Download Dynamics of Disease Transmission Learning Objectives Students

Dynamics of Disease Transmission
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Learning Objectives
Students should learn about
Epidemiological triad of disease transmission
Factors effecting disease transmission
Routes of transmission
Models of disease causation
Iceberg concept of infection
Dynamics Of Disease Transmission
Like all creatures ,human falls victim to death & disease.
Disease is defined in Webster new collegiate dictionary as a condition in
which body health is impaired.
This led to fundamental questions
Where do diseases come from ?
Is ill health inevitable?
What can be done to prevent the onset of disease?
Dynamics Of Disease Transmission
Diseases has been classically described as the result of an
epidemiologic triad
Host.
Agent.
Environment: Physical, chemical, biological.
Factors like stress are harder to classify
Dynamics Of Disease Transmission
The balance of forces that determines an individual state of health at a
given time is a kind of dynamic equilibrium.
A potentially harmful change in any of the components of the system
may not lead to detectable disease if other parts of the system have the
capacity to compensate for the insult.
Models of Disease Causation
(1) The Epidemiologic Triangle (Triad)
(2) The Web of Causation
(3) The Wheel
(2) The Web of Causation
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This model is best used in the study of chronic diseases, where the
disease agent is often not known.
Effects never depend on single isolated cause but as a result of chains
of causation, interlinked.
The web of causation provides a model which shows a variety of
possible interventions that could be taken to prevent & control disease.
Removal or elimination of just one link or chain may be sufficient to
control disease.
(3) The Wheel
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The wheel consists of a host (human), which has genetic make-up as its
core.
Surrounding the host is the environment (Biological – Physical – Social).
For hereditary diseases (sickle cell anemia, G-6-P-D), the genetic core
will be large
For measles or influenza, the state of immunity of the host & the
biological environment will contribute more.
Carriers
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It occurs either due to inadequate treatment or immune response,
the disease agent is not completely eliminated, leading to a carrier
state.
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It is “an infected person or animal that harbors a specific infectious
agent in the absence of discernible (visible) clinical disease and
serves as a potential source of infection to others.
Three elements have to occur to form a carrier state:
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The presence in the body of the disease agent.
The absence of recognizable symptoms and signs of disease.
The shedding of disease agent in the discharge or excretions.
Incubation and Latent periods
Incubation period: time from exposure to development of disease. In
other words, the time interval between invasion by an infectious agent
and the appearance of the first sign or symptom of the disease in
question.
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Latent period: the period between exposure and the onset of
infectiousness (this may be shorter or longer than the incubation
period).
Herd immunity
Level of immunity in a population which prevents epidemics even if
some transmission may still occur
Presence of immune individuals protects those who are not themselves
immune
Disease occurrence in populations
Sporadic: occasional cases occurring at irregular intervals
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Endemic: continuous occurrence at an expected frequency over a
certain period of time and in a certain geographical location
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Epidemic or outbreak: occurrence in a community or region of cases
of an illness with a frequency clearly in excess of normal expectancy
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Pandemic: epidemic involves several countries or continents, affecting
a large population
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Epidemic
“The unusual occurrence in a community of disease, specific health
related behavior, or other health related events clearly in excess of
expected occurrence”
(epi= upon; demos= people)
Epidemics can occur upon endemic states too.
Endemic
It refers to the constant presence of a disease or infectious agent within
a given geographic area or population group. It is the usual or expected
frequency of disease within a population.
(En = in; demos = people)
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Pandemic and Exotic
An epidemic usually affecting a large proportion of the population,
occurring over a wide geographic area such as a section of a nation, the
entire nation, a continent or the world, e.g. Influenza pandemics.
Exotic diseases are those which are imported into a country in which
they do not otherwise occur, as for example, rabies in the UK.
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