection. Even before his experiments, Simond had suspected a blood-sucking vector, because he had noted, on the skin of patients near their buboes, pink nodules, which contained plague bacilli [2]. Simond additionally observed bacilli in the stomachs of fleas that had fed on infected rats.

Discovery of Effective Therapies

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

Antiserum

The first application of antiserum to the treatment of patients is credited to Yersin [5], who used serum developed with the assistance of his Parisian colleagues Calmette, Roux, and Borrel. He administered antiserum to 23 Chinese patients in Hong Kong in 1896, with good results; only two died, giving a remarkably low mortality rate of 9% [33]. Subsequent uses of antiserum by Yersin and other workers in this field were less successful, and some efforts were deemed to be failures. The difficulty in evaluating this research stems from the lack of standardization of batches of serum, which were developed in different animals in different countries, and administered in varying doses. Meyer reviewed the literature on antisera [36], making use of decades of studies in Asia, Africa, and South America. From a compilation of >20 000 patients, Meyer gave results for untreated patients and patients who received antisera. The mortality rate for all patients receiving antiserum was 35%, whereas untreated patients in these same reports had a mortality rate of 82%. Although the studies were not controlled or randomized, the overall impression was that antiserum therapy was beneficial. The superiority of sulphonamides and, later, streptomycin, however, was quickly appreciated, with the effect that interest in antiserum therapies declined precipitously during the 1940s.

Sulphonamides

The first use of sulphonamides in patients was reported in 1938 in East Africa [10] and in India [11]. In 1937, Carman described African patients who received intramuscular injections of the sulphonamide Prontosil. Although only three of six treated patients survived, the report made a convincing argument for the drug's efficacy by comparing these patients with nine consecutive fatalities in similar patients at the same hospital before the drug was available. Furthermore, Carman related how the three deaths occurred in patients who were treated after 4–5 days of illness, whereas the three surviving patients had been ill for only 1–3 days after the onset of their illnesses [10]. Also in 1937, Vine administered Prontosil injections to three Indian patients with plague. They all survived, whereas two of four patients not receiving this treatment died [11]. In 1939 in the Belgian Congo, van Hoof [37] reported that one of two children treated with sulphanilamide survived. Also in 1939, Chopra et al. [38] reported that a 6-year-old girl in India was treated successfully with sulphapyridine. In Madagascar up to early 1940, Girard reported that 37 patients with bubonic plague received injections of sulphapyridine (Dagenan), with the result that only nine, or 24%, died [39]. On the other hand, all eight patients with pneumonic plague who received the sulphonamide died [39]. During an epidemic of plague in 1940 in Egypt, Kamal et al. [40] reported that sulphanilamide or sulphapyridine was given orally to 81 patients, of whom 27, or, 33% died. When antiserum was combined with either drug, even better results were obtained; there were only 12 deaths out of 69 treated patients, or 17% [40]. Subsequent studies in 1943–1944 in India with sulphadiazine or sulphathiazole demonstrated that, of 180 patients, only 50, or 28%, died [41]. This mortality rate was compared with 58% for 165 patients treated with intravenous iodine, which is considered to be an ineffective therapy, and who thus constituted a control group. In cases of pneumonic plague, which advances inexorably to death within 3 days after the onset of symptoms, attempts to save patients with sulphonamides were unsuccessful in 1941 [39], but three patients were successfully treated with sulphapyridine in 1947 in Madagascar [42].

Streptomycin

The first use of streptomycin was reported in 1947 by Videla [14], who described five patients with plague in Argentina. All of them survived, with two being discharged promptly after therapy, and three having more prolonged hospital courses. Also in 1947, with reports being published in 1948, patients in Palestine and India were treated with streptomycin [13, 43-48]. In Palestine, Haddad and Valero [43] treated three severely ill patients who had failed to respond to sulphonamide, with the result that all survived. In India, Sokhey and Wagle [44] reported that only four of 118 bubonic plague patients treated with streptomycin, or 3%, died. These authors updated this experience in 1953 to include 148 patients, of whom only six, or 4%, died [45]. In addition to the lower mortality rate with streptomycin therapy, the authors noted faster defervescence of patients treated with streptomycin, with an average of 50 h, as compared with 85–89 h for those treated with sulphadiazine or sulphamerazine [45]. One of the authors, Wagle [13], an astute investigator, performed quantitative blood cultures to show that three of the fatal cases treated with streptomycin had high-grade bacteraemia. Wagle called this condition severe septicaemic plague, indicating that the patients were probably already in septic shock that was not amenable to reversal with antibiotic therapy. Also in India, Karamchandi and Rao [46] described five patients treated with streptomycin. They all survived, despite the fact that they were all moribund, and the condition of three of them had worsened while they were receiving sulphathiazole during the preceding day. Having been semiconscious or comatose before treatment, these patients were observed to regain consciousness, with marked improvement, within 36 h after the start of streptomycin treatment. These authors updated their experience in the following year to include 15 moribund plague patients treated with streptomycin, of whom 3, or 20%, died [47]. These physicians were impressed with both the efficacy of the drug and its superiority to sulphonamides, because all of their patients were moribund, and three of the survivors had failed to respond to sulphadiazine. Furthermore, all three deaths occurred in patients who had presented after the third day of illness, with death occurring during the first day of treatment, whereas five survivors described in 1948 all presented for treatment after only 1 day or 2 days of illness [47]. In another report from India, Datt Gupta related that seven of 24 plague patients treated with streptomycin, or 29%, died [48]. This relatively high mortality rate was explained by the study design, whereby only severe cases were selected to receive streptomycin. Furthermore, two of the deaths occurred within 12 h after admission, indicating that the patients probably had advanced disease before treatment [48]. Thus, 1947 was a watershed year: all bubonic plague patients fortunate to receive streptomycin within the first 2 days of symptoms survived, whereas deaths were reported only in some ‘moribund’ cases who had symptoms for at least 3 days or had failed to respond to sulphonamides [47]. For pneumonic plague, in which sulphonamides had failed to save any lives among eight patients in 1941 [39], streptomycin therapy in 1948 in Madagascar was reported to give recovery in one patient [49], and in 1949 to give recovery in another two patients [50].

Lack of penicillin efficacy

In the quest for effective therapies, there was general agreement among investigators that penicillin was useless [13, 36, 45, 51, 52]. Infected mice and guinea pigs failed to respond to penicillin injections, despite the fact that the strains of Y. pestis were uniformly susceptible to penicillin, with sensitivity of 1–5 units/mL [36]. Meyer et al. [36] explained that penicillin inhibited bacteria well, but much higher concentrations of drug, approximately 10 000 units/mL, were required to kill bacteria. Penicillin was also reported to be without benefit in human plague [13]. This is a peculiar discrepancy between in vitro susceptibility and clinical performance, because other bacteriostatic antibiotics, such as tetracyclines and chloramphenicol, are effective against plague. The usual reasons for an antibiotic to fail clinically are antibiotic resistance in vitro and failure of a drug to penetrate into infected tissues, neither of which pertained to plague. Because of this failure of penicillin to show efficacy against plague, no other β-lactam antibiotic has been tested in humans with plague, despite experimental efficacy in animal infections of ampicillin, amoxycillin, and newer cephalosporins [53-55].

Chloramphenicol and oxytetracycline

The first applications of chloramphenicol and oxytetracycline (Terramycin) were reported in Madagascar in 1953 [56]. These antibiotics were as effective as streptomycin, but not better than streptomycin. Their usefulness was in providing additional drugs that were available for oral administration in outpatients. Chloramphenicol had the advantage of good penetration into cerebrospinal fluid for the rare complication of plague meningitis. In addition to oxytetracycline and chlortetracycline, other drugs in this class, including tetracycline and doxycycline, later came into use and have benefited many patients.

Trimethoprim–sulphamethoxazole

After trimethoprim–sulphamethoxazole was shown to be active in vitro, as well as effective in experimental animals, Ai et al. [17] treated 12 patients in Vietnam. All survived, with defervescence occurring in 2–5 days. Subsequently, in a prospective randomized comparison with streptomycin, trimethoprim–sulphamethoxazole treatment resulted in the cure of five patients, but defervescence was not as prompt as with streptomycin [18].

Gentamicin

A need arose to find a replacement for streptomycin as the drug of choice, because the manufacturers of streptomycin discontinued it following reports of toxicity to the kidneys and ears of patients, as well as to fetuses during pregnancy. In a retrospective analysis, gentamicin or gentamicin plus tetracycline was administered to 18 patients in the USA between 1985 and 1999, with the excellent result that all survived [24]. Additionally, in a randomized controlled comparison of gentamicin and doxycycline in Tanzania in 2002, 35 patients received gentamicin. Only two patients, or 6%, died, both on the first day of treatment [57]. Thus, gentamicin could take the place of streptomycin as a drug of choice for plague.

Fluoroquinolones

The newest antimicrobial drugs to be employed for plague are fluoroquinolones. Ciprofloxacin and other fluoroquinolones were effective in animal studies [54, 58, 59]. Although prospective comparative studies have not been carried out in humans, one patient in the USA and five patients in Algeria were reported to be successfully treated with ciprofloxacin [27, 60].

Rarity of antibiotic resistance

Only two persons with plague have ever been reported to be infected with streptomycin-resistant Y. pestis. Both were in Madagascar in 1995, and their survival could be attributed to receiving trimethoprim–sulphamethoxazole concomitantly with streptomycin [25, 26]. Both cases were demonstrated to be caused by bacteria that harboured resistance plasmids that were transferable from other bacterial species. Fortunately, these resistant strains did not persist in the environment, because all subsequent cases of plague have been caused by antibiotic-susceptible bacteria [61, 62].

A Modern Perspective on Plague Therapies

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

This history of therapies suggests that antiserum was effective enough to lower the mortality rate approximately two-fold, from c. 70% to 35%. Sulphonamides were even better, reducing mortality rates to approximately 20%, and streptomycin was a breakthrough, reducing it further to approximately 7%. Streptomycin probably achieved the maximal antimicrobial benefit, as this has not been improved on with the currently favoured drugs, gentamicin and doxycycline [57]. The worldwide mortality rate of patients reported to the WHO in 2000–2009 was actually 7% [63], which could be lowered further only by patients receiving treatment more promptly after the onset of their symptoms. For bubonic plague, treatment started within 3 days after the onset of symptoms is nearly always effective, whereas, for pneumonic plague, treatment must be started within 24 h after the onset of symptoms to be life-saving [56]. None of the studies of therapeutic efficacy was carried out as a randomized controlled trial, so the results were potentially biased by disease severity and arbitrary selection criteria. Only in later decades were objective comparisons of therapies using randomized designs performed for trimethoprim–sulphamethoxazole vs. streptomycin and doxycycline vs. gentamicin [18, 57].

Fluctuations in Incidence

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

The WHO estimated that India suffered >6 million deaths from plague in 1900–1909. This incidence far exceeded that of any other country, representing a high-water mark for the disease in the last century, to be followed by stepwise declines in subsequent decades, reaching <1000 cases a year in 1955 [7, 16]. The decline was attributable to public health measures for rodent control, insecticides, vaccines, and natural cycles in rodent populations, despite some years with increases in the 1940s, owing to disruptions caused by World War  II and the partition of India [7].

During the war in Vietnam, a breakdown in public health services and the creation of refugees led to an increase that peaked at >5000 reported cases a year in 1967 [16]. In 1965–1975, Vietnam's 29 003 reported cases accounted for 80% of the total cases in the world.

Coincident with a reduction in the incidence in Vietnam after the war, Africa's reported cases showed increases starting in 1982, leading to that continent suffering >90% of the world's total cases during the 1990s and up to the present [23]. The countries of Madagascar, Congo and Tanzania experienced most disease, which was attributed, in part, to deforestation, urbanization, and civil wars.

Serological Diagnosis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

In the 1950s, measurements of antibodies in the blood of patients as indicators of an immunological response to plague became possible when serological tests using the fraction 1 antigen of Y. pestis were developed [15]. Previously, a laboratory diagnosis of plague had required cultures or observation with a microscope of bacteria from buboes or blood, but these specimens and techniques were often unavailable in plague-endemic places. Diagnosis by elevated levels of blood antibodies or a rise in antibody level between the early and convalescent phases of disease greatly expanded the numbers of patients that could be confirmed as having the infection.

Plague Vaccines

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

The development of vaccines got an early start in 1897, when Waldemar Haffkine (1860–1930) showed that a heat-killed culture of plague bacteria protected rabbits against experimental infection. This preparation was tested in humans in India, with >20 million doses being given, resulting in observations of reduced incidence and mortality in immunized persons [6]. In 1931, Georges Girard and Jean Robic developed a live attenuated non-pigmented strain of the plague bacillus in Madagascar called EV [9]. This vaccine or similar live attenuated bacteria with designations including EV76, EV NIIEG and Tjiwide were administered to millions of people in Madagascar, Indonesia, Vietnam, and the Soviet Union [8, 64]. In an effort led by Meyer, starting in 1939 [12], the US Army developed a formalin-killed bacterial cell vaccine that was given to more than a million American servicemen deployed to Vietnam. Controversies remain over how much protection these vaccines provided; besides, all produced local or systemic reactions that were sometimes serious [65]. By the end of the 20th century, these vaccines were rarely used and became commercially unavailable.

Laboratory Correlates of Clinical Severity and Mortality

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References

Clinical observations of patients had suggested that, after ≥3 days of fever and bubo enlargement, some progressed to septicaemia and a fatal outcome. In India, Wagle [13] reported higher mortality rates when patients' blood cultures grew >1200 colonies/mL. In Vietnam, measurement of plasma endotoxin with the sensitive Limulus test showed that patients with endotoxaemia were severely ill and had greater numbers of blood bacteria by quantitative culture than patients without endotoxaemia [18].

Plasmids

The 1980s was the decade in which it was discovered that extrachromosomal DNA in plasmids was responsible for virulence. Y. pestis harbours three virulence plasmids—pFra, encoding the antiphagocytic capsular protein fraction 1 and the murine toxin that enables bacteria to survive in the flea gut; pCD (calcium dependency for growth at 37°C), also called pYV for Yersinia virulence, encoding V antigen and Yersinia outer proteins, which disrupt phagocytosis and reduce inflammation; and pPla, encoding a plasminogen activator that allows bacteria to spread in tissues by dissolving fibrin clots [20-22, 66-68]. Earlier genetic studies had hinted at plasmid-mediated virulence in Y. pestis. In 1976, Beesley et al. [69] suggested extrachromosomal inheritance of the virulence determinants of pesticin, coagulase, and fibrinolytic factor, and in 1973 Kol'tsova et al. [70] demonstrated that pesticin could be transmitted as an episome by conjugation from one bacterial cell to another. The evolutionary implication of mobile genetic elements determining virulence was that ancestral progenitor bacteria acquired, from contact with other donor organisms, their abilities to disseminate by flea bites and to kill people rapidly. The most likely progenitor of plague is Yersinia pseudotuberculosis, because this species shares >90% of its chromosomal DNA sequences with Y. pestis [19]. Although Y. pseudotuberculosis has the pYV plasmid, it must have acquired both the pFra and pPla plasmids in order to evolve into our present-day Y. pestis.

Palaeomicrobiology

Three biotypes of Y. pestis—Antigua, Medievalis, and Orientalis—are based on glycerol fermentation and nitrate reduction, and have been postulated to have originated historically in sequence, with Antigua being the oldest, starting to cause Justinian plague in the sixth century, Medievalis starting to cause the Black Death in the 14th century, and Orientalis originating in southern China in about 1890, spreading along shipping routes to become the predominant biotype today. By the use of PCR and known DNA sequences from banks of DNA from modern Y. pestis strains, it has been possible to diagnose plague biotypes in the remains of victims from pandemics in past centuries. The specimen most likely to harbour intact ancient DNA is dental pulp from teeth recovered from graves in places known to have suffered plague mortality. Diagnostic DNA sequences of both plasmids and the chromosomes were demonstrated in teeth of remains in London of the 14th century [30, 31], remains in France of the seventh to ninth and 18th centuries [29], and remains in Germany of the sixth century [28]. These DNA sequences were nearly identical to modern sequences of genes that determine virulence, suggesting that ancient strains were ancestors of today's bacteria, and that plague's current status of relatively subdued endemicity might be attributable to changes in other genetic features that govern virulence. Reconstruction of the pPla plasmid from London graves showed some matches with the Medievalis biotype but not with the Orientalis biotype, whereas analysis of genes from graves in France and Germany indicated that ancient strains from both the Justinian and medieval eras resembled the Orientalis biotype [28, 29, 71]. Further evidence for the Orientalis biotype came from the finding of Orientalis-specific sequences in these teeth by genotyping, and observation of deletion of the gene for glycerol dehydrogenase, the enzyme responsible for glycerol fermentation in the other biotypes [72, 73]. Thus, the previously held notion that ancient epidemics were caused by the Antigua and Medievalis biotypes needs to be revised.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Discovery of Yersinia pestis as the cause of Plague: Yersin as the Underdog
  5. Role of James Lowson in the Discovery
  6. Yersin's Observation that Rats were Infected with the Plague Bacillus
  7. Discovery of Effective Therapies
  8. A Modern Perspective on Plague Therapies
  9. Fluctuations in Incidence
  10. Serological Diagnosis
  11. Plague Vaccines
  12. Laboratory Correlates of Clinical Severity and Mortality
  13. Acknowledgements
  14. Transparency Declaration
  15. References
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