Pathogenicity and Virulence in Bacterial Diseases

Pathogenicity

  • Pathogenicity is the capacity of a bacteria to produce disease.
  • A bacteria will generally be either pathogenic or non-pathogenic, although opportunistic bacteria can blur this line.
  • Pathogenicity depends on:
    • The bacteria's ability to enter the host.
    • Attachment to host tissues.
    • Evasion of host defenses.
    • Causing damage to tissues, thereby producing symptoms.
  • Host: An organism, usually a human, that harbors another organism on or in itself.

Virulence

  • Virulence refers to the severity of sickness or the degree of pathology or damage caused to tissues.
  • Virulence depends on both bacterial and host features.
  • Examples:
    • Cold virus: Less virulent.
    • Influenza A: More virulent.
    • Rabies: Very virulent (100% fatality rate if untreated).
  • Virulence is measured by case fatality rates:
    • Rabies: Case fatality rate of 1 (100%).
    • Cold virus: Case fatality rate of approximately 0.0000.
    • Influenza: Case fatality rate of approximately 0.001 to 0.002 (0.1% to 0.2%).
  • Pathogenicity = Infectivity \times Virulence
    • Pathogenicity is a combination of how easily the bacteria spreads and how able it is to cause disease.
    • Example: The common cold is very infective but not very virulent.
  • Virulence is also dependent on characteristics of the host.
    • Example: 90% of COVID-19 deaths occurred in people with pre-existing illnesses like diabetes, high blood pressure, and obesity.

Factors Determining Virulence

  • Does the microorganism cause disease? (Determines pathogenicity).
  • How many microorganisms are needed to cause disease? (Indicates infectivity).
  • How many microorganisms are needed to kill? (Indicates virulence).
  • How long does it take for a person to die from the infection? (Shorter time indicates higher virulence).

Extremely Virulent Diseases

  • Rabies and Ebola are examples of very virulent diseases.
  • Ebola:
    • Hemorrhagic fever spread by direct contact with body secretions.
    • Interferes with blood vessel cell walls, causing bleeding.
    • Death usually occurs within seven days.
    • Case fatality rate approaching 90%.
  • Rabies:
    • Acquired from an animal bite.
    • Symptoms: insomnia, confusion, paralysis, hallucinations, agitation, excessive salivation, difficulty swallowing, hydrophobia.
    • Case fatality rate of 100%.

Infective Dose (Virulence Factor)

  • Infective dose refers to the number of microorganisms required to enter the body to cause disease.
  • The body can usually destroy a few cells, but a large number of cells may overwhelm the defenses.
  • Stomach acid (pH 2-3) kills most bacteria.
  • Eating food with many pathogenic bacteria can cause illness because the acid isn't sufficient to kill all the bacteria.
  • The more virulent an organism, the fewer organisms are required to evade the host.
  • Gastrointestinal infections:
    • Shigella: Only 100 microorganisms on contaminated food are needed to cause gastroenteritis.
    • Salmonella: Requires millions of microorganisms to cause illness.