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

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

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.

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

The Epidemiologic Triangle Time

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??

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

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…

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??

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

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, ).

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

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

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

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

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

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

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

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

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.

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

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

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.

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

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

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

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

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