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