EPIDEMIOLOGY SESSION 5.pptx

Experimental method of epidemiology
•Common experimental studies include:
I. Intervention studies
II. Clinical trials
III. Prophylactic trials

Experimental method of epidemiology
• An experimental study is an investigation in which the
researcher wishes to study the effects of exposure to or
deprivation of a defined factor, and designs a situation in
which subjects (persons, animals, communities, etc.) will be
exposed to or deprived of the factor
• If the investigator compares subjects exposed to the factor
with subjects not exposed to it, the study is a controlled
experiment.

Experimental method of epidemiology cont.
 Intervention or experimental studies attempt to change a variable in one
or more groups of people e.g.
• Testing a new treatment in a selected group of patients or
• Eliminating a dietary factor thought to cause allergenic
 The effects of the intervention are measured by comparing the outcome
in the experimental group with that in the control group

Randomized control trials
 Randomized control trials (or randomized clinical trial) are
epidemiological intervention or experiment to study a new
preventive or therapeutic regime
• Study subject from a selected reference population are
randomly selected and randomly allocated to groups i.e.
Treatment and control
• Exposure randomly assigned
• Exposure status not known by subjects- i.e subjects are
blinded
• Exposure status not known to person assigning it to the
subjects
• Comparing the outcome in the two group or more groups
assesses groups and the results

Randomized control trials cont’d
 The intervention being assessed may be
• A new drug or
• New regimen or assessing the value of new therapies e.g. Rice-
based against glucose
OR
• Management procedures

Field trial
• These involve people who do not have disease but presumed to be at
risk
• Data collection takes place in the field among non-institualized people
in the population
• Since the population is disease free and the purposes is to prevent,
disease occurrence that may occur with relatively low frequency e.g
vaccination trail etc.
• NB. Subjects are allocated to group as in randomized control trials

Community trials
• In this form of experiment the treatment group is communities rather
than individuals
• It is appropriate for a disease, which has their origins in social
conditions, which in turn can most easily be influenced by
intervention directed at group behavior as well as individuals
• Random allocation of communities is not possible

Epidemiological tools used to collect data
 Checklists
• This is a structured person’s observation or evaluation of a
performance
• Simple lists of criteria that can be marked as present or absent, or can
provide space for observer comments
• These tools can provide consistency over time or between observers
• Used for evaluating databases, the use of library space, or for
structuring peer observations of instruction sessions

Epidemiological tools used to collect data cont’d
 Interviews
• In-Depth Interviews include both individual interviews (e.g., one-on-
one) as well as “group” interviews (including focus groups)
• The data can be recorded in a wide variety of ways including
stenography, audio recording, video recording or written notes
• In depth interviews differ from direct observation primarily in the
nature of the interaction
• In interviews it is assumed that there is a questioner and one or more
interviewees
• The purpose of the interview is to probe the ideas of the interviewees
about the phenomenon of interest.

 Observation
• Sometimes, the best way to collect data through observation
• This can be done directly or indirectly with the subject knowing or
unaware that you are observing them
• Researcher may choose to collect data through continuous
observation or via set time periods depending on your project

 Surveys or Questionnaires
• Surveys or questionnaires are instruments used for collecting data
in survey research
• They usually include a set of standardized questions that explore a
specific topic and collect information about demographics,
opinions, attitudes, or behaviors

 Examining records
• Useful educational records include archival documents, journals,
maps, videotapes, audiotapes, and artifacts

Session4:Utilize different methods of
diseases prevention and control
• Related Tasks :
• Define disease prevention and disease control
• Describe different methods and levels of disease prevention
• Define disease intervention
• Develop interventions plan for disease prevention and control
• Implement interventions for disease prevention and control

INTRODUCTION TO DISEASE PREVENTION
AND CONTROL
• The major purpose in investigating the epidemiology of
diseases is to learn how to prevent and control them.
• Epidemiology plays a central role in disease prevention by
identifying those modifiable causes.

Define disease prevention and disease control
 Prevention
• Inhibiting the development of a disease before it occurs
• In epidemiology, prevention‘ includes measures that interrupt or
slow the progression of a disease
 Prevention of Disease
• Any activity which reduces the possibility of occurrence, or the
burden of morbidity, disability or mortality of a disease
 Control of Disease
• Reduction of disease prevalence to a level where it is no longer a
public health problem

Levels of disease prevention
 Four levels of prevention have been identified in epidemiology:
I. Primordial Prevention
II. Primary Prevention
III. Secondary Prevention
IV. Tertiary Prevention

Primordial Prevention
Preventing agents or risk factors; preventing the interaction between
host, agent and environmental factors, so that disease may not occur.
 Mainly deals with underlying conditions, and reflects more on
non-communicable diseases.
In contrast, primary prevention deals with a specific agent or causal
factor.
• Efforts and interventions at the primordial prevention level involve
anticipation of disease occurrence and modification of the conditions
responsible for the occurrence, before disease happens.

Primordial Prevention cont’d
 Whenever possible, these efforts should be evidence-based, drawing from solid
research and experience in other areas/countries
• Examples of primordial prevention interventions include
• Policy and public health interventions that discourage, limit, and/or
prohibit cigarette smoking
• Cigarette smoking can lead to high blood pressure, strokes, or lung
cancer
• Environmental interventions that reduce air pollution, the greenhouse
effect, acid rain, and ozone layer depletion can also result in a reduction of
the prevalence and severity of respiratory problems in the general
population

Key summary on Primordial Prevention
 Phase of disease: Underlying economic, social, and environmental
conditions leading to causation
 Aim: Establish and maintain conditions that minimize hazards to
health
 Actions: Measures that inhibit the emergence of environmental,
economic, social and behavioral conditions.
 Target: Total population or selected groups; achieved through public
health policy and health promotion.

Primary Prevention
Primary Prevention refers to those activities that are undertaken to
prevent the disease and injury from occurring.
Primary prevention also means preventing healthy people from
becoming ill.
The main objectives of primary prevention are promoting health,
preventing exposure and preventing disease.
It works with both the individual and the community.

Primary Prevention
It may be directed at the host, to increase resistance to the agent (such
as immunization or cessation of smoking), or may be directed at
environmental activities to reduce conditions favorable to the vector
for a biological agent, such as mosquito vectors of malaria.
Primary prevention helps to lower disease incidence and control
disease.

Components of primary prevention
Primary prevention has 3 components. These are
A. Health promotion
B. Prevention of exposure
C. Prevention of disease

Primary Prevention cont’d
A. Health promotion
 Consists of general non-specific interventions that enhance health
and the body's ability to resist disease.
Improvement of socioeconomic status, provision of adequate food,
housing, clothing, and education are examples of health promotion.
B. Prevention of exposure
 Is the avoidance of factors which may cause disease if an individual
is exposed to them.
 Examples can be provision of safe and adequate water, proper excreta
disposal, and vector control.

Primary Prevention
C. Prevention of disease:
Is the prevention of disease development after the individual has
become exposed to the disease causing factors.
Immunization is an example of prevention of disease.
Immunization acts after exposure has taken place.
Immunization does not prevent an infectious organism from invading
the immunized host, but does prevent it from establishing an infection.
 If we take measles vaccine, it will not prevent the virus from entering
to the body but it prevents the development of infection/disease.

Strategies in Primary prevention
The population or "mass” strategy
 Directed to the whole population with the aim of reducing average
risk
The high-risk-individual strategy
 Directed to people at high risk as a result of particular exposures

Key Summary on primary prevention
 Phase of disease-Specific causal factors
 Aim-Reduce the incidence of disease
 Actions-Protection of health by personal and communal efforts, such
as enhancing nutritional status, providing immunizations, and
eliminating environmental risks.
 Target-Total population, selected groups and
individuals at high risk; achieved through public
health programmes

Secondary Prevention
• Secondary prevention: identifying/detecting individuals who are
already infected with a given disease as early as possible, in order to
stop the disease from spreading/developing further.

Secondary Prevention is the early diagnosis and management to
prevent complications from a disease.
 It includes steps to isolate cases and treat or immunize contacts to
prevent further epidemic outbreaks.
The objective of secondary prevention is to stop or slow the
progression of disease so as to prevent or limit permanent damage.
 Secondary prevention can be achieved through detecting people who
already have the disease as early as possible and treat them.
It is carried out before the person is permanently damaged.

Secondary Prevention cont’d
Infected individuals should be diagnosed and treated as early as
possible, to increase recovery rates and reduce disability, morbidity,
and mortality rates.
• Screening for early diagnosis and treatment can be done for sub-
clinical diseases using laboratory tests.
• Clinical examination can be done to discover early manifestation of
disease, which is easier to reverse.

• Treatment can be provided via drugs, lifestyle modification,
or by natural remedies.
 Examples:
• Prevention of blindness from Trachoma
• Early detection and treatment of breast cancer to prevent its
progression to the invasive stage, which is the severe form of
the disease.

Key Summary on Secondary prevention
 Phase of disease: Early stage of disease
 Aim: Reduce the prevalence of disease by shortening its duration
 Action: Measures available to individuals and communities for early
detection and prompt intervention to control disease and minimize
disability (e.g. through screening programs).
 Target: Individuals with established disease; achieved through early
diagnosis and treatment

Tertiary prevention
Tertiary Prevention involves activities directed at the host but also at
the environment in order to promote rehabilitation, restoration, and
maintenance of maximum function after the disease and its
complications have stabilized.
 Is targeted towards people with permanent damage or disability.
Tertiary prevention is needed in some diseases because primary and
secondary preventions have failed, and in others because primary and
secondary prevention are not effective.

Tertiary prevention cont’d
It has two objectives:
I. Treatment to prevent further disability or death
II. To limit the physical, psychological, social, and financial impact
of disability, thereby improving the quality of life.
This can be done through rehabilitation, which is the retraining of
the remaining functions for maximal effectiveness.

Tertiary prevention cont’d
Example:
 When a person becomes blind due to vitamin A deficiency, tertiary
prevention (rehabilitation) can help the blind or partly blind person
learn to do gainful work and be economically self supporting.
 Providing a wheelchair, special toilet facilities, doors, ramps, and
transportation services for paraplegics are often the most vital factors
for rehabilitation.

Key Summary on Tertiary prevention
 Phase of disease: Late stage of disease (treatment, rehabilitation)
 Aim: Reduce the number and/or impact of complications
 Action: Measures aimed at softening the impact of long-term disease
and disability; minimizing suffering; maximizing potential years of
useful life.
 Target: Patients; achieved through rehabilitation

Define disease intervention
• Intervention
• The act of intervening, interfering or interceding with the intent of
modifying the outcome
• In medicine, an intervention is usually undertaken to help treat or cure
a condition
• For example, early intervention may help children with autism to
speak
• Acupuncture as a therapeutic intervention is widely practiced
• From the Latin intervene means to come between

Define disease intervention
• Intervention is a set of planned activities that aim at stopping or
controlling/reducing progression of disease
• Intervention can be health promotion, prevention or treatment or
rehabilitation

Define intervention cont’d
• Manipulation of the subject or subject's environment for the purpose of
modifying one or more health-related event, determinants or behavioral
processes
• Examples of interventions include
• Drugs, devices procedures (e.g., surgical techniques);
• Delivery systems (e.g., telemedicine, face-to-face interviews);
• Strategies to change health-related behavior (e.g., diet, cognitive therapy,
exercise, development of new habits);
• Treatment strategies; prevention strategies; and, diagnostic strategies

Types of intervention
Interventions can be classified into two broad categories:
i. Preventive interventions
 Are those that prevent disease from occurring and thus reduce the
incidence (new cases) of disease
 Eg. Vaccines, Nutritional interventions, Maternal and neonatal
interventions, Education and behaviour change, Environmental
alterations, Vector and intermediate host control, Drugs for the
prevention of disease and Injury prevention

Types of intervention
ii. Therapeutic interventions
 Are those that treat, mitigate, or postpone the effects of disease, once it
is under way, and thus reduce the case fatality rate or reduce the
disability or morbidity associated with a disease.
 Some interventions may have both effects.
 Eg.
• Treatment of infectious diseases, Surgical and radiation treatment
• Diagnostics to guide therapy ie MRDT
• Control of chronic diseases

Steps in Developing disease interventions
1. Identify high-risk/high-volume diseases or problems.
2. Identify best practices.
3. Define existing practice patterns and outcomes across the current
variation from best practices.
4. Identify and implement interventions to promote best practices.
5. Document that best practices improve outcomes.
6. Document that outcomes are associated with improved health-related
quality of life.

Developing disease intervention
• Healthcare Planning
• The process of identifying key objectives and choosing among alternative
means for achieving them
• Evaluation of Healthcare Services
• The process of determining the relevance, effectiveness, efficiency and impact
of activities in a systematic way in line with the agreed-upon objectives

Health Care planning cycle
• This is a cyclical and repetitive process that constitutes different levels of
interventions for the purpose of making rational decisions
• The process consists of the following:
• Measurement or assessment of the burden of illness
• Identification of the cause of illness
• Measurement of the effectiveness of different community interventions
• Assessment of their efficiency in terms of resources used
• Implementation of interventions
• Monitoring of activities
• Reassessment of the burden of illness to determine whether it has been
altered

Healthcare Interventions cycle

Components of health care planning cycle
• Burden of illness
• Measurement of overall health status of the community is the first
step in the planning process
• The measurements can include prevalence rates, incidence rates,
different measures of mortality, and the number of cases of
different diseases
• The process of measuring the burden of illness must include
different diseases

Components of health care planning cycle
• Causation
• After measuring the burden of disease in the community, it is
important to identify the major preventable causes of disease so
that intervention strategies can be developed
• Wherever possible interventions should have the prevention of
disease as their primary focus but, normally it is not always
possible.

Components of health care planning cycle cont’d
• Measuring effectiveness of different interventions
• It is important to measure the effectiveness of an intervention
through indicators or measurement of health status
• Common measurements of health used in the planning process
include morbidity and mortality measures
• They are used to allocate resources appropriately and equitably

• Measures of morbidity and mortality
• Prevalence rate, incidence rate, incidence density and Uses of
Morbidity and Mortality Statistics
• Crude Death Rate
• Total deaths in defined population in a given time period
divided by the total population
• Crude Death Rate = No. of deaths in one year / Total
population × 1,000

• Maternal mortality rate
• All maternal deaths occurring during pregnancy or within 42 days
after termination of pregnancy in a year, divided by the total
number of live births in that year per 100,000
• Maternal Mortality Rate
• = No. of pregnancy-related deaths in time period/100,000 live
births

• Efficiency
• This is a measure of the relationship between the results achieved
and the effort expended in terms of money, resources and time
• It provides the basis for the optimal use of resources and involves
the complex inter-relationship of costs and effectiveness of an
intervention
• This is the area where epidemiology and health economics are
applied together
• There are two main approaches to the assessment of efficiency:
• Cost-Effectiveness
• Cost-Benefit Analysis

• These two measures are important in prioritizing which
intervention is best especially for developing countries
• Cost-Effectiveness Analysis
• Compares the ratio of financial expenditure and effectiveness
• Dollars per case prevented, dollars per life-year gained, dollars per
quality adjusted life year gained, etc.

• Cost-Benefit Analysis
• In this measure, both the denominator and numerator are expressed
in monetary terms.
• The health benefits (e.g. lives saved) are measured and given a
monetary value.
• If the cost-benefit analysis shows that economic benefits of the
program are greater than the costs, the program should be seriously
considered.
• The measurement of efficiency requires many assumptions, and it
should be used very cautiously; it is not value-free and can serve
only as a general guideline

Implement interventions for disease prevention and control
• Implementation
• The fifth stage in planning process begins by determining a specific
intervention and takes into account the problems likely to be faced
in and by the community
• For example
• if a planned intervention involves screening women for breast
cancer using mammography, it is important to ensure that the
necessary equipment and personnel are available

Implementation interventions for disease prevention and control cont’d
• This stage involves setting specific quantified targets,
• For example
• ‘To reduce the frequency of smoking in young women from 30% to
20% over a five year period’
• This type of target-setting is essential for assessing the success of
an intervention

Components of the Healthcare Planning Cycle cont’d…
• Monitoring
• Monitoring is the continuous follow-up of activities to ensure that
they are proceeding according to plan
• Monitoring must be directed to requirements of specific program,
the success of which may be measured in a variety of ways using
short-, intermediate- and long-term criteria

Reassessment of the Burden of Illness
• Reassessment is final step in the healthcare planning process, and the
first step in the next cycle of activity
• Reassessment requires a repeat measurement of the burden of illness
in the population
• For example, repeated surveys of population blood pressure levels

The concrete examples of activities undertaken at each
stage of the Healthcare Planning Cycle
The Case of Hypertension
Stage in
Planning Cycle
Activity
Burden Population surveys of blood pressure and
control of hypertension
Aetiology
(causation)
Ecological studies (salt and blood pressure)
Observational studies (weight and blood
pressure)
Experimental studies (weight reduction)
Community
effectiveness
Randomized controlled trials
Evaluation of screening programs
Studies of compliance

The Case of Hypertension cont’d…
Stage in Planning
Cycle
Activity
Efficiency Cost-effectiveness studies
Implementation National control programs for high
blood pressure
Monitoring Assessment of personnel and
equipment
Effect on quality of life
Reassessment Re-measurement of population
blood pressure levels

SESSION 5: BASIC CONCEPTS OF EPIDEMIC
DISEASES
• Related Tasks :
• Explain concept of epidemic
• Identify risk factors for an epidemic
• Explain types of epidemics
• Identify disease pattern
• List steps to control epidemics

Introduction to Disease Epidemics
• The magnitude of a particular disease present in a specific population
may remain stable for long period of time or it may alternatively rise
and fall due to fluctuations in the number of susceptible individuals
and the nature and extent of their exposure to disease agents.
• Endemic Disease:
• Diseases which are continuous and/or habitually transmitted in
populations throughout the year (such as malaria)
• Endemicity denotes the habitual presence of a disease in a
community.

Concept of epidemic
• Epidemic: The occurrence of more cases of a specific disease in a
population that is clearly in excess of the expected incidence in a
specified period of time.
• The number of cases that constitute an epidemic will vary with the
type of disease.
• In some epidemic-prone diseases such as cholera and poliomyelitis,
one case is considered an epidemic.
• In order to say that there is an epidemic, it is necessary to know the
level of endemicity of the disease.

Concept of epidemic
• In the USA a disease such as malaria one case is an epidemic since
malaria is already eradicated in USA.
• Diseases like Ebola do not occur habitually in human populations. A
single case will constitute an epidemic in any part of the world (such
as the outbreak in Zaire in May 1995).
• In Tanzania, it is important to know the average acceptable numbers
of cases for endemic diseases (such as malaria) from a prior year’s
records before deciding that an epidemic is occurring.

Concept of epidemic
• Pandemic Disease :This is expressed when an epidemic spreads to
affect many countries globally.
• Modern epidemiology arose out of the study of so-called ‘classical
epidemics’, such as plague, smallpox, cholera, typhus, typhoid fever
and dietary deficiencies.
• Some of these epidemics remain an important threat to many tropical
countries.
• Frequently Encountered Disease Epidemics: Poliomyelitis , Measles ,
Mumps ,Rubella , Hepatitis A , Streptococcal infections ,
Meningococcal meningitis , Food poisoning

Concept of epidemic
• For Tanzania and other tropical countries the most important cause of
epidemics are infectious diseases.
• For other countries (Iraq, Pakistan, Guatemala, etc.) it is also important to
consider road accidents, drug addiction, poisoning, etc. as epidemics that
can affect mortality and morbidity.
• Poisoning and neurological disability epidemics have been reported as a
result of ingestion of wheat products treated with methyl- and ethyl-
mercuric compounds. The wheat was intended only for use as seed and was
so treated to prevent fungus growth.
• Other disease outbreaks involving the nervous system (Konzo) have been
reported from Mozambique and Tanzania and were later found to be
associated with the consumption of certain types of cassava with high
content of cyanide.

Epidemics of Emerging/Re-Emerging
Diseases
• ‘New’ diseases such as Lassa fever and Legionnaire’s or Veteran’s disease
continue to pose problems from time to time in certain parts of the world.
HIV/AIDS is also now recognized as a world-wide problem.
• Lassa fever: a viral disease transmitted from rodents and was first
recognized in 1969. Where three nurses contracted it in Nigeria and two of
them died.
• Ebola, which is a viral disease, was first recognized in Southern Sudan and
Zaire in 1976. Subsequently it was found in Southern Sudan in 1979.
• An Ebola virus Haemorrhagic fever outbreak has also been reported in Zaire
in May 1995.

Diseases
• Legionnaire’s disease: outbreak of pneumonia at a convention of the
American Legionnaires in Philadelphia in 1976. There were 29 deaths.
A gram-negative bacillus was identified as the causative agent
Legionella pneumophila.
• AIDS: An immune deficiency disorder (Acquired Immune Deficiency
Syndrome) brought about by infection with the Human
Immunodeficiency Virus (HIV).
• It was described for the first time in 1981 among men who have sex
with men (MSM) and intravenous drug users in the USA.

Diseases
• Later the epidemic was found to spread among heterosexual
populations in Africa and elsewhere.
• The disease has a long incubation period (5-10 years) and is
transmitted through sexual contact, blood transfusion and unsterile
skin piercing instruments (e.g., needles) including injections.
• Vertical transmission from mother-to-child is also an important route.

Risk factors for an epidemic
• The number of susceptible individuals exposed to a source of infection
that become infected
• The period of time over which susceptible individuals are exposed to
the source
• The minimum and maximum incubation periods of the disease
• The level of contact between infected and susceptible individuals.

Types of epidemics
• common source outbreak: a type of epidemic outbreak
where the affected individuals had an exposure to a common
agent.
• propagated outbreak: a type of epidemic outbreak where the
disease spreads person-to-person. Affected individuals may
become independent reservoirs leading to further exposures.
• NB pandemic: A disease that hits a wide geographical area
and affects a large proportion of the population.

Types of epidemics
• Common Source Epidemics:
• Occurs when a group of people are exposed to the same causative
agent.
• If the period of exposure to the agent is brief and essentially
simultaneous for all persons contracting the disease, the epidemic is
called a ‘point source epidemic’.
• All persons are affected by the same source and person-to-person
transmission does not occur.

Types of epidemics
• Common Source Epidemics:
• Common source epidemics are not necessarily caused by infectious agents;
they may also result from common exposure to noxious agents in the
environment.
• Examples include: The Bhopal disaster: a large industrial catastrophe
occurring in 1984 at the Union Carbide India Limited pesticide plant. Over
500,000 people were exposed to harmful gas and toxins leaked. Chemicals
continue to pollute groundwater in the area.
• The Chernobyl disaster: A nuclear accident that occurred in 1986 in the
Ukraine. A series of explosions occurred in one of the nuclear reactors, and
radioactive materials polluted the surrounding areas.
• Other examples might include children swimming in a chemically polluted
river or factory workers exposed to extreme heat or volatile chemicals.

Types of epidemics
• Propagated (Progressive) Epidemic :
• Propagated epidemics result from transmission of an infectious agent
from an infected host to a susceptible one.
• The transmission can either be direct (e.g. infectious hepatitis or
measles) or indirect through a vector, as in malaria and yellow fever.
• Transmission of the infecting organism continues until the number of
susceptible individuals is depleted, or until susceptible individuals are
no longer exposed to infected persons or intermediary vectors.

Types of epidemics
• Propagated (Progressive) Epidemic :
• There are three important aspects of person-to-person transmission of disease, and
they include:
• Generation time: The time interval between receipt of infection and maximal
infectivity for both clinical and subclinical infection
• Herd immunity: The decreased probability of a group of people or community to
develop an epidemic upon introduction of an infectious agent.
• The decreased probability is due to the presence of a high proportion of immunes
although there may be a certain number of persons who are individually
susceptible to the agent.
• Secondary Attack Rate: The proportion of contacts who get a communicable
disease as a consequence of contact with the index case within the accepted
incubation period.

Common Source Epidemics vs. Propagated
Epidemics
• The curve of onsets for a common source epidemic shows a
rapid rise and fall within one incubation period, whereas new
cases in a propagated epidemic continue to develop beyond
one incubation period.
• If you look at the epidemic curve you can see that those
affected have different times of onset of symptoms.
• This is because of individual differences in the level of
immunity, and the exposure to different doses of the infective
agent.

Epidemics
• In the curve of a propagated epidemic, usually a gradual rise to a peak may
be observed, followed by a gradual fall in the number of new cases.
• This is because, as the number of cases increases, the number of
susceptible falls below a critical level so that the number of new cases
begins to fall.
• The shape of the epidemic curve in this type of epidemic reflects several
factors including the population size and composition, the proportion of
susceptible in the population, the number of cases at the start of the
epidemic, the contact rate between the infected persons and the susceptible
individuals, the infectivity or pathogenicity of the disease agent and the
incubation period of the disease.

Epidemics
• Sometimes it may be difficult to identify the nature of an epidemic
from the shape of the epidemic curve alone.
• The typical common source epidemic curves may be affected by the
continued development of cases through persistent contamination of
the source, or exposure occurring repeatedly or by a long and variable
incubation period.
• The shape of the curve may also vary depending on the size of the
population exposed, the type of source distribution and the extent of
use or the extent of contact with the susceptible population.

Epidemics
• The typical shape of a point source epidemic may be modified by
presence of more than one disease agent, each with a different
incubation period, or if secondary cases (person to person
transmission) follows exposure to the original point source.
• Conversely, a propagated epidemic can create a rapidly rising and
rapidly falling epidemic curve similar to that of a common source
epidemic. This is especially so when the disease has a short incubation
period and is highly infectious (e.g. cholera).

Identify disease pattern
• The timing of onset of cases of disease in a population tends
to follow one of four patterns:
• 1. Cases may occur in a sporadic fashion
• 2. Cases may occur regularly at a fairly constant level. The
disease is often referred to as being endemic.
• 3. Cases may occur in time clusters, a pattern typical of
outbreaks or epidemics.
• 4. If an epidemic takes international proportions and affects a
large proportion of the population, it is termed a pandemic.

Steps to control epidemics
• The following are crucial steps in the investigation of an epidemic.
• A. Verification of the diagnosis:
• Investigation of an epidemic starts with a study of the affected as well
as the unaffected persons.
• For this reason it is necessary to establish criteria for labelling persons
as “cases” or “non-cases”.
• Next clinical and laboratory studies should be done to confirm the
diagnosis.

• A. Verification of the diagnosis:
• Always consider whether the initial reports are correct, otherwise you
may start a large scale investigation of a "false epidemic".
• You should always consider new, enthusiastic or inexperienced staff
and the enthusiastic use of new equipment.
• Also consider the possibility of using faulty reagents in the laboratory
which can produce false positive results.

• B. Establishment of the existence of an epidemic:
• Existence of an epidemic is established by comparing
the current disease incidence with past levels of the
disease.
• If this is clearly in excess of the expected incidence,
then there is an epidemic.

• Sometimes it may be difficult to determine whether an
epidemic exists due to the following reasons:
• (a) Recent and marked fluctuations in the number of
cases and populations although the incidence rate may
remain constant
• (b) Gross exaggeration in the number of cases due to
misdiagnosis, duplicate reporting by hospitals, health
centres, etc.

• (c) Normal seasonal variation in the occurrence of a disease may give
an impression of an epidemic when few or no cases occurred until
recently.
• Comparison of incidence rates during the same season in previous
years will reveal the expected frequency of disease and clarify whether
the observed frequency is unusual.
• (d) Completeness of recording
• (e) Manufactured epidemic - special surveys and efforts to find cases
and innaparent infections

• C. Description of the epidemic:
• The description should answer the questions:
• - who is affected? (in terms of age, sex, occupation etc.)
• - where does that individual live? (in terms of geographic
location)
• - when was that individual affected? (time of onset of
symptoms)

• The cases are then plotted by time of onset of
symptoms to determine the epidemic curve;
• and by location to determine their geographical
distribution (spot map).
• One may also draw/plot occurrence of cases in relation
to presumed antecedent exposure, e.g. place where
lunch was taken, or where they went for recreation etc.

• D. Formulation and testing of hypothesis:
• (i) From the description of the epidemic; i.e. the epidemic
curves, spot maps and the personal characteristics, identify
the type of epidemic - common source against propagated
epidemics.
• (ii) Consider possible source or sources from which disease
may have been contracted, e.g. contaminated food or water
supplies, breeding sites of insect vectors, etc.

• D. Formulation and testing of hypothesis:
• (iii) Design an epidemiological study to test the hypothesis that the source
is/or is not a particular factor.
• Usually a case control study or a retrospective cohort study design is used
for this type of investigation depending on the nature of the epidemic.
• (a) Use a case-control study design when only some of the population
members of the affected community can be identified and interviewed. e.g.
large communities such as villages, districts etc.
• (b) Use a retrospective cohort study design when all the population
members of the affected community can be identified and interviewed, e.g.
small and closed communities such as schools, hostels, army barracks etc.

• (iv) Compare cases with controls with regard to exposure to the
postulated source by using a case-control study design, or compare
exposed populations with non-exposed populations with regard to
attack rates by using a retrospective cohort study design.
• (v) Carry out statistical tests to determine the most probable source
(Odds ratio or relative risk and their 95% confidence intervals)
• (vi) When appropriate confirm the epidemiologic findings by
laboratory tests: i.e. on samples of blood, faeces etc. or samples of
suspect food, water etc.

• E. Further investigations and analysis:
• Up until this point in time the epidemic has been studied passively by
the use of reports and incoming information.
• For proper control of the epidemic the next step should include active
investigation of the cases in more detail and of the remainder of the
population.
• (i) Case finding - search for unreported cases
• (ii) Detection of subclinical cases. (iii) Detection of carriers or
reservoirs of infection, if any

• These three groups can be important sources of continued spread of
the disease agent to new susceptible hosts.
• Further investigation may utilize the following methods, note that the
methods are not exhaustive:
• - stool culture and examination
• - blood smears for parasites
• - serum for antibody titres
• - food and water for toxic substances
• - surveillance for infection in wild and domestic animals

• F. Control of the Epidemic:
• This is done right from the time the existence of the
epidemic is established.
• The principles of control are:
• - controlling the source of the agent
• - interrupting the methods of transmission
• - enhancing the defense mechanisms of the host

•G. Report writing for the investigation:
• - scientific report for publication if possible
•- preliminary report for the ministry of health for
action
•- simplified report for community health
education

• Dissemination of Findings :
• Convey the report to higher Ministry of Health
(relevant division/program, senior/top management)
• Disseminate report to the Council Health
Management Team (CHMT).
• If epidemic has been confirmed, convey report to
World Health Organisation (WHO) through top
management (i.e., MoH).

EPIDEMIOLOGY SESSION 5.pptx

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