1. Epidemiological triad Agent, Host, Environment Model
A.Prof. Dr.
Maha A. AL-Nuaimi
Ph.D. Com.Med

2. OBJECTIVES OF THE LECTURE
By the end of this lecture students will be able to:
Explain epidemiologic triads as a model of study of disease causation
Describe importance of studying epidemiologic triads and its implications for public health. 2

3. What is the epidemiological triad
Is a model for explaining the agent causing a disease and the personal & environmental conditions that allow its spread among population.
Why … How …
Epidemiologists effectively study in order to fight the spread of disease.

4. Does a disease occurs randomly? How & Why the disease occur??

5. The Epidemiologic Triangle
Time

6. DISEASE AGENT
Agent is defined as a substance, living or non living or a force, tangible or intangible, the excessive presence or relative lack of which may initiate or perpetuate a disease process.
Biological Agents
Physical Agents
Chemical Agents
Nutritional Agents.. What are the commonest nutritional disorders in our community??

7. Types of Agents Biological Chemical Physical Helminths Endogenous ??
Heat/cold
Protozoans
Exogenous ??
Light / radiation
Fungi
Noise
Bacteria
pressure
Rickettsia
Viral
Prion

9. Terms to know Pathogenicity Infectivity Virulence Antigenicity
Describes the potential ability and strength of a pathogenic substance to cause path. changes.
Infectivity
The capability of the pathogen or agent to enter, survive, and multiply in the host… → INFECTION
Virulence
The capacity and strength of the disease to produce severe and fatal cases of illness
Antigenicity
The ability of the agent to induce antibodies in the host. As antitoxin, lysine, complement fixating substances…

10. Agent with High infectivity, Pathogenicity with low Antigenicity ….
→ high incidence and high prevalence.
Agent with High infectivity, low pathogenicity
→ sub acute or sub clinical symptoms. Incidence.. & Prevalence??

12. Types of Host Factors Personal trait. Immunity & susceptibility
Genetic and chromosomal constituents.
Behavioral
Occupational
Cultural
etc!

13. Environment factors physical factors (climate, water, soil),
Are suitable extrinsic factors that collect the agent and host .
physical factors (climate, water, soil),
biological factors (insects, sanitation)
socio-economic factors (H. services, SES, ).

14. Chain of disease occurrence & Transmission
APPLIED FOR CD ..
Disease occurs when the pathogen or agent leaves the reservoir through a portal or exit and is spread by one of several modes of transmission ?? to the portal of enlet of the host.
Breaks in the chain of transmission will stop the spread of disease
Etiological agent/pathway
Source / Reservoir
Mode of transmission
Host

15. Natural History of Disease
2/4/2019
Chapter 2: General Concepts
2/4/2019
Natural History of Disease
Progression of disease in an individual over time
GerstmanGerstman
Gerstman
Chapter 2
Chapter 2 15 15
Epidemiology

16. Infectious disease epidemiology
The possible outcomes of exposure to an infectious agent
exposure
No foothold
Carriage
Sub-clinical infection
Clinical infection
Death
Immunity
Carriage
Non immunity

17. Does the mere presence of agent is necessary and always sufficient to cause the disease condition ??

18. Disease Factors in general:
Necessary cause: The cause must be present for the outcome to happen.
Sufficient is very rare in medicine cause.

19. Types of disease pathway:
1. Direct causation (with out any intermediate step).
2. Indirect causation (a factor causes a disease, but only through an intermediate steps).
- Which one is always present in human biology?
disease
factor
Factor 2
factor3
factor
disease

20. Web of Causation Disease social organization phenotype behaviour
microbes
Disease
genes
environment
Unknown factors
workplace

21. Web of Causation - CHD CHD stress medications genetic susceptibility
smoking
lipids
CHD
gender
physical activity
Unknown factors
inflammation
blood pressure

23. Epidemics arise when HAE factors are not in balance due to
New agent
Change in existing agent (infectivity, pathogenicity, virulence)
Change in the no. of susceptible in the population.
Environmental changes that affect transmission of the agent or growth of the agent.

24. Homeostatic Balance A H E H E A E A H E A H E H A At equilibrium
Agent becomes more pathogenic E H A
The proportion of susceptibles in population decreases E A H
At equilibrium
Steady rate
Environmental changes that favor the agent E A H
Environmental changes that favor the host E H A 24

25. Factors Influencing Disease Transmission
Agent
Environment
Infectivity
Pathogenicity
Virulence
Immunogenicity
Antigenic stability
Survival
Weather
Housing
Geography
Occupational setting
Air quality
Food
Age
Sex
Genotype
Behaviour
Nutritional status
Health status
Host

26. The AIM Epidemiologic Triangle
The aim of the epidemiologist is to break one of the arms of the triangle, which disrupts their connection.
An epidemic can be stopped when one of the elements of the triangle is interfered with, altered, changed or removed from existence.

27. Agent, Host and Environmental Factors Associated with HIV Infection
APPLY THIS ON NCD ….

28. Factors Influencing Disease Transmission
Agent
Environment
Cigarettes
Other cigarettes
Products.
Social factors ES
ENV. Cont.
in&outdoor
Age
Sex
Genotype
Behaviour
Health status
Suscept.Host

29. Is it necessary to know the agent, to start the control intervention ??
Understanding causal mechanisms is essential for effective public health intervention
– John Snow

30. If there is no agent , does the disease occur??
Eradicated disease

31. Thanks