Lecture 2

• Pathogen

  Organism that causes disease in an immunocompetent host
• Pathogenicity

  The ability of an organism to cause disease in an immunocompetent host
• Virulence

  A relative measure of pathogenicity
• ID50 (Infectious Dose50)

  The number of bacteria needed to cause an infection in 50% of exposed animals
• LD50 (Lethal Dose50)

  The dose that is lethal to 50% of exposed animals
• MLD (Minimum Lethal Dose)

  The minimum dose that is lethal
• Differences in susceptibilities of individual animals, immune status of hosts, and breed susceptibilities can affect whether an animal develops severe disease

  
• Infection

  The presence and replication of an obligate pathogen within a host, implying overt disease or capability of causing disease
• Types of infection

  • Apparent infection (acute or chronic)
  • Inapparent infection (normal flora/colonization)
  • Carrier infection
• Events in establishment of infection

  1. Encounter

  2. Entry

  3. Multiplication and spread

  4. Damage
• Encounter

  The initial meeting of the microorganism and the host
• Possible outcomes of initial encounter

  • Do not gain entry or are rapidly eliminated
  • Colonize to form normal flora
  • Cause disease
• Exogenous infection

  Infection obtained from an external source
• Endogenous infection

  Infection due to normal flora present in or on the host body
• Entry of pathogens

  1. Ingress (without crossing epithelial barrier)

  2. Penetration (by penetrating deeper into tissues)
• Ingress

  • Inhalation, ingestion, direct infection of mucus membranes
• Penetration

  • Pathogen-dependent (via virulence factors)
  • Pathogen-independent (vector-borne, catheters, blood transfusions)
• Inoculum size

  The number of organisms in the initial exposure, which is a prime determinant of infection outcome
• Incubation period

  The time between exposure and onset of disease, during which bacteria are multiplying
• Factors affecting multiplication and spread

  • Nutritional requirements
  • Temperature
  • Bacteremia
  • Lymphatic spread
  • Fascial plane spread
• Three ways microbes damage the host are: release of endotoxin, production of exotoxin, and damaging host response (inflammation, immune escape, immunopathology)
• Endotoxin

  The constitutive part of the Gram-negative cell wall, also called lipopolysaccharide (LPS)
• Exotoxin

  Secreted bacterial proteins with specific and often different actions
• Mechanisms of damaging host response

  • Inflammation
  • Immune escape
  • Immunopathology
• A good history, physical exam, and clinical work-up can help determine the encounter, entry, multiplication/spread, and damage caused by an infection