Chapter 14: Pathogenesis of Infectious Diseases

							Chapter 14: Pathogenesis of
   Infectious Diseases
       Group 1 BSN 2-F
Pathogenesis
  -the steps or mechanisms involved in
  the development of a disease.
Pathogenicity
  -the ability to cause disease.
Infectious Diseases
  -diseases that are caused by
  pathogenic microorganisms such as
  bacteria, viruses, parasites or fungi.
Infection
  -colonization by a pathogen(according
  to microbiologists)
     Why Infection does not
     always occur?
• The microbe may land at an anatomic site
  where it is unable to multiply.
• Many pathogens must attach to specific
  receptor sites before they are able to
  multiply and cause damage. If they land at a
  site where such receptors are absent, they
  are unable to cause disease.
• Antibacterial factors that destroy or inhibit
  the growth of bacteria may be present at
  the site where a pathogen lands
• The indigenous microflora of that site
  may inhibit growth of the foreign
  microbe by occupying space and using
  up available nutrients.
• The indigenous microflora at the site
  may produce anti-bacterial factors that
  destroy newly arrived pathogen.
• The individual’s nutritional and overall
  health status often influences the
  outcome of the pathogen-host
  encounter.
• The person may be immune to a
  particular pathogen.
• Phagocytic WBCs present in the blood
  and other tissues may engulf and
  destroy the pathogen before it has an
  opportunity to multiply, invade and
  cause disease.
   Four Periods in the Course
   of Infectious Disease
Incubation period
- the time that elapses between arrival
   of the pathogen and the onset of
   symptoms.
Prodromal period
- the time during which the patient feels
   “out of sorts”, but is not yet
   experiencing actual symptoms of the
   disease.
Period of illness
 - the time during which the patient
 experiences the typical symptoms
 associated with that particular
 disease.
Convalescent period
 - time during which the patient
 recovers.
  Localized vs Systemic
  Infections
Once an infectous process is initiated, the
 disease may remain localized to one
 site or it may spread. Pimples, boils,
 and abscesses are examples of
 localized infections.
When the infection has spread
 throughout the body, it is referred to
 as systemic or generalized infection.
    Acute, Subacute and
    Chronic Diseases
Acute disease
 - has a rapid onset, usually followed by
 a relatively rapid recovery.
Chronic disease
 - has an insidious(slow) onset and lasts a
 long time.
Subacute disease
 - come on more suddenly than chronic
 disease, but less suddenly than an acute
 disease.
   Symptoms vs Signs of a
   disease
A symptom of a disease is defined as
 some evidence of a disease that is
 experienced or perceived by the
 patient; something that is subjective. It
 may be symptomatic or asymptomatic.

A sign of a disease is defined as some
 type of objective evidence of a disease.
Latent Infections
 - an infectious disease that may go
 from being symptomatic to be
 asymptomatic and then some time
 later go back to becoming
 symptomatic.
Primary vs Secondary
Infections
One infectious disease may
 commonly follow another, in which
 case the first disease is referred to
 as a primary infection and the
 second disease is referred to as a
 secondary infection.
   Steps in the Pathogenesis
   of Infectious Diseases
1. Entry of the pathogen into the body.
2. Attachment of the pathogen to some
   tissues within the body.
3. Multiplication of the pathogen.
4. Invasion or spread of the pathogen.
5. Evasion of host defenses.
6. Damage to host tissues.
             Virulence
Virulence is a measure or degree of
  pathogenicity. Different species or even
  different strains of the same species
  vary in their ability to cause disease;
  thus, some are more virulent than
  others. Some strains of a particular
  species may be virulent, whereas other
  strains of the same species are
  avirulent.
Virulent
- capable of causing disease

Avirulent
- not capable of causing disease
VIRULENT FACTORS
 - properties of pathogens that allow
 them to escape various host defense
 mechanisms and cause disease.
Attachment:
Receptor and Integrin- molecules on the
 surface of a host cell that a pathogen is
 able to recognize and attach to.
Adhesin and Ligand- molecules on the
 surface of a pathogen that is able to
 bond to a particular receptor.
Obligate Intracellular Pathogens
 - pathogens that must live within
 host cells to survive and
 multiply.
Facultative Intracellular
Pathogens
- Pathogens that are capable of both
  an intracellular and extracellular
  existence
- They are also able to survive within
  phagocytes
Intracellular Survival Mechanisms
- 2 most important phagocytes in the
  body or “Professional Phagocytes”:
     - Macrophages & neutrophils
- Once phagocytized, most
  pathogens are destroyed within the
  phagocytes, however, certain
  pathogens are able to survive and
  multiply within phagocytes after
  being ingested.
- Some pathogens has a cell wall that
  resist digestion
       -> myobacterial cell walls contain
  waxes that protect the organism from
  digestion.
- Others prevent fusion of lysosomes with
  phagocytic vacuole.
- Other pathogens produce phospho
  lipases that destroy the phagosome
  membrane that prevents lysosome-
  phagosome fusion.
- Encapsulated bacteria that gain
  access to the blood stream or
  tissues are protected from
  phagocytosis, they are able to
  multiply, invade, and cause disease
- Non-encapsulated bacteria are
  phagocytized and killed.
Flagella
- Considered virulence factors
  because flagella enable flagellated
  bacteria to invade aqueous areas
  of the body which unflagellated
  bacteria are unable to reach.
- Enable bacteria to avoid
  phagocytosis.
Exoenzymes
- Are produced by pathogens
- Enables pathogens to evade host
  defense mechanisms, invade, or
  cause damage to body tissues
- Exoenzymes include necrotizing
  enzymes, coagulase, kinases,
  hyaluronidase, collagenase,
  hemolysins and lecithinase.
Necrotizing Enzymes
 - Exoenzymes that destroy tissues
Coagulase
- Binds to prothrombin, forming a
  complex called staphylothrombin
- In the body, coagulase enables S.
  Aureus to clot plasma and form a
  sticky coat fibrin around themselves
  for protection from phagocytes,
  antibodies, and other host defense
  mechanisms
Kinases
- Opposite effect of coagulase
- Are enzymes that dissolves clots;
  pathogens that produce kinases are
  able to escape from clots.
Hyaluronidase
-called “spreading factor”
- Enables pathogens to spread
   through connective tissues by
   breaking down hyaluronic acid (a
   polysaccharide that holds tissue
   cells together.)
Collagenase
-breaks down collagen (supportive
  protein found in tendons, cartilage,
  bones.
-enables pathogens to invade tissues
Hemolysins
-enzymes that cause damage to the
  host red blood cells
-provides pathogens with a source of
  iron
Lecithinase
-breaks down phospholipids called
   lecithin
-is destructive to cell membranes of
   red blood cells and other tissues
TOXINS
-poisonous substances released by
   pathogens that enables them to
   damage the host.
-2 major types:
   1.) endotoxins- integral parts of the
   cell wall of gram negative bacteria.
   Can cause adverse physiological
   effects such as fever and shock.
***Septic shock-results from gram
   negative sepsis.
2.) exotoxins- toxins produced within
   cells and then released from the
   cells.
   -poisonous proteins secreted by a
   variety of pathogens.
a) neurotoxins- affects central
    nervous system.
b) Enterotoxins- toxins affecting GI
    tract.
c) Exoliative Toxins- causes the
    epidermal layers of skin to slough
    away.
d.) Leukocidins
- Toxins that destroy WBC
       Mechanisms by Which Pathogens
       Escape Immune Responses
a) Antigenic Variation
- Pathogens that are able to periodically
   change their surface antigens.
- By the time that the host has produced
   antibodies in response to pathogen’s
   surface antigens, those antigens are shed
   and new ones appear in their place.
b) Camouflage and Molecular Mimicry
- Camouflage: pathogens are able to conceal
  their foreign nature by coating themselves with
  host proteins.
- Molecular Mimicry: pathogen’s surface antigens
  closely resemble host antigens and are not
  recognized being foreign
C) Destruction of Antibodies
- Pathogens that produce an enzyme that
  destroys IgA antibodies
- These pathogens are capable of destroying
  some of the antibodies that the host’s immune
  system has produced in an attempt to destroy
  them.