1

Example of a Web of Causation

Susceptible Host Infection Tuberculosis

Vaccination Genetic

Overcrowding Malnutrition

Tissue Invasion and Reaction

Exposure to Mycobacterium

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

airborne food, water contaminated with agent

Skin and hair Injury to skin or membranes

– AIDS, rabies leptospirosis

8

Agent - host relationship

Agent

Reservoir

Mode of transmission

Host

Habitat where agent survives or propagates

Many diseases have multiple reservoirs and modes of transmission

9

Natural History of Disease

The process by which diseases occur and progress in humans

10

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

11

Typical course of infectious disease

Susceptible

Host

No infection

SubclinicalDisease

Death

ClinicalDisease

Recovery

Exposure Onset

Incubation period

TIME

12

Induction + latency = incubation

SusceptibleHost

SubclinicalDisease

ClinicalDisease

ExposureClinical onset

Incubation

TIME

Induction

Disease onset

Latency

13

Latency and infectiousness

SusceptibleHost

SubclinicalDisease

ClinicalDisease

Infection Clinical onset

Incubation

TIME

Latent

DeathRecovery

Infectious Non-infectious

14

Transmission Mechanisms

In fectedhost

Susceptib lehost

Vector

Indirect

D irect

15

“Iceberg” concept of infectious disease in populations

EXPOSURE WITHOUT INFECTION

INFECTION WITHOUT CLINICAL ILLNESS

MILD ILLNESS

SEVEREDISEASE

DEATH

CLINICALDISEASE

SUB CLINICALDISEASE

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

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

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

• Spread between animals

• Often involves vectors or carriers

Propagated Epidemic

22

Summary

• Simple description of disease occurrence is the first step in epidemiological investigations

• Disease patterns