3rd Epid. pract.

AGENT- HOST-ENVIRONMENT By:Dr.Yossra K.Al-RobaiaayAssistant professorFICMS (FM)

INFECTIOUS DISEASE CAUSATION

Epidemiologic Triad:Agent,Host and Environment

The epidemiologic triangle or triad is the traditional model of infectious diseases causation.
Disease is the result of forces within a dynamic system consisting of: 3 components
an external agent
a susceptible host
an environment that brings the host and agent together.

Epidemiologic triad

Agent

Host
Environment


Age
Sex
Genotype
Behaviour
Nutritional status
Health status
Infectivity
Pathogenicity
Virulence
Immunogenicity
Antigenic stability
Survival
Weather
Housing
Geography
Occupational setting
Air quality
Food

Factors Influencing Disease Transmission

A classic example of a vector is the Anopheles mosquito. As the mosquito ingests blood from an infected host, it picks up the parasite plasmodium. The plasmodium are harmless to the mosquito. However, after being stored in the salivary glands and then injected into the next human upon which the mosquito feeds, the plasmodium can cause malaria in the infected human. Thus, the Anopheles mosquito serves as a vector for malaria.

Host (Man)

Agent (P falciparum)
Environment
(Pool of Water, Vector)

The process of determining disease occurrence commonly requires analytic epidemiology triad of Agent, Host and Environment after the first descriptive person, place, and time ( determine what, who, when, and where). Such information can be a great help in determining how and why the disease has occurred.


These three factors constitute the epidemiologic triad. They interact in a variety of ways that result in various states of health in an individual or a community.






Thus, the traditional model of disease transmission can be useful to identify areas of potential intervention to reduce disease prevalence, whether infectious or non-infectious.

Agent

In infectious disease models, the term "agent" originally referred to the entity or micro-organism (e.g. virus, bacterium) capable of causing the disease. As a general rule, the agent must be present for the disease to occur. However, the mere presence of the agent is not always sufficient for the disease to occur.


As the scope of epidemiology has expanded, the concept of "agent" has also grown to include chemical and physical components. This model works well with infectious diseases and accidents/injuries and most non-infectious diseases.

types of Agents

• Biological (micro-organisms)
• Physical (temperature, radiation, trauma, others)
• Chemical (acids, alkalis, poisons, tobacco, medications / drugs, others)
• Nutritional (under- or over-nutrition)

AGENT FACTORS

• BIOLOGICAL AGENTS

1.BIOLOGICAL AGENTS

• Virus
• bacteria
• Fungus
• protozoa
• ectoparasite


These agents exhibit certain host related biological properties such as :

• INFECTIVITY

The ability of agent to attack, adapt, live and multiplicate in the host

• PATHOGENICITY

The ability of agent to produce a local or general reaction in the host

• VIRULENCE

The ability to elicit a severe clinical manifestation

• ANTIGENICITY

The agent’s ability to stimulate host production of antibody such as agglutinin, opsonin, precipitin, antitoxin, lysine, complement fixating substance,etc.

Disease with high antigenicity can be prevented by immunization

Example :
Typhoid fever, : highly antigenic
Tuberculosis : doubtful
Influenza virus has lots of strain : rather difficult to develop an effective vaccine

Agent with high infectivity and pathogenicity but low antigenicity will cause a relatively high disease prevalence in the community


Agent with high infectivity but low pathogenicity usually produce a mild or sub clinical symptom and carrier

• TROPISM

The agent preference to attack and stay in special location in the host
Cholera : digestive tract
Staphylococcus : mostly in the skin
Herpes zoster : nervous system
Poliomyelitis : anterior-horn cells of spinal cord

NUTRIENTIONAL AGENT

Any excess or deficiency of the intake of nutritive elements may result in nutritional disorders,

PEM, anaemia, goitre, obesity & vitamin deficiencies are some of the current nutritional problems in our country.

PHYSICAL AGENT

Exposure to excessive heat, cold, humidity, pressure, radiation, electricity, sound, etc may result in illness


CHEMICAL AGENTS

ENDOGENOUS : Some of the chemicals may be produced in the body as a result of derangement of function, e.g.,
• urea (ureamia),
• serum bilirubin (jaundice),
• ketones (ketosis),
• uric acid (gout),
• calcium carbonate (kidney stones), etc.

allergens

• heavy metals
• fumes
• dust
• gases
• insecticides, etc.

Host factors

"Host" refers to the organism (e.g. human) capable of being infected by a specific agent.
There are intrinsic factors that influence an individual's exposure, susceptibility or response to a causative agent.

HOST FACTORS

The host factors may be classified as :
1. Demographic characteristics such as age, sex, ethnicity,religon.
• Biological characteristics:
• Physiological : e.g, pregnancy
• genetic factors : e.g, sickle cell disease
• bio chemical levels of the blood (cholesterol).

3. Social & Economic characteristics such as education, occupation, stress, marital status, housing.

4. Life style factors such as personality traits, living habits, physical exercise, use of alcohol, drugs, smoking.

ENVIRONMENTAL FACTORS

• The study of disease is really the study of man & his environment.
• The environment can be classified as Internal & external environment.
• The external or the macro environment is defined as all that which is external to the individual human host. They are categorized as follows.


1. Physical environment.
2.Biological environment.
3.psychosocial environment.

PHYSICAL ENVIRONMENT

The term “physical environment” is applied to non living things & physical factors. (air, water, soil, housing, climate, geography, heat, light, noise, radiation, etc.)with which man is in constant interaction.

• Man’s victory over his physical environment has been responsible for most of the improvement in health during the past century.
• However this has added to a host of other problems like air pollution, noise pollution, urbanization, radiation hazards etc, human exposure to electromagnetic energy, etc.

BIOLOGICAL ENVIRONMENT

The biological environment is the universe of living things which surrounds man.

The living things are bacteria, virus, insects, rodents, animals & plants.

These are constantly working for their survival, in this process some of them act as disease producing agents, reservoirs of infection, intermediate hosts & vectors of disease.

When the harmonious relationship is disturbed, ill health results.

PSYCHOSOCIAL ENVIRONMENT
The psychosocial environment encompasses cultural values, customs, habits, beliefs, attitudes, morals, religion, education, lifestyles, community life, health services, social& political organization.


Man is in constant interaction with that part of the social environment known as “people”.

He is a member of a social group.

The behaviour of a person can affect another.

Conflict & tension between members of a group can yield a great distress. The impact of social environment has both positive & negative aspects on the health of individuals.

Psychosocial factors can also affect negatively man’s health & well being. For example poverty, urbanization, migration & exposure to stressful situations such as bereavement, loss of employment, birth of a handicapped child may produce feelings of anxiety, depression, anger, frustration.

Agent,host and environmental factors interrelate in a variety of complex ways to produce disease in humans.
When searching for causal relationships, all three components must be looked at and analyse their interactions to find practical and effective prevention and control measures.