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Disease Principles
• Hugh B. Fackrell
• Filename: DiseasePrinciples.ppt
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Outline
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Infection vs Disease
Pathogenicity
Virulence
Pathogenic factors
Latency Dormancy
Communicability
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Saprophyte
• nutrient source is non-living
• can become parasite
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Parasitism
• host/microorganism interaction
• Mutualism - both host and parasite benefit.
• Commensalism - the parasite does no damage
to the host.
• Pathogen - the parasite damages the host.
• Opportunism - the parasite takes advantage of
the weakened condition of the host.
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Microbial Infection vs Disease
• Infection: colonization of the body with microbe
– usually non pathogenic
– indigenous or commensal
– beneficial
• Disease: breach of host defenses
– microbes infect tissues not normally exposed
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Infection: Benefits
• Metabolites
– E. coli makes vitamin K
• Bacterial antagonism
– E.coli blocks colonization of gut by S. aureus
– evidence: antibiotic sterilization before
abdominal surgery
– subsequent Staphylococcal infections
– enteritis
• Encourage immune system
– axenic animals have poor immunity
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Infection: Adverse Effects
• Staphylococcal infection
– Produces penicillinase
– Concurrent infection of Neisseria gonorrhaeae
– Becomes disease gonorrhea
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Microbial Disease
• Microbial disease is the exception
– imbalance favours the microbe
• 3% of all microbes pathogenic
– majority of known microbes
• >95% do not cause disease
– virtually unknown
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Location of Microbe in Host
• Location often decides outcome of
Infection vs disease
• Streptococcus
– Infection: nasopharynx
– Disease: heart
– bacterimiae after tooth extraction
• E. coli
– Infection gut
– Disease: cystitis in urinary tract
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Contagious Transmission
• can be transmitted from one host to another
(communicable)
• some infections acquired from indigenous
flora are categorized as communicable.
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Communicability
• Communicable: spread directly or indirectly
from one host to another
– chickenpox, measles, tuberculosis, typhoid
fever
• Contagious: easily communicable
– eg chickenpox,measles, sore throat
• Non communicable: Not spread from host
to host
– tetanus
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Dormancy
• Latency = Dormancy: causative microbe
remains inactive in the host for some time
but later becomes active to produce the
signs and symptoms of the disease
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Carrier State
• Individual infected
– results from a previous disease state (may be
temporary)
– the host is a true carrier
• microbe in balance with that individual
• No overt signs or symptoms
• reservoir for infection of others
–Typhoid Mary
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Koch’s Postulates
• The organism should be found in all cases of
the disease and its distribution in the body
should be in accordance with the lesions
observed.
• The organism should be cultivated outside
the body of the host, in pure culture, for
several generations of the pathogen.
• The disease should be reproduced in
susceptible animals.
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Etiology
Science of the causes of disease
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the nature of the host - species
the condition of the host
the nature of the disease agent
the transmission of the agent
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Etiology of Disease
• Specimen from patient containing infectious
agent cultured.
• Pure culture obtained and described; identified
if possible.
• Inoculation and observation of test animal.
• Many organs removed and cultured.
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Etiology of Disease (2/2)
• Isolation and identification of the test
organism.
• Inoculation of a second test animal.
• Culture of second test animal.
• Antibodies in blood of human or animal.
• Immunity developed to the infecting agent
in recently recovered animal.
• Animal protected by vaccine or toxoid.
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Pathogenicity
• Ability or potential to cause disease
attributed to a genus or species
• Dependent on ability to
– enter the host
– adapt and multiply in the host
– exit from the host
– transmit to new host
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Keppie Smith
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Portal of Entry
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May multiply at entry site> lesion
Often enters multiplies elsewhere
2/3 Respiratory
1/3 intestine, urethra, conjunctiva and skin
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Multiplication in Host
• Rate of multiplication different in vivo and
in vitro
• time to overt symptoms
• Carrier state
• reservoir gall bladder
• Temperature
– viruses
– aspirin
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Exit from Host
• Usually by same route a entry
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Bacterial transmission
• Droplets
• Fomites
• Direct contact
– sexual
– non sexual
• Bites
– insects
– animals
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Types of Pathogens
• pathogens characterized as “frank” Salmonella
• opportunistic pathogens - E. coli in urinary
tract
• non-pathogens - such as Serratia marcescens
may become infectious agents
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Virulence
• DEGREE of pathogenicity shown by a
specific strain of an organism
– C. diptheriae > diptheria
• variables include:
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number of infecting bacteria
route of entry into body
specific host defense mechanisms
non-specific host defense mechanisms
virulence factors of the bacterium
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Virulence Measurement
• Measures the pathogenicity of a isolate
– variable among strains
• Measure of Virulence
– Median Dose
– Minimum Effective Dose
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Median Dose
Amount that
affects half the
population
[Pathogen]
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Cause of Virulence Variation
• Dose of pathogen
• Virulence/Pathogenic factors
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Dose of Pathogen
• Typhoid Fever
– S. typhosa contaminated water
– 1-100 bacteria no effect
• Boils
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Staphylococcus aureus
just on surface 7 x 10 6cells
on suture or scratch ~1000 cells
“stictch abcesses”
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“Pathogenic Factors”
• Gene mutation
– S. pneumoniae- capsule
– Shigella > dysentery
• Lysogeny
– scarlet fever
– diptheria
• Capsules
– Klebsiella > polysaccharide
– Anthrax > protein capsule
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