1.15. The Black Plague (Yersinia pestis)

Question 1

Describe the 5 challenges that a microbe must overcome to cause infection/disease

Answer 1

1.Maintain a reservoir (a human, animal or environmental source in which a microbe can exist and from which it can be transmitted) •An infectious disease spread from animals reservoirs such a rodents (e.g. plague), or contaminated animal products e.g. leather or wool (e.g. anthrax) to man is called a ZOONOTIC infection (‘zoonosis’)
2.Gain access to a new host
•Portal of entry (skin, mucous membranes; respiratory tract; gastrointestinal tract); mode of transmission (contact, traumatic/parenteral inoculation; vector-borne; inhalation; ingestion).
•A vector is a living creature that transmits infection from one host to another (e.g. mosquito, flea, tick, etc.)
3.Adherence: non-specific (e.g. hydrophobicity, Van der Waal’s forces etc.) & specific mechanisms (e.g. adhesion organelles such as pili, fimbriae and adhesins)
4.Mechanisms of disease causation
a.Adherence
b.Invasion (‘aggressins’)
c.Toxins (exotoxins, endotoxin)
d.Immunologically-mediated damage (cytokines, superantigens, hypersensitivity-mediated reactions)
5.Exiting from one host, entering another, or returning to the reservoir

Question 2

List the countries that are still actively reporting plague worldwide

Answer 2

• developing countries
  1. India?
  (refer to notes and edit)

Question 3

Zoonosis in the plague cycle

Answer 3

• Wild rodents encroach on human habitat: food, weather, deforestation, etc.
• Domestic rodents then at risk
• Humans encroach on rodent habitat: agriculture, hunting, residence
• Domestic animals can also intrude, get infected, be source of human infection

Question 4

Bacteriology & properties

Answer 4

• Gram negative bacillus
• Obligate pathogen: does not survive freely in the environment
• Unable to replicate in nature outside a host (flea or mammal)
• Survive drying for few days; survival prolonged in dried blood & secretions

Question 5

Pathogenesis:

Answer 5

• Virulence factors:
• Temperature-dependent coagulase -< 30 C
• Fraction 1 antigen (capsular glycoprotein) –37 C
• Low calcium response at 37 C mediated by a 70 kb plasmid is important for adaptation to IC environment
• V & W antigens (maintain bacteriostasis& aid IC survival)
• Yersinia OMPs (Yops): type E & K important for virulence
• Other: antigen 4 or ‘pH 6’ antigen; pigment production, LPS endotoxin & plasmid-encoded murine exotoxin (lethal only to mice & rats)
• Fleas are cold blooded > ingestion of organisms from blood of a suitable reservoir results in clotting of blood in flea’s proventriculus(foregut) > blood containing many organisms is regurgitated during subsequent feeding attempts > perpetuation of infection cycle
• F1 Ag enables organism to evade phagocytosis when produced at 37 C > immune evasion
• Intracellular survival negotiated by ‘low calcium response’ and V & W Ags

Question 6

Diagnosis:

Answer 6

• Bubo aspirates; blood cultures; sputum (? pneumonic); CSF (? meningeal)
• Microscopy (Wayson, Giemsa / Gram stains); culture
• Serology
• PCR

Question 7

Treatment:

Answer 7

• Untreated, case fatality rate > 50%
• Antibiotics e.g. aminoglycosides; doxycycline; fluoroquinolones
• Supportive therapy
• Infection prevention and control, as appropriate

Question 8

Prevention:

Answer 8

• Surveillance
• Public education
• Rodent and flea control
• Chemoprophylaxis for close contacts with plague pneumonia & individuals exposed in laboratory accidents
• Infection prevention & control

Question 9

Yersinia pestis. Microbial Pathogenesis -the Five Challenges

Answer 9

1.Maintain a reservoir
- Human, animal, environmental Y pestis reservoir: rodent (rats), gerbils –zoonosis!
2.Gain access (via vectors: fleas) to a new host (animal/human) Portal of entry (inoculation into host) ; mode of transmission (vector bite)
3.Adherence: non-specific & specific mechanisms (? role of CCR5 receptor)
4.Mechanisms of disease causation:
•Establishing infection: evading host defenses (Fraction 1 Ag; low calcium response plasmid; V & W antigens and intracellular survival)
•Invasion of tissues (buboes, lungs, etc.)
•Toxins (exotoxins [murine exotoxin], endotoxin [LPS]); immunological damage (inflammatory cytokines)
5. Exiting from one host, entering another (human-human; animal-human; flea to animal/human returning to the reservoir (rodents)