Download File - Faculty Of Medicine

Faculty of Medicine
Introduction to Community Medicine Course
(31505201)
Unit 4 Epidemiology
Introduction to Epidemiology.
Definition , History of Epidemiology Purpose/Use of Epidemiology
Concepts in the infectious diseases
By
Hatim Jaber
MD MPH JBCM PhD
20+23 -10-2016
Presentation outline
Time
Introduction to unit 4 Epidemiology
12:00 to 12:10
Public Health & Epidemiology
12:10 to 12:20
Key Words
12:20 to 12:30
Main components
12:30 to 12:40
Concepts in the infectious diseases
Introduction to unit 4 Epidemiology
• Definition , History of Epidemiology
Purpose/Use of Epidemiology
• Disease Causation
• Levels of prevention
• Concepts in the infectious diseases
• Measurements of Morbidity and Mortality
• Sources of Data and methods of data
collection
• Epidemic Investigation and Management
• Epidemiological Surveillance
Epidemiology
• Epi = upon
• Demos = population
• Logos = study of
Definition
• The study of the distribution and determinants
of health related states or events in specified
human populations and its application to the
control of health problems
•
Last, 1988
Epidemiological Principles
• •Diseases (or other health events) don’t occur at
random
• •Diseases (or other health events) have causal
and preventive factors which can be identified
• Diseases and health have a distribution
• Epidemiology focuses on populations rather than
individual persons, tissues or organs.
• « The art of epidemiological thinking is to
draw conclusions from
data »
George W. Comstock
imperfect
Epidemiology and
Preventive Medicine
• Epidemiology is the central science of public health,
and preventive medicine is a clinical approach to public
health practice. Epidemiology can provide the
preventive medicine practitioner with the following:
• - Information on the state of the health of the
population.
• - Methods for identifying possible determinants of
health and disease within individuals.
• - Appropriate population groups for interventions.
• - Understanding the origins of public health
recommendations.
Why have chronic diseases increased
in importance?
• Epidemiologists investigate both infectious and chronic (noncommunicable) diseases
• Favorable demographic changes and public health successes
during the 1900’s (quality and availability of food, water, housing,
sanitation; communicable disease control)
•
•
•
•
•“Epidemic transition” (i.e. change)
–Lower overall death rate
–Greater life expectancy
–Shift in major causes of death (from infectious to noncommunicable)
• –Change in lifestyles (access & economy)
Defining characteristics of chronic
diseases
•
•
•
•
•
•
•
•Prolonged course of illness
•Multiple risk factors
•Long latency period
•Non-contagious origin (non-communicable)
•Uncertain etiology (causation)
•Functional impairment or disability
•Incurability
Key Words:
• The study of the distribution and
determinants of health related states or
events in specified human populations and
its application to the control of health
problems.
• Distribution : Time, place, person
Key Words:
• The study of the distribution and
determinants of health related states or
events in specified human populations and
its application to the control of health
problems.
• Determinants : Cause, risk factors
• The study of the distribution and
determinants of health related states or
events in specified human populations and
its application to the control of health
problems.
• Population = Public health
• The study of the distribution and
determinants of health related states or
events in specified human populations and
its application to the control of health
problems.
• Application = Information for action
Major components of the definition
• 1. Population. The main focus of epidemiology
is on the effect of disease on the population
rather than individuals. For example malaria
affects many people in Ethiopia but lung cancer
is rare. If an individual develops lung cancer, it is
more likely that he/she will die.
• Even though lung cancer is more killer,
epidemiology gives more emphasis to malaria
since it affects many people.
Major components of the definition
• 2. Frequency. This shows that epidemiology is mainly a
quantitative science. Epidemiology is concerned with the
frequency (occurrence) of diseases and other health related
conditions. Frequency of diseases is measured by morbidity and
mortality rates.
• 3. Health related conditions. Epidemiology is concerned not only
with disease but also with other health related conditions because
every thing around us and what we do also affects our health.
Health related conditions are conditions which directly or indirectly
affect or influence health. These may be injuries, births, health
related behaviors like smoking, unemployment, poverty etc.
• 4. Distribution. Distribution refers to the geographical distribution
of diseases, the distribution in time, and distribution by type of
persons affected. (where, When and who)
Major components of the definition
• 5. Determinants. Determinants are factors which
determine whether or not a person will get a
disease.
• 6. Application of the studies to the promotion of
health and to the prevention and control of
health problems. This means the whole aim in
studying the frequency, distribution, and
determinants of disease is to identify effective
disease prevention and control strategies.
Natural history of disease
• The “natural history of disease” refers to the
progression of disease process in an individual
over time, in the absence of intervention.
• There are four stages in the natural history of a
disease. These are:
• 1. Stage of susceptibility
• 2. Stage of pre-symptomatic (sub-clinical) disease
• 3. Stage of clinical disease
• 4. Stage of recovery, disability or death
Four stages in the natural history
• 1. Stage of susceptibility
• In this stage, disease has not yet developed, but the groundwork has been
laid by the presence of factors that favor its occurrence.
• Example: unvaccinated child is susceptible to measles.
• 2. Stage of Pre-symptomatic (sub-clinical) disease
•
•
•
•
•
•
In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body. The disease
can only be detected through special tests since the signs and symptoms of the
disease are not present.
Examples:
• Detection of antibodies against HIV in an apparently healthy person.
• Ova of intestinal parasite in the stool of apparently healthy children.
The pre-symptomatic (sub-clinical) stage may lead to the clinical stage, or may
sometimes end in recovery without development of any signs or symptoms.
3. The Clinical stage
• At this stage the person has developed signs and symptoms of the
disease. The clinical stage of different diseases differs in duration,
severity and outcome. The outcomes of this stage may be recovery,
disability or death.
• Examples:
• 􀂃 Common cold has a short and mild clinical stage and almost
everyone recovers quickly.
• 􀂃 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives.
• 􀂃 Rabies has a relatively short but severe clinical stage and almost
always results in death.
• 􀂃 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated.
4. Stage of recovery, disability or
death
• Some diseases run their course and then resolve completely
either spontaneously or by treatment. In others the disease
may result in a residual defect, leaving the person disabled for
a short or longer duration. Still, other diseases will end in
death.
• Disability is limitation of a person's activities including his role
as a parent, wage earner, etc
• Examples:
• • Trachoma may cause blindness
• • Meningitis may result in blindness or deafness. Meningitis
may also result in death.
Aims of Epidemiology
1. Describe the health status of population
2. Explain the etiology of disease
3. Predict the frequency and distribution of
disease
4. Control diseases in populations
Uses of Epidemiology
•
•
•
•
•
•
Determine the magnitude and trends
•Identify the etiology or cause of disease
•Determine the mode of transmission
•Identify risk factors or susceptibility
•Determine the role of the environment
•Evaluate the impact of the control measures
Uses of Epidemiology
•
To make a community diagnosis. Epidemiology helps to identify and describe
health problems in a community (for example, the prevalence of anemia, or
the nutrition status of children).
•
To monitor continuously over a period of time the change of health in a
community. (for example, the effect of a vaccination programme, health
education, nutritional supplementation).
•
To practice surveillance for a specific disease in order to be able to act
quickly and so cut short any outbreak (example cholera).
• To investigate an outbreak of a communicable disease, analyze the reasons
for it, plan a feasible remedy and carry it out, and monitor the effects of the
remedy on the outbreak.
• To plan effective health services. Effective services, interventions and
remedies all depend on accurate community data.
Epidemiologist Core Functions
•
•
•
•
•
•
Public health surveillance
•Outbreak investigations
•Data analysis
•Evaluation of disease control programs
•Communication
•Management and teamwork
Epidemiologic triad
•Demographic characteristics
•Biological characteristics
•Socioeconomic characteristics
Host
Agent
•Biological agents
•Physical agents
•Chemical agents
•Nutrient agents
•Mechanical agents
•Social agents
Environment
•Physical environment
•Biological environment
•Social environment
Host, Agent, Environment
Terminology and Definitions
•
•
•
•
•
•
•
•
•
•
Infection
Contamination
Infestation
Contagious disease
Incidence and prevalence of
infectious diseases
Epidemic
Endemic
Hyperendemic
holoendemic
Pandemic
•
•
•
•
•
•
•
•
•
Exotic
Sporadic
Attack rate
Primary/secondary cases
Zoonosis, epizootic and
enzootic
Nosocomial infection
Opportunistic infection
Eradication
Elimination
Terminology and Definitions (cont.)
• Virulence
• Reproductive rate of
infection
• Host
• Vector (source)
• Reservoir
•
•
•
•
•
Incubation period
Infectivity period
Serial interval
Latent period
Transmission Probability
ratio
Infectious Diseases Epidemiology
Definition
• Communicable disease (infectious disease) –
is an illness due to a specific infectious agent
or its toxic products that arises through
transmission of that agent or its products from
an infected person, animal, or reservoir to a
susceptible host, either directly or indirectly
through an intermediate plant or animal host,
vector, or the inanimate environment.
Components of the infectious process
• The infectious process of a
specific disease can be
described by the following
components, which
constitute of the chain of
disease transmission.
•
•
•
•
•
•
1. The Agent
2. Its reservoirs
3. Its portal of exits
4. Its mode of transmission
5. Its portals of entry
6. The human host
Components of the infectious process
• I. The Agents
• The agents in the infectious process range from
viral particles to complex multi-cellular
organisms
• II. Reservoirs
• A reservoir is an organism or habitat, in which an
infectious agent normally lives, transforms,
develops and/or multiplies. Reservoirs for
infectious agents may be humans, animals, plants
or other inanimate objects.
Types of reservoirs
Reservoir
Human
reservoir
Animal
reservoir
Non-living
reservoir
Human reservoir
Human reservoir
cases
•Primary case
•Index case
•Secondary cases
According to spectrum of disease:
•Clinical cases
(mild/severe-typical/atypical)
•Sub-clinical cases
•Latent infection cases
Type:
•Incubatory
•Convalescent
•healthy
Duration:
•Temporary
•Chronic
carriers
Portal of exit:
•Urinary
•Intestinal
•Respiratory
•others
Reservoir
• Habitat in which the disease normally lives and
multiplies
• •People
-Symptomatic - Smallpox
-Asymptomatic - HIV
• •Animals (Zoonoses)
-Brucellosis
-Plague
• •Environmental
-Histoplasmosis
-Legionnaires’ bacillus
Components of the infectious process
• Some diseases with human reservoirs are: Most bacterial and viral
respiratory diseases HIV/AIDS/Sexually Transmitted Infections (STIs),
measles, typhoid etc.
• All infected humans, whether showing signs and symptoms of the disease
or not, are potential sources of infection to others.
• A person who does not have apparent clinical disease, but is a potential
source of infection to other people is called a Carrier. An example of carrier
is a person infected with HIV. A person infected with HIV might not have the
signs and symptoms but he/she is capable of transmitting the infection to
others
• Some diseases are transmitted to human beings from animals. These
diseases are called zoonoses. Examples: Rabies, anthrax, etc.
Components of the infectious process
• III. Portal of Exit
• Portal of exit is the way the infectious agent
leaves the reservoir.
• Possible portals of exit include all body secretions
and discharges: Mucus, saliva, tears, breast milk,
vaginal and cervical discharges, excretions (feces
and urine), blood, and tissues.
• For example feces is the portal of exit for the eggs
of hook worm.
IV. Mode of Transmission
IV. Mode of Transmission
Modes of transmission include the various mechanisms
by which agents are conveyed to other susceptible
hosts. Transmission may be direct or indirect.
• 1. Direct Transmission
• 1.1 Direct contact: Occurs when there is contact of skin,
mucosa, or conjunctiva with infectious agents directly from
person or vertebrate animal, via touching, kissing, biting,
passage through the birth canal, or during sexual
intercourse. Example: HIV/AIDS/STIs, rabies
• 1.2 Direct Projection: is transmission by projection of
saliva droplets during coughing, sneezing, singing, spitting
or talking. Example: common cold
• 1.3 Transplacental: is transmission from mother to fetus
through the placenta. Example: syphilis, HIV/AIDS
Indirect transmission
• 2. Indirect transmission The following are the different types of indirect
transmission.
• 2.1 Vehicle-borne: Transmission occurs through indirect contact with
inanimate objects fomites: bed sheets, towels, toys, or surgical
instruments; as well as through contaminated food, water, IV fluids etc.
• 2.2 Vector-borne: The infectious agent is conveyed by an arthropod to a
host. Vectors may be biological or mechanical.
-Biological vector: A vector is called biological vector if the agent
multiplies in the vector before transmission. • Example: anopheles
mosquito is a biological vector for malaria.
-Mechanical vector: A vector is called mechanical vector if the agent is
directly infective to other hosts, without having to go through a period of
multiplication or development in the vector. The vector simply carries the
agent by its body parts( leg, proboscis etc) to convey it to susceptible
hosts. Example: Flies are mechanical vectors for the transmission of
trachoma.
Indirect transmission
• Airborne: which may occur by dust or droplet nuclei (dried
residue of aerosols) Example: Tuberculosis. When
pulmonary tuberculosis patients cough, they emit many
aerosols which consists the agents of tuberculosis. When
these aerosols dry droplet nuclei will be formed. These
droplet nuclei will remain suspended in the air for some
time. When another healthy susceptible individual breaths
he/she will inhale the droplet nuclei and become infected
with tuberculosis.
• Examples: -Nasal mucosa is portal of entry for common
cold
-Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI. Susceptible human host:
• The susceptible human host is the final link in the infectious
process. Host susceptibility or resistance can be seen at the
individual and at the community level.
• Host resistance at the community (population) level is called herd
immunity.
• Herd immunity can be defined as the resistance of a population to
the introduction and spread of an infectious agent, based on the
immunity of a high proportion of individual members of the
population, thereby lessening the likelihood of a person with a
disease coming into contact with b susceptible.
• Example - If 90 % of the children are vaccinated for measles, the
remaining 10 % of the children who are not vaccinated might not
become infected with measles because most of the children (90 %)
are vaccinated. That means transmission from infected person to
other susceptible children will not be easier.
Basic Epidemiological Terms
• Disease Incidence: Percentage of population
that contracts a disease in a given time
period
• Disease Prevalence: Percentage of
population that has the disease during a
given time period
Levels of disease
Basic Epidemiological Terms
• Sporadic Disease: occurs only occasionally
(i.e. Polio in US)
• Endemic Disease: constantly present in
population (i.e. common cold or ear infection)
• Epidemic Disease: Many people acquiring a
disease in a short time period (i.e. Influenza,
Gonorrhea, AIDS)
• Pandemic Disease: Worldwide Epidemics (i.e.
Influenza and AIDS)
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)
Hyperendemic and holoendemic
• The term “hyperendemic” expresses that the
disease is constantly present at high incidence
and/or prevalence rate and affects all age groups
equally.
• The term “holoendemic” expresses a high level of
infection beginning early in life and affecting most
of the child population, leading to a state of
equilibrium such that the adult population shows
evidence of the disease much less commonly than do
the children (e.g. malaria)
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.
Sporadic
• The word sporadic means “scattered about”. The
cases occur irregularly, haphazardly from time to
time, and generally infrequently.
• The cases are few and separated widely in time and
place that they show no or little connection with each
other, nor a recognizable common source of infection
e.g. polio, meningococcal meningitis, tetanus….
• However, a sporadic disease could be the starting
point of an epidemic when the conditions are
favorable for its spread.
Attack rates and primary/secondary cases
• Attack rate: proportion of non-immune exposed
individuals who become clinically ill.
• Primary (index)/secondary cases: The person who
comes into and infects a population is the primary
case. Those who subsequently contract the infection
are secondary cases. Further spread is described as
"waves" or "generations".
Zoonosis, epizootic and enzootic
• Zoonosis is an infection that is transmissible under
natural conditions from vertebrate animals to man,
e.g. rabies, plague, bovine tuberculosis…..
• An epizotic is an outbreak (epidemic) of disease in an
animal population, e.g. rift valley fever.
• An Enzotic is an endemic occurring in animals, e.g.
bovine TB.
Nosocomial infections
• Nosocomial (hospital acquired) infection is an
infection originating in a patient while in a
hospital or another health care facility.
• It has to be a new disorder unrelated to the
patient’s primary condition.
• Examples include infection of surgical
wounds, hepatitis B and urinary tract
infections.
Opportunistic infection
• This is infection by organisms that take the
opportunity provided by a defect in host
defense (e.g. immunity) to infect the host and
thus cause disease.
• For example, opportunistic infections are very
common in AIDS. Organisms include Herpes
simplex, cytomegalovirus,
• M. tuberculosis….
Basic Epidemiological Terms
• Virulence: the severity of disease that the
agent causes to the host
• The Host: is the organism that is susceptible
to the effect of the agent. The status of the
host is very important and is generally
classifiable as: susceptible, immune or
infected. The host’s response can vary from
showing no effect to manifesting subclinical
disease, atypical symptoms, straight forward
illness or severe illness.
Basic Epidemiological Terms
• The Portal of Exit: is a pathway by which the
agent can leave the source. This pathway is
usually related to the pathway where the
agent is localized.
Basic Epidemiological Terms
• A Portal of Entry: a pathway into the host that
gives the agent an access to the tissue where
it can multiply or act. Often the agent enters
the host the same way it left the source.
Basic Epidemiological Terms
• The Environment : is the conditions or
influences that are not part of either the agent
or the host, but that influence their
interaction. A wide variety of factors including
physical, climatologic, biologic, social and
economic factors can come into play.
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential
source of infection for others
Carriers
•
It occurs either due to inadequate treatment or
immune response, the disease agent is not completely
eliminated, leading to a carrier state.
FEATURES OF CARRIER
•
Three elements have to occur to form a carrier state:
1. The presence in the body of the disease agent.
2. The absence of recognizable symptoms and signs of
disease.
3. The shedding of disease agent in the discharge or
excretions.
4.
As a source of infection to others
CARRIERS
• TYPE
A) Incubatory
B) Convalescent
C) Healthy
• DURATION
A)Temporary
B)Chronic
Opportunistic infection
• This is infection by organisms that take the
opportunity provided by a defect in host
defense (e.g. immunity) to infect the host
and thus cause disease. For example,
opportunistic infections are very common in
AIDS. Organisms include Herpes simplex,
cytomegalovirus,
• M. tuberculosis….
Cases
• A case is defined as “a person in the
population or study group identified as having
the particular disease, health disorder, or
condition under investigation”
Virulence and Case Fatality Rate
• Virulence: is the degree of pathogenicity; the disease evoking
power of a micro-organism in a given host. Numerically
expressed as the ratio of the number of cases of overt
infection to the total number infected, as determined by
immunoassay. When death is the only criterion of severity,
this is the case fatality rate.
•
Case fatality rate for infectious diseases: is the proportion of
infected individuals who die of the infection. This is a
function of the severity of the infection and is heavily
influenced by how many mild cases are not diagnosed.
Serial interval and Infectious period
• Serial interval: (the gap in time between the onset
of the primary and the secondary cases) the interval
between receipt of infection and maximal infectivity
of the host (also called generation time).
• Infectious (communicable) period: length of time a
person can transmit disease (sheds the infectious
agent).
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.
• Latent period: the period between exposure and the
onset of infectiousness (this may be shorter or
longer than the incubation period).
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to
susceptible individuals during a contact.
TPR of differing types of contacts, infectious agents,
infection routes and strains can be calculated.
•
•
•
•
Transmission probabilities:
p00: tp from unvaccinated infective to unvaccinated
susceptible
p01: tp from vaccinated infective to unvaccinated
susceptible
p10: tp from unvaccinated infective to vaccinated
susceptible
p11: tp from vaccinated infective to vaccinated susceptible
TPR (cont.)
• To estimate the effect of a vaccine in reducing
susceptibility, compare the ratio of p10 to p00.
• To estimate the effect of a vaccine in reducing
infectiousness, compare the ratio of p01 to p00.
• To estimate the combined effect of a vaccine,
compare the ratio of p11 to p00.
Eradication and Elimination
• Termination of all transmission of infection by the
extermination of the infectious agent through surveillance
and containment.
• Eradication is an absolute process, an “all or none”
phenomenon, restricted to termination of infection from the
whole world.
•
The term elimination is sometimes used to describe
eradication of a disease from a large geographic region.
Disease which are amenable to elimination in the meantime
are polio, measles and diphtheria.
And Enjoy