Chapter 14

• Pathogenesis


  The steps or mechanisms involved in the development of a disease
• Pathogenicity


  The ability to cause disease
• Infection


  Colonization by a pathogen; the pathogen may or may not go on to cause disease
• Infectious disease


  A disease caused by a microbe
• Pathogens


  • Microbes that cause infectious diseases
• Why infection does not always occur

  1. Microbe lands at a site where it is unable to multiply

  2. Pathogen must attach to specific receptor sites before multiplying and causing damage

  3. Antibacterial factors present at site

  4. Indigenous microflora inhibit growth of foreign microbe

  5. Indigenous microflora produce antibacterial factors

  6. Individual's nutritional and health status influences outcome

  7. Person may be immune to pathogen

  8. Phagocytes destroy pathogen
• Periods/phases in course of infectious disease

  • Incubation period
  • Prodromal period
  • Period of illness
  • Convalescent period
• Localized infection


  Disease remains localized, e.g. pimples, boils, abscesses
• Systemic infection


  Infection spreads throughout the body, e.g. miliary tuberculosis
• Acute disease


  Rapid onset, relatively rapid recovery, e.g. measles, mumps, influenza
• Chronic disease


  Slow onset, lasts a long time, e.g. tuberculosis, leprosy, syphilis
• Subacute disease


  Comes on more suddenly than chronic, less suddenly than acute, e.g. bacterial endocarditis
• Symptom


  Subjective evidence of disease experienced by patient, e.g. aches, pains, nausea
• Sign


  Objective evidence of disease, e.g. elevated blood pressure, abnormal lab results
• Symptomatic disease


  Patient experiences symptoms
• Asymptomatic disease


  Patient does not experience any symptoms
• Latent infection


  Infectious disease goes from symptomatic to asymptomatic and back, e.g. syphilis, herpes
• Primary infection


  First infectious disease that occurs
• Secondary infection


  Infectious disease that commonly follows a primary infection, e.g. bacterial pneumonia after viral respiratory infection
• Steps in pathogenesis of infectious diseases

  1. Entry of pathogen into body

  2. Attachment of pathogen to tissue

  3. Multiplication of pathogen

  4. Invasion or spread of pathogen

  5. Evasion of host defenses

  6. Damage to host tissue
• Virulent


  Pathogenic, capable of causing disease
• Avirulent


  Nonpathogenic, not capable of causing disease
• Virulence


  Measure or degree of pathogenicity
• Virulence factors


  Attributes that enable pathogens to attach, escape destruction, and cause disease
• Virulence factors

  • Adhesins (ligands)
  • Pili (bacterial fimbriae)
• Obligate intracellular pathogens


  Pathogens that must live within host cells to survive and multiply, e.g. Rickettsia, Chlamydia, Plasmodium, Babesia
• Facultative intracellular pathogens


  Capable of both intracellular and extracellular existence, e.g. Mycobacterium tuberculosis
• Capsules and flagella


  Considered virulence factors, e.g. Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Neisseria meningitidis
• Exoenzymes released by bacteria

  • Necrotizing enzymes
  • Kinases
  • Collagenase
  • Lecithinase
  • Coagulase
  • Hyaluronidase
  • Hemolysins
• Endotoxins


  Part of cell wall structure of Gram-negative bacteria, can cause serious adverse effects
• Exotoxins


  Poisonous proteins secreted by pathogens, e.g. neurotoxins, enterotoxins, exfoliative toxin, erythrogenic toxin, leukocidins
• Mechanisms by which pathogens escape immune responses

  • Antigenic variation
  • Camouflage and molecular mimicry
  • Destruction of antibodies
• Adhesins (ligands)


  special molecules on the surface of pathogens – are considered to be virulence factors because they enable pathogens to recognize and bind to particular host cell receptors.
• Pili (bacterial fimbriae)


  are considered to be virulence factors because they enable bacteria to attach to surfaces, such as tissues within the human body.