1. Example of a Web of Causation
Overcrowding
Malnutrition
Exposure to Mycobacterium
Susceptible Host
Infection
Tuberculosis
Tissue Invasion and Reaction
Vaccination
Genetic

2. Roles of hosts Maintenance host
maintains infection within endemic area
Secondary host
Amplifier host
increases disease risk
pigs and Japanese B encephalitis
Incidental
Vector

3. 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

4. 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:

5. 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

6. 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)

7. Routes of Entry Mouth and nose Skin and hair
airborne
food, water contaminated with agent
Skin and hair
Injury to skin or membranes
AIDS, rabies leptospirosis

8. Agent - host relationship
Habitat where agent survives or propagates
Reservoir
Mode of transmission
Many diseases have
multiple reservoirs and modes of transmission
Host

9. Natural History of Disease
The process by which diseases occur and progress in humans

10. 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
Clinical Stage:
Symptoms present
Resolution Stage:
Problem resolved. Returned to health or chronic state or death
Primary Prevention
Secondary Prevention
Tertiary Prevention

11. Typical course of infectious disease
TIME
Subclinical
Disease
Death
Susceptible
Host
Clinical
Disease No
infection
Recovery
Incubation period
Exposure
Onset

12. Induction + latency = incubation
TIME
Susceptible
Host
Subclinical
Disease
Clinical
Disease
Induction
Latency
Incubation
Clinical onset
Exposure
Disease onset

13. Latency and infectiousness
TIME
Susceptible
Host
Subclinical
Disease
Clinical
Disease
Death
Recovery
Latent
Non-infectious
Infectious
Incubation
Infection
Clinical onset

14. Transmission Mechanisms

15. “Iceberg” concept of infectious disease in populations
DEATH
CLINICAL DISEASE
SEVERE DISEASE
SUB CLINICAL DISEASE
MILD ILLNESS
INFECTION WITHOUT CLINICAL ILLNESS
EXPOSURE WITHOUT INFECTION

16. Endemic Disease Disease present in population or region at all times
Usually low and predictable level
Enzootic used for some animal diseases

17. Seaonality of disease Human leptospirosis in U.S.A

18. Sporadic Disease Infrequent disease occurrence
Irregular and unpredictable

19. Epidemic Disease Incidence exceeds expected
Usually infectious disease or poisoning
In animals, occasionally referred as epizootic disease
Point source or propagated

20. Point Epidemic Single common exposure Does not spread
Foodborne disease outbreaks

21. Propagated Epidemic Spread between animals
Often involves vectors or carriers

22. Summary
Simple description of disease occurrence is the first step in epidemiological investigations
Disease patterns