Best PDF of epidemolgy cheak hear studentstudentes

1. Epidemiology
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By Nigusie SH.(Bsc., MPHE)

2. Session objectives
• At the end of this session students will be able
to
– Define and classify epidemiology
– Describe levels of disease occurrence
– Describe levels of disease prevention
– Explain infectious disease cycle
– Discuss different models of disease causation
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3. Introduction
What is Epidemiology?
Greek: EPI - Upon
DEMOS - People
LOGOS - Study of, Body of Knowledge
“The study of that which falls upon the common
people.”
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4. Introduction …
Definition
 Epidemiology is the study of the frequency,
distribution, and determinants of health-related
states or events in specified populations, and the
application of this study to the control of health
problems.
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5. Classification of Epidemiology
1. Descriptive epidemiology
– concerned with describing diseases and other health
related conditions by person, place and time
– answers the questions who, where and
when.
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6. 2. Analytic epidemiology
₋ concerned with identification of causes and other
factors
₋ It answers the questions why and how.
₋ Involves explicit comparison of groups
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7. 7
SCOPE OF EPIDEMIOLOGY:
Since 5th
century
Middle of
20th century
Past 25 yrs
recently
Epidemic of communicable diseases
Endemic communicable diseases &
Non communicable diseases
Chronic diseases,
injuries,
birth defects
MCH
Occupational health
Env’tal health
Health related behavior
Genetic markers of disease risks
▲ +
▲+
▲+
▲+
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8. Use/applications of Epidemiology
• Clarify the natural history of disease.
• Describe the health status of the population.
• Found disease causation
• Provide basic information about what causes or
sustains disease in populations.
• Guide healthcare policy and health planning.
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9. Use/applications of Epidemiology…
• Assist in the management and care of health
and disease in individual
• Evaluations of preventive, diagnostic and
therapeutic programmes and technologies.
• Define standards and ranges for normal values
of biological and social measures.
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10. The two basic assumptions in epidemiology are:
1. Human disease does not occur at random:
 There are patterns of occurrence in which some
behavioral and environmental factors (exposures)
increase the risk of acquiring/developing a particular
disease among group of individuals
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Basic epidemiological assumptions
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11. 2. Human disease has causal and preventive
factors
 that can be identified through systematic
investigation.
 Thus, identifying these factors creates opportunity for
prevention and control of disease
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Basic epidemiological assumptions
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12. Level of disease occurrence
• Diseases occur in a community at different levels
• Some diseases are usually present in a community
at a certain predictable level, this is called the
expected level
• But at times disease may occur in excess of what
is expected
• There are various term used to describe the level
occurrence of disease at a particular place, person
and specified time
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13. • Generally the level of disease occurrence
classified as
Expected levels
Excess of what is expected
Terms used to describe the expected level of
disease occurrence includes
 Endemic
 Hyper-endemic
 Sporadic
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Level of disease occurrence… cont’d
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14.  Endemic: a persistent level of low to moderate
occurrence of disease at a particular place ,person
and time
 Hyper-endemic: A persistently high level of
occurrence of disease at a particular geographical
area ( place) ,at a particular group of individuals
(person ) and at specified period ( time)
 Sporadic: These are occasional cases occurring at
irregular intervals
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Expected level of disease occurrence
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15. • Epidemic: occurrence of disease in excess of
what is expected in a limited period.
• Outbreak: same as epidemic, However, ‘outbreak’
is usually used when diseases happen in a more limited
geographic area.
• If an outbreak of a diseases spreads quickly to more
people than experts would expect and moves into a
large geographic area, it is often then called an
epidemic.
• Pandemic: an epidemic spread over several
countries or continents, affecting a large number
of people. 15
Excess occurrence of disease
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16. The Natural History of Diseases
(NHD)
 refers to the progression of a disease process in an
individual over time, in the absence of intervention
 The process begins with exposure to the causative agent
 Without medical intervention, the process ends with
recovery, disability, or death.
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17. The stages of NHD
Four stages
1. Stage of susceptibility
2. Stage of subclinical disease
3. Stage of clinical disease
4. Stage of outcomes
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18. 18
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Fig 1 Natural History of Disease Timeline

19. Healthy person
Sub clinical disease
Clinical disease
Disability
Recovery
Recovery
Death
Fig 2 – A schematic diagram of the natural history of
diseases and their expected outcomes. 19
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20. Level of disease prevention
Disease prevention means to interrupt or slow
the progression of disease.
Fluctuation in patterns of morbidity and mortality
of disease over time indicate that causes of
disease are preventable.
Hence, epidemiology plays a central role in
disease prevention by identifying those
modifiable causes.
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21. Level of disease prevention….
Levels of disease prevention;
A. Primordial
B. Primary
C. Secondary
D. Tertiary
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22. Levels of disease prevention
Levels Phase of the disease
Primordial Underlying condition leading to causation
Primary Specific causal factors
Secondary Early stage of the disease
Tertiary Late stage of the disease
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23. A. Primordial prevention
• Deals with underlying conditions leading to exposure to
causative factors
• It aims to modify the conditions that generate and
structure the unequal distribution of health damaging
exposures, susceptibilities and health protective
resources among the population.
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24. Primordial prevention …
• Primordial prevention consists of actions and
measures that inhibit the emergence of risk
factors
• The main intervention in primordial prevention is
through individual and mass education.
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25. Primordial prevention…
Examples:
 National policies and programmes on nutrition involving
the agricultural sector, the food industry, and the food
import-export sector
 Comprehensive policies to discourage smoking
 Programmes to promote regular physical activity
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26. B. Primary prevention
Is prevention of occurrence of disease.
• It Has three phases
I. Promotive –enhance health & body’s ability to
resist disease. It is non specific.
–E.g. Socioeconomic status, proper diet
II. Prevent exposure –limit contact with agents of
disease
–E.g. Safe water supply, Bed net
III. Prevent disease -limit development of disease
after exposure to agents
–E.g. immunization, prophylaxis
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27. C. Secondary prevention
• Prevent progression of disease after development
• Limit complications of disease
• Stop or slow progression
• Use early detection & treatment of disease
E.g. Early detection and treatment of malaria from a
patient with acute fever.
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28. D. Tertiary prevention
• Reduce impact of established disease outcome
(disability)
• It is rehabilitative care for abilities affected with the
disease
• It is restoration of person’s physical, psychological,
social &emotional abilities
E.g.
Reduce stigma towards fistula, HIV/AIDS
Wheel chair provision and hearing aid for patients
affected by leprosy
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29. Infectious disease cycle
• Is the process by which agent leaves its
reservoir or host through a portal of exit, is
conveyed by some mode of transmission, and
enters through an appropriate portal of entry
to infect a susceptible host.
• Simply it is the chain of infection
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30. Chain of Infection
• Chain of infection- is a model used to
understand the infection process which has
different components
• Understanding the characteristics of each link
provides insight for methods to prevent the
spread of infection.
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31. Chain of Infection … Cont’d
• Infection-implies that the agent has achieved
entry and begun to develop or multiply, whether or
not the process leads to disease.
• Disease- is a condition in which clinically apparent
onset of the problem was observed
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32. Components of Chain of Infection
1. Causative Agent
2. Reservoir host
3. Portal of exit
4. Mode of transmission
5. Portal of entry
6. Susceptible host
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33. Chain of disease
Transmission
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34. 1. Causative Agent
• Is a particle which range from small viral
particles to complex multi cellular organisms
(parasites)
• Host –agent interaction is characterized by
− Infectivity
− Pathogenicity
− Virulence
− Immunogenicity
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35. • Infectivity: The ability of the agent to invade and
multiply in the host (the ability to produce infection )
• Pathogenicity: the ability to produce clinically apparent
infection
• Virulence: the ability of an infectious agent to cause severe
disease
• Immunogenicity: the infection ability to produce
specific immunity
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36. Case
• When individuals are infected by measles
virus for the first time they develop specific
antibody against the agent and they will not
be susceptible for the next time.
– Which characteristic of host-agent interaction is
best described by this statement? Why?
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37. 2. Reservoir
• Is the habitat in which an infectious agent normally
lives, grows, and multiplies.
• Can be human, animal and environmental
reservoir
• Human reservoirs may be symptomatic or carriers
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38. Types of Carriers
a) Healthy or asymptomatic carriers
 Persons whose infection remains unapparent through
out its course.
E.g. In polio virus, meningococcal and hepatitis virus
infections
b) Incubatory or precocious carriers
 Persons who shed the pathogens during the
incubation period
E.g. Measles
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39. Types of Carriers…
c) Convalescent carriers
• These are those who continue to harbor the
infective agent after recovering from the illness.
E.g. Diphtheria
d) Chronic carriers
• The carrier state persists for a long period of time
E.g. Typhoid fever
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40. 3. Portal of exit is the way the infectious agent
leaves the reservoir host and these includes all body
secretions and discharges
4. Mode of transmission is the various
mechanisms by which agents are conveyed to a
susceptible host
• Can be
– Direct (direct contact, droplet spread)
– Indirect (airborne, vehicle borne, vector borne)
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41. 5. Portal of entry
• Is through which an infectious agent enters
a susceptible host
For Example:
 Nasal ------ ------ -Common cold
 Injury site ------ ---Tetanus
 Skin ------ ------ -- Hookworm infection
 Respiratory ------ --Tuberculosis
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42. 6. Susceptible host
• The chain of infection may be interrupted if the agent
does not find a susceptible host.
• Host susceptibility can be seen at
• Individual level – individual immunity and genetics
• Community level - herd immunity
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43. Models of disease causation
• In epidemiology, there are several models of disease
causation that help understand disease process
• The most widely applied models are:
– The epidemiological triad (triangle)
– The web of causation
– The wheel and
– The sufficient cause and component causes models
(Rothman’s component causes model)
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44. The epidemiological triad (triangle)
• Depicts a relationship among three key factors
• This model holds true for infectious disease which has
specific agent
• Is the best known, but most dated model of communicable
disease
• A change in any of the components will alter an existing
equilibrium
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45. The epidemiological triad (triangle) ….
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Fig. Two versions of The epidemiological triad model (epidemiologic triangle
and beam balance)
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46. The web of causation model
• The process that actually generates disease or leads to
injury is more complex
• This complexity is better portrayed by the web of causation
• There is no single cause &causes of disease are interacting
• Was developed especially to enhance understanding of
chronic disease, such as cardiovascular disease but it also
be applied to the study of injury and communicable disease
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47. Web of Causation
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48. The Wheel of Causation model
• The wheel consists of a hub (host or human) which has
genetic makeup as its core
• Surrounding the host is the env’t, divided into biological,
social and physical
• The relative sizes of the d/t components of the wheel depend
upon the specific disease under consideration
• For hereditary disease, the genetic core would be relatively
large
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49. The Wheel of Causation model…
• For conditions like measles, the state of immunity of the
host and biological sector would contribute more heavily
• In contrast to the web of causation, the wheel model does
encourage separate delineation of host and env't factors, a
distinction useful for epidemiologic analyses
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50. The Wheel of Causation model
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51. 51
• Rothman's model has emphasized that the causes of disease
comprise a collection of factors
• These factors represent pieces of a pie, the whole pie
(combinations of factors) are the sufficient causes for a
disease
• a disease may have more than one sufficient cause
Rothman’s Component Causes and Causal Pies Model
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52. Rothman’s Component Causes…
• The factors represented by the pieces of the pie in this
model are called component causes
• Each single component cause is rarely a sufficient cause by
itself, but may be necessary cause
• Control of the disease could be achieved by removing one of
the components in each "pie" and if there were a factor
common to all "pies“ (necessary cause) the disease would
be eliminated by removing that alone
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53. Necessary Vs Sufficient
• Necessary: the disease will not occur without the
presence of the factor
Example: Mycobacterium TB for TB
• Sufficient: the presence of the factor always result in
disease
Example: Rabies virus for rabies
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54. Causal Relationships
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Types of Causal Relationships
• There are four types of causal relationship in disease
causation.
a) Necessary and sufficient cause
b) Necessary but not sufficient cause
c) Sufficient but not necessary cause
d) Neither sufficient nor necessary cause
Disease Causation

55. Causal Relationships
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a) Necessary and sufficient cause – without the
factor, disease never develops; with the factor,
disease always develops (this situation rarely
occurs).
E.g. HIV, Rabies
Disease Causation

56. Causal Relationships
b) Necessary but not sufficient cause – the factor in itself is
not enough to cause disease.
o Multiple factors including main factor are required,
o E.g. Tuberculosis
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57. Causal Relationships
C) Sufficient but not necessary cause – the factor alone
can cause disease, but so can other factors in its
absence.
E.g. sun can cause burn without the presence of the other
factor such as fire
radiation exposure vs cancer
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58. Causal Relationships
d) Neither sufficient nor necessary cause–
o the factor cannot cause disease on its own, nor
o is it the only factor that can cause that disease.
o This is the probable model for chronic disease
relationships.
o Example: High fat diet and heart disease,
hypertension, diabetes and so on.
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59. Rothman’s Component Causes and Causal Pies Model
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Which factor is/are necessary cause/s?

60. Thank you all!
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