Modes of Transmission ■ Fleabite ■ Inhalation of salivary droplets of infected humans or cats

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Overview - 3 Types of Plague

Modes of Transmission

  • ■ Fleabite

  • ■ Inhalation of salivary droplets of infected humans or cats

  • ■ Cat’s scratch

  • ■ Ingestion of the bacillus

  • ■ Contact with infected body fluids

Plague and World History

  • 1320 B.C. -- 1st mention of plague is in the Bible Philistines stole the ark of the covenant from the Israelites and plague ensued:

  • [t]he Lord’s hand was heavy upon the people of Ashdod and its vicinity. He brought devastation upon them and afflicted them with tumors. And rats appeared in their land, and death and destruction were throughout the city…[T]he Lord’s hand was upon that city, throwing it into great panic. He afflicted both young and old with and out break of tumors in the groin.

Plague and World History

  • 541-700 A.D. – 1st Pandemic, Justinian Plague

  • ■ Began in Pelusium Egypt and

  • spread to the middle East

  • & Europe with estimated population

  • losses of 50-60% in N. Africa,

  • Europe & S. Asia.

  • ■ A.D. 558-654: The 2nd through 11th epidemics occurred

  • in 8-12 year cycles.

Plague and World History

  • 1346-1666 – 2nd Pandemic, Black Death

  • Plague believed to have entered Europe along trade

  • routes from central Asia by fleas in bundles of imported fur.

  • 1347-1351: 1st 5-year epidemic killed an estimated 17-28 million people, 30-40% of Europe’s population at the time.

  • Epidemics continued throughout the period in 2-5 year cycles until 1480 and then with less frequency until late 17th century.

  • The plague stimulated significant

  • advance in medical practice including

  • the beginnings of clinical research,

  • reorganization of hospitals.

Plague and World History

  • 1855 – 3rd Pandemic, China

  • Began in the Yünnan province and troops from the war in that area helped to spread plague down the southern coast to Hong Kong and Canton by 1894 and Bombay by1898.

  • By 1900 steamships had helped to spread the disease to Africa, Austrailia, Europe Hawaii, India, Japan, the Middle East, the Phillipines, the U.S, and S. America. By 1903, 1 million people/year were dying of plague in India.

Plague and World History

  • 1855 – 3rd Pandemic, China (cont’d)

  • Stable enzootic foci were established on every inhabited continent except Austrailia and outbreaks continue until today though greatly reduced because of advances in public health practices and drugs.

  • Significantly, it is during the 3rd Pandemic that the bacterium responsible for plague as well as its vectors and reservoirs were identified.

History of Yersina Pestis

  • First discovered in 1894 during the Hong Kong Plague by two independent investigators (Alexandre Yersin and Shibasaburo Kitasato) within days of each other.

  • Kitasato was initially credited for the discovery but Yersin’s bacillus proved to be the infecting bacillus.

  • Yersin drained buboes of deceased plague victims and identified the bacillus with microscopy.

  • Confirmed the involvement of the bacillus in infection by injecting healthy rodents with infected lymph isolates. Healthy animals developed plague symptoms and dissection showed blood and organs to be filled with the isolated bacillus.


  • Nomenclature: the plague bacillus has had four name changes.

  • Classifications:

  • Bacterium Pestis until 1900

  • Bacillus Pestis until 1923

  • Pasturella Pestis (after Yersin’s mentor)

  • Yersinia Pestis in 1970 until present day

Identification of Vectors and Reservoirs

Early Prevention and Treatment Methods

  • Yersin isolated serum from horses immunized with the bacillus for treatment of human plague in 1896.

  • W.M Haffkine created and used an effective preventative vaccine for the Plague containing killed bacteria during the Manchurian outbreak (1910).

  • L.T Wu characterized pneumonic plague during the 1910 Manchurian outbreak and helped to institute protective measures against aerosol spread of the disease.

Modern Plague

  • For most of the 20th century the occurrence of plague has sharply decreased though not disappeared. Plague remains an enzootic infection of rats, ground squirrels and other rodents in every inhabited continent except Austrailia.

  • Why has Plague decreased so sharply?

  • The WHO credits public health prevention protocols and the development of effective antibiotics.

Modern Plague

  • WHO reports 1000-3000 cases of plague worldwide annually

  • (avg.1700cases/year for the last 50 years) (conservative estimate)

  • Plague is largely underreported in countries with limited surveillance and laboratory capabilities.

  • Modern plague is primarily occurrent in rural areas with poor sanitation and large rodent populations.

  • Urban cases are increasingly rare. Los Angeles1924 was the last U.S urban outbreak.

Modern Plague - Distribution

Modern Plague - Timeline

  • 1900 - Plague officially arrives in the U.S. Infected corpse of Chinese Laborer found in San Francisco hotel basement.

  • 1924 - Los Angeles. 33 cases, 31 fatal. 1st case, Mexican American male, misdiagnosed with STD, family and neighbors contract plague and die with in two weeks. Many die before public health measures were taken.

  • 1967-72 - Vietnam. Defoliation, disturbance of the ecosystem and economic losses during the war contributed to outbreak. Singular contributor to the 1967 jump in worldwide plague vases.

  • 1992 - Arizona. 1 confirmed case, 1 fatality. Misdiagnosis of pneumonia. Post-mortem lab tests confirmed Y. Pestis. Family cat identified as source.

■ 1997

  • ■ 1997

  • Zambia- Jan. Namwala region 90 cases of bubonic plague, 22 fatal.

  • Heavy rains drove rats into inhabited regions.

  • Mozambique - Aug. Tete province. 115 reported cases of bubonic.

  • No reported deaths.

  • Malawi - Oct. Southern region, 43 reported cases, 17 of which were seropostive. >60% cases were children. No reported deaths.

  • Mozambique - Nov. August outbreak continues and extends to 225 cases.

  • No reported deaths.

  • 1999

  • Namibia - May Northwest region, 39 reported cases, 8 deaths.

  • Malawi - July Reported cases in 22 villages, 74 suspected cases total.

  • No reported deaths.


  • 2001

  • Zambia - Mar. Nyanje and Petauke regions 436 suspected cases, 11 deaths. Y. Pestis postively identified.

  • 2002

  • India - Feb. Hat Koti village. 16 cases of pneumonic plague including 4 deaths. Cases linked to one villager.

  • Malawi- June. Nsanje region and 26 nearby villages. 71 reported cases of bubonic plague.

  • 2003

  • New York - Nov. 2 cases. Couple contracts plague in Santa Fe, NM and travels to New York. Neither case fatal but man required double foot amputation.

Trends in Human Plague in the U.S.

  • From 1899-1926 Plague in the U.S. was an urban epidemic involving domesticated rats with cases most prevalent in California and Hawaii.

  • Late 1940’s saw a jump in infection in the Southwestern states which remain hotspots for infection.

  • Static annual infection levels from 1925-1964 (avg.<2 cases/year). 1965 saw increases that would carry into the 80’s.

Trends in Human Plague in the U.S

  • Seasonal distribution: 1926-1979 81.5% of plague cases occurred in May-September. Prior to 1926 most were Sept.-Oct.

  • 1970-1980

  • 53% cases ♀

  • 59% were < 20 years old.

  • Similar age distribution from 1926-1969

  • Racial dist.: 1970-79 35% of cases were in Indians (1.4 Indians per 100,00 compared to 0.1non-Indians).

Molecular Biology of Yersinia Pestis

Bacteriological Characteristics

  • Gram negative

  • Non-motile

  • Enterobacteriacae

  • Non-spore forming coccobacillus

  • Facultative anaerobe

  • Obligate parasite

Bacteriological Characteristics

  • .5-.8 uM in diameter

  • 1-3 uM long

  • Grows optimally at 28° C and a pH of 7.2-7.6

  • Bacterial cell wall

  • Protein Envelope (F1)

Evolution of Y. Pestis

  • There are 11 species of Yersinia

  • 3 pathogenic species of Yersinia

    • Yersinia Pseudotuberculosis - enteropathogen
    • Yersinia Enterocolitis - enteropathogen
    • Yersinia Pestis - systemic pathogen
  • Y. Pestis evolved from Y.pseudotuberculosis 1500-4000 years ago

    • 90% chromosomal DNA relatedness
    • Physiological and antigenic similarities

Y. Pseudotuberculosis Y. Pestis

  • Disease

    • Bubonic Plague
  • Transmission

    • Flea, mammal, never found in free environment
  • Pseudogenes

    • YadA, Inv, O antigen
  • Extrachromosomal DNA

    • pCD1
    • pFra
    • pPst
  • Pathogenicity Islands

Biovars of Y.Pestis

  • 3 biovars named based on their ability to convert nitrate to nitrite and ferment glycerol

    • Antiqua (1st pandemic)
    • Medievalis (2nd pandemic)
    • Orientalis (3rd pandemic)


  • 4500 genes

  • One chromosome- 4.6Mb

    • Pathogenicity islands (pgm locus)
  • Three plasmids

    • pFra – 96.2 kb
    • pPst1- 9.7 kb
    • pCD1- 70.3kb

Chromosome Pgm locus

  • 102 kb pathogenicity island in the chromosome

  • contains three hemin storage genes (Hms)

    • These genes only found in Y.pestis
    • They are essential for flea blockage
    • Allow Y. Pestis to store large quantities of hemin (iron source) in its outer membrane
  • Bacteria requires iron in order to cause infection

  • An inorganic iron transport system (ybt) (a sidephore) is encoded within Pgm locus

  • Ybt chelates iron bound to eukaryotic proteins and transports them to the bacterium


  • 60-280 kb (depending on the strain)

  • Found only in Y. Pestis

  • Contains gene for murine toxin (Ymt)

    • Two forms of a protein toxic for mice and rats
    • Proposed as B-adrenergic antagonist
    • Ymt transcription is threefold higher at 26°C(flea) than at 37°C(mammal).
    • This gene is believed to be essential for flea colonization


  • Contains structural and regulatory genes that encodes F1 protein capsule (caf1)

    • F1 is a proteinacious capsule that forms at 37°C (mammal) but not 26°C (flea)
    • F1 capsule makes bacterium resistant to phagocytosis by monocytes
    • Resistance to phagocytosis by monocytes allows journey from dermis to viscera

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