1 example of a web of causation susceptible host infectiontuberculosis vaccination genetic...
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Example of a Web of Causation
Susceptible Host Infection Tuberculosis
Vaccination Genetic
Overcrowding Malnutrition
Tissue Invasion and Reaction
Exposure to Mycobacterium
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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 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
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Exit routes - urogenital Organism established in urinary tract
leptospirosis splash droplets meat works
Semen Ova (Salmonella enteritidis) Venereal
organisms often not resilient in environment Treponema pallidum (syphilis) Neisseria gonorrhoeae:
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Exit routes - skin and hair Direct contact - skin and hair
Lice, mites Skin detritus, scabs
Pox viruses Herpes simplex
Vector-borne transfer Malaria
Secondary skin contamination
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Exit routes - products Milk - Bovine TB, brucellosis Meat/offal Cadavers, products of disease processes
effusions, discharges from lesions e.g. draining abscesses
anthrax, clostridiosis (gaseous oedema)
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Routes of Entry Mouth and nose
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
Reservoir
Mode of transmission
Host
Habitat where agent survives or propagates
Many diseases have multiple reservoirs and modes of transmission
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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
Primary Prevention
Secondary Prevention
Tertiary Prevention
Pre-exposure Stage:
Factors present leading to problem development
Preclinical Stage:
Exposure to causative agent: no symptoms present
Clinical Stage:
Symptoms present
Resolution Stage:
Problem resolved. Returned to health or chronic state or death
Exposure to AgentSymptom Development
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Typical course of infectious disease
Susceptible
Host
No infection
SubclinicalDisease
Death
ClinicalDisease
Recovery
Exposure Onset
Incubation period
TIME
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Induction + latency = incubation
SusceptibleHost
SubclinicalDisease
ClinicalDisease
ExposureClinical onset
Incubation
TIME
Induction
Disease onset
Latency
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Latency and infectiousness
SusceptibleHost
SubclinicalDisease
ClinicalDisease
Infection Clinical onset
Incubation
TIME
Latent
DeathRecovery
Infectious Non-infectious
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Transmission Mechanisms
In fectedhost
Susceptib lehost
Vector
Indirect
D irect
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“Iceberg” concept of infectious disease in populations
EXPOSURE WITHOUT INFECTION
INFECTION WITHOUT CLINICAL ILLNESS
MILD ILLNESS
SEVEREDISEASE
DEATH
CLINICALDISEASE
SUB CLINICALDISEASE
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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
0
20
40
60
80
100
120
140
160
180
200
NE
W C
AS
ES
1 2 3 4 5 6 7 8 9 10 11 12
MONTH OF YEAR
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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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• Spread between animals
• Often involves vectors or carriers
Propagated Epidemic
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Summary
• Simple description of disease occurrence is the first step in epidemiological investigations
• Disease patterns