Download Host Parasite Relationships

Hostparrel/med/notes/1109/
Host Parasite Relationships
Organisms:
Two types of organisms:
1. Saprophytes (Non-living host).
2. Parasites (Living host).
Parasitic organisms are further divided into three categories.
1. Pathogenic (Produce disease).
2. Commensal (Live on host, Cause no harm, Part of normal flora,
Protective, Often beneficial, produce vitamins).
3. Oppurtunist (Low virulence, Normal Flora, Disease due to
abnormality in host, Change in natural habitat).
Definitions:
1. Pathogenicity: The capacity of bacteria to cause disease.
Bacteria differ in their ability to cause disease.
2. Commensals: Those bacteria that normally infect humans
after birth and cannot penetrate normal host defenses
unless they are impaired.
3. Opportunists: Those organisms that normally co-exist with
the host but may cause disease when normal defense is
broken e.g., Peumococci, Staphylococci.
4. Virulent organisms: Those that nearly always produce
disease when they infect.
5. Virulence: Indicates degree of pathogenicity. Includes two
features, ability to infect & severity of disease produced.
6. Carriers / Chronic Infections. Most bacteria are eradicated
after treatment (typhoid and diphtheria). Bacteria that
produce plague and anthrax are only present during active
disease. Others can lead to dormant infections (agents of
syphilis and brucellosis) which may relapse later.
Important to distinguish the two terms infection & disease.
 Infection: Process by which microbe enters into relationship with
host.
 Disease: Power of the infecting microbe to cause disturbance of the
physiologic functions of the host with different signs and
symptoms.
 Disease occurs when an infecting organism invades an area of
the body that is normally sterile or when a microbe that is not
part of the normal flora becomes established in a site where it can
cause harm to the host.
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Bacteria cause infection by 3 basic mechanisms:
1. Invasion of tissues
2. Production of toxins
3. Hypersensitivity reactions
Microbes enter the host (& produce infection) via either:
Gastrointestinal route (GIT)
Respiratory tract (RT)
Skin and Mucous Membranes
Genitourinary tract
Blood
Occasionally through broken skin or wounds.
Following infection microbes may multiply in:
 Site of entry i.e. locally
 Lymph node
 Blood (transmitted to other parts of the body producing
bacteraemia or viraemia).
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Microbes enter the host (& produce infection) via either:
 Gastrointestinal route (GIT)
 Respiratory tract (RT)
 Skin and Mucous Membranes
 Genitourinary tract
 Blood
 Occasionally through broken skin or wounds.
Following infection microbes may multiply in:
 Site of entry i.e. locally
 Lymph node
 Blood (transmitted to other parts of the body producing
bacteraemia or viraemia).
Pathogenicity of a organism is determined by its virulence (ability to
produce disease).
1. Secretion of toxin.
 Endotoxin
 Exotoxin
2. Presence of capsule.
3. Secretion of extracellular enzymes. Produced by Gram-positives
e.g. hyaluronidase (spreading factor), coagulase (clots plasma) and
collagenase (breaks down collagen in muscles).
4. Adherence. Some bacteria have pili or fimbrae that adhere to
specific receptors ,e.g. N.gonorrhoea and E.coli.
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Properties of Exotoxins and Endotoxins.
Exotoxins
Endotoxins
Bacteria
Gram-positive
Gram-negatives
Diffusability
Nature
Released in
medium
Protein
Remains with
Cell wall
LPS
Effect of Heat
Antigenecity
Labile
High
Stable
Weak
Toxicity
Strong
Weak
Specificity
High
Low
Toxoid Form
Yes
No
Host Resistance to Infection
 Non-specific Resistance (Natural /Innate)
1. Skin. Physical barrier. Sebaceous glands produce sweat with
lysozyme.
2. Respiratory Tract. Ciliated cells and Mucous produced. Nasal
hairs. Reflex of coughing.
3. Gastrointestinal Tract. Normal flora in mouth and stomach. Acid
and enzymes produced in stomach.
4. Genital Tract. Stratified epithelium (in vagina). Acid produced by
lactobacilli.
5. Conjunctiva. Reflex of blinking. Tears act to clear (washing) and
have lysozyme.
6. Phagocytic system. Polymorphs and macrophages.
7. Chemical constituents of blood, e.g. complement.
 Specific Resistance.
 Directed against specific microbes.
1. Humoral antibodies.
2. Cell Mediated Immunity. Acquired immunity.
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Host Resistance to Infection
 Other factors.
1. Age. Extremeties of age, more susceptible.
2. Hormonal factor. Increase in corticosteroids suppress antibodies.
Therefore more susceptible to disease.
3. Species factor. Some microbes have specific host range, e.g.
Mycobacterium avium pathogenic for birds not humans.
4. Genetic factor. Genetic defects such as agammaglobulinemia
increase susceptibility to infection.
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