Transcript Basic Concepts - Lampang Rajabhat University

Example of a Web of Causation
Overcrowding
Malnutrition
Exposure to
Mycobacterium
Susceptible Host
Infection
Tuberculosis
Tissue Invasion
and Reaction
Vaccination
Genetic
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Roles of hosts
• Maintenance host
– maintains infection within endemic area
• Secondary host
• Amplifier host
– increases disease risk
– pigs and Japanese B encephalitis
•Incidental
•Vector
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Exit routes - mouth and nose
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Saliva on solids and liquids (fomites)
Saliva direct to new host
Excessive nasal/salivary secretion
Lacrimal
Faeces on solids and liquids
Aerosolised faeces in dust
Vomitus
3
Exit routes - urogenital
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Organism established in urinary tract
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leptospirosis
splash droplets
meat works
Semen
Ova (Salmonella enteritidis)
Venereal
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organisms often not resilient in environment
Treponema pallidum (syphilis)
Neisseria gonorrhoeae:
4
Exit routes - skin and hair
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Direct contact - skin and hair
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Skin detritus, scabs
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Pox viruses
Herpes simplex
Vector-borne transfer
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Lice, mites
Malaria
Secondary skin contamination
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Exit routes - products
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Milk - Bovine TB, brucellosis
Meat/offal
Cadavers, products of disease processes
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effusions, discharges from lesions e.g. draining
abscesses
anthrax, clostridiosis (gaseous oedema)
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Routes of Entry
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Mouth and nose
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airborne
food, water contaminated with agent
Skin and hair
Injury to skin or membranes
–
AIDS, rabies leptospirosis
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Agent - host relationship
Agent
Habitat where agent survives
or propagates
Reservoir
Mode of
transmission
Many diseases have
multiple reservoirs and
modes of transmission
Host
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Natural History of Disease
The process by which
diseases occur and
progress in humans
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Natural History of Disease
Exposure to Agent
Symptom
Development
Pre-exposure
Stage:
Factors present
leading to
problem
development
Preclinical
Stage:
Exposure to
causative
agent: no
symptoms
present
Primary
Prevention
Clinical
Stage:
Resolution
Stage:
Symptoms
present
Problem resolved.
Returned to health
or chronic state or
death
Secondary
Prevention
Tertiary
Prevention
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Typical course of infectious disease
TIME
Susceptible
Host
Subclinical
Disease
No
infection
Death
Clinical
Disease
Recovery
Incubation
period
Exposure
Onset
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Induction + latency = incubation
TIME
Susceptible
Host
Subclinical
Disease
Clinical
Disease
Induction
Latency
Incubation
Exposure
Clinical onset
Disease onset
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Latency and infectiousness
TIME
Susceptible
Host
Latent
Subclinical
Disease
Clinical
Disease
Infectious
Death
Recovery
Non-infectious
Incubation
Infection
Clinical onset
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Transmission Mechanisms
Infected
host
Direct
Susceptible
host
ct
re
di
In
Vector
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“Iceberg” concept of infectious disease
in populations
DEATH
CLINICAL
DISEASE
SUB CLINICAL
DISEASE
SEVERE
DISEASE
MILD ILLNESS
INFECTION WITHOUT
CLINICAL ILLNESS
EXPOSURE WITHOUT INFECTION
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Endemic Disease
• Disease present in population or
region at all times
• Usually low and predictable level
• Enzootic used for some animal
diseases
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Seaonality of disease
Human leptospirosis in U.S.A
200
180
160
NEW CASES
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
MONTH OF YEAR
8
9
10
11
12
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Sporadic Disease
• Infrequent disease occurrence
• Irregular and unpredictable
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Epidemic Disease
• Incidence exceeds expected
• Usually infectious disease or
poisoning
• In animals, occasionally referred
as epizootic disease
• Point source or propagated
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Point Epidemic
• Single common exposure
• Does not spread
• Foodborne disease outbreaks
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Propagated Epidemic
• Spread between animals
• Often involves vectors or
carriers
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Summary
• Simple description of disease
occurrence is the first step in
epidemiological investigations
• Disease patterns
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