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Transcript
Principles of
Communicable
Diseases Epidemiology
Ass. Prof. Dr. Wijdan Akram
Community Medicine
Department
Objectives
Definition of epidemiology
The epidemiologic triad
Definition of communicable diseases
Importance of studying communicable
diseases epidemiology
 Terminology
 Dynamics of disease transmission
(chain of infection):




 Human reservoir or source
 Modes of transmission
 Susceptible host
Definition of Epidemiology
Epidemiology is the study of the
distribution and determinants of
health-related states and events
in populations, and the
application of this study to
control health problems.
What is infectious disease epidemiology?
Epidemiology
Deals with one population
Risk  case
Identifies causes
Infectious disease epidemiology
 Two or more populations
 A case is a risk factor
 The cause often known
(www)
What is infectious disease epidemiology?
Two or more populations
1. Humans
2. Infectious agents
Helminths, bacteria, fungi, protozoa, viruses, prions
3. Vectors
1. Mosquito (protozoa-malaria), snails (helminthsschistosomiasis)
2. Blackfly (microfilaria-onchocerciasis) – bacteria?
4. Animals
1. Dogs and sheep/goats – Echinococcus
2. Mice and ticks – Borrelia
(www)
What is infectious disease epidemiology?
A case is a risk factor …
Infection in one person can be transmitted to others
(www)
Routes of transmission
Direct
Indirect
 Skin-skin
 Herpes type 1
 Mucous-mucous
 Food-borne
 STI
 Across placenta
 toxoplasmosis
 Through breast milk
 HIV
 Salmonella
 Water-borne
 Hepatitis A
 Vector-borne
 Malaria
 Air-borne
 Chickenpox
 Sneeze-cough
 Influenza
Exposure
 A relevant contact – depends on the agent
Skin, sexual intercourse, water contact, etc
(www)
Some Pathogens that Cross the Placenta
Modes of Disease Transmission
Exposure to Infectious Agents
No infection
Death
Clinical
Carrier
Sub-clinical
Immunity
Carrier
No immunity
Outcome
(www)
Timeline for Infection
Dynamics of
infectiousness
Latent
period
Infectious
period
Non-infectious
Susceptible
Time
Dynamics of
disease
Incubation
period
Symptomatic
period
Non-diseased
Susceptible
Time
(www)
Transmission
Cases
 Index – the first case identified
 Primary – the case that brings the infection into a population
 Secondary – infected by a primary case
 Tertiary – infected by a secondary case
T
S
Susceptible
Immune
Sub-clinical
P
S
S
T
Clinical
(www)
Person-to-Person Transmission
Data from Dr. Simpson’s studies in England (1952)
Measles Chickenpox
Rubella
Children exposed
Children ill
251
201
238
172
218
82
attack rate
0.80
0.72
0.38
Attack rate =
ill
expose
d
(www)
Epidemiologic Triad-Related Concepts
Infectivity (ability to infect)
(number infected / number susceptible) x 100
Pathogenicity (ability to cause disease)
(number with clinical disease / number infected) x 100
Virulence (ability to cause death)
(number of deaths / number with disease) x 100
All are dependent on host factors
Chain of Infection
Horton & Parker: Informed Infection Control Practice
(www)
Infectious Disease Epidemiology:
Major Differences
A case can also be an exposure
Subclinical infections influence
epidemiology
Contact patterns play major role
Immunity
There is sometimes a need for
urgency
(www)
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
Infectious Disease Model
Pathogen
Host
disease
Environment
Definition of communicable
diseases
 A communicable disease is an illness
due to a specific infectious (biological)
agent or its toxic products capable of
being directly or indirectly transmitted
from man to man, from animal to man,
from animal to animal, or from the
environment (through air, water, food,
etc..) to man.
Importance of Studying Communicable
Diseases Epidemiology
 Changes of the pattern of infectious
diseases
 Discovery of new infections
 The possibility that some chronic
diseases have an infective origin.
Infection
 Infection is the entry and development or
multiplication of an infectious agent in the
body of man or animals. An infection does
not always cause illness.
 There are several levels of infection
(Gradients of infection):
 Colonization (S. aureus in skin and normal
nasopharynx)
 Subclinical or inapparent infection (polio)
 Latent infection (virus of herpes simplex)
 Manifest or clinical infection
 latent infection:
is a situation in which a virus is present in the
body, but it remains dormant, not causing
any overt symptoms. The patient is still
infected with the virus, and he or she can
pass the virus on to others when they are
exposed to the dormant virus .Latent
infections can also be activated, causing
symptoms and illness to emerge again. A
classic example of a latent infection is herpes
simplex ,which periodically flares up to cause
cold sores before going dormant again .
 Some infections *can never be fully flushed
from the body, becoming latent with the use
of medications and other measures to control
the virus and inhibit replication. In these
cases, the latent infection may periodically
flare up in response to environmental cues .
Latent infections can also be caused when* a
virus mutates, becoming impossible to
eradicate, or when a course of treatment is
not completed, allowing a virus to remain
dormant in the body .
contamination
 The presence of an infectious agent
on a body surface, on or in clothes,
beddings, toys, surgical instruments
or dressings, or other articles or
substances including water and food
Infestation
 It is the lodgment, development and
reproduction of arthropods on the
surface of the body or in the clothing,
e.g. lice, itch mite. This term could be
also used to describe the invasion of
the gut by parasitic worms, e.g.
ascariasis.
Contagious disease
 A contagious disease is the one that
is transmitted through contact.
Examples include scabies, trachoma,
STD and leprosy.
Host
 A person or an animal that affords
subsistence or lodgement to an
infectious agent under natural
conditions. Types include: an obligate
host, definitive (primary) host,
intermediate host and a transport
host.
Definitive host:
The definitive host is the one in which a parasite
reaches sexual maturity and undergoes
.reproduction. It is mostly a vertebrate
Intermediate host
development of the parasite occurs but it does
not reach sexual maturity, is referred as
: intermediate host and is usually invertebrate
Paratenic or transport host :Sometimes the
parasite enters a host in which it does not
undergo any development but remains alive till
it gains entry in the definitive host or
intermediate host. Such a host is termed as
paratenic or transport host or a carrier host.
These hosts are important for the completion of
the life cycle of certain parasites as they are
believed to bridge the ecological gap between
the intermediate and the definitive
host
Vector of infection
 An insect or any living carrier that
transports an infectious agent from
an infected individual or its wastes to
a susceptible individual or its food or
immediate surroundings. Both
biological and mechanical
transmissions are encountered.
Reservoir
 Any person, animal, arthropod, plant,
soil, or substance, or a combination
of these, in which an infectious agent
normally lives and multiplies, on
which it depends primarily for
survival, and where it reproduces
itself in such a manner that it can be
transmitted to a susceptible host. It is
the natural habitat of the infectious
agent.
Incidence and prevalence of
infectious diseases
 Incidence of an infectious disease: number of new
cases in a given time period expressed as percent
infected per year (cumulative incidence) or number
per person time of observation (incidence density).
 Prevalence of an infectious disease: number of cases
at a given time expressed as a percent at a given
time. Prevalence is a product of incidence x duration
of disease, and is of little interest if an infectious
disease is of short duration (i.e. measles), but may be
of interest if an infectious disease is of long duration
(i.e. chronic hepatitis B).
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, occuring 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.
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 epizootic is an outbreak (epidemic) of
disease in an animal population, e.g. rift
valley fever.
 An Enzootic 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 infetions.
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….
Control
 Concept of control:
The term disease control describes ongoing
operations aimed at reducing:
 The incidence of disease
 The duration of disease and consequently
the risk of transmission
 The effects of infection, including both the
physical and psychosocial complications
 The financial burden to the community.
 Control activities focus on primary
prevention or secondary prevention,
but most programs combine both.
control
elimination
eradication
Eradication and Elimination
 Termination of all transmission of infection by the
extermination ‫ ابادة‬of 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.
Monitoring
 Monitoring is "the performance and
analysis of routine measurements aimed
at detecting changes in the environment
or health status of population" (Thus we
have monitoring of air pollution, water
quality, growth and nutritional status,
etc).
 It also refers to on -going measurement
of performance of a health service or a
health professional, or of the extent to
which patients comply with or adhere to
advice from health professionals.
Surveillance
 surveillance means to watch over with
great attention, authority and often with
suspicion
 According to another, surveillance is
defined as "the continuous scrutiny
(inspection) of the factors that
determine the occurrence and
distribution of disease and other
conditions of ill-health"
Objectives of Surveillance
 The main objectives of surveillance are:
 (a) to provide information about new and
changing trends in the health status of a
population, e.g., morbidity, mortality, nutritional
status or other indicators and environmental
hazards, health practices and other factors that
may affect health
 (b) to provide feed-back which may be expected
to modify the policy and the system itself and
lead to redefinition of objectives, and
 (c) provide timely warning of public health
disasters so that interventions can be mobilized.
Control of infectious diseases
(the 4 “C”s
Control
Cases
Contacts
Carriers
Diagnosis
notificationstandard
isolation
strict
protective
disinfection
treatment
follow up
release
observation
detection
Community
Epidemiological
Investigation &
containment
Reproductive rate of infection:
 Reproductive rate of infection: potential for
an infectious disease to spread. Influential
factors include:
• *the probability of transmission between an
infected and a susceptible individual;
*frequency of population contact; *duration
of infection;
•
*virulence of the organism
• *population immune proportion .
Dynamics of disease Transmission
(Chain of Infection)
I
Source or Reservoir
II
Modes of transmission
III
Susceptible host
(I): Source or Reservoir
 The starting point for the occurrence of a communicable
disease is the existence of a reservoir or source of
infection.
 The source of infection is defined as “the person,
animal, object or substance from which an infectious
agent passes or is disseminated to the host (immediate
source). The reservoir is “any person, animal,
arthropod, plant, soil, or substance, or a combination of
these, in which an infectious agent normally lives and
multiplies, on which it depends primarily for survival,
and where it reproduces itself in such a manner that it
can be transmitted to a susceptible host. It is the
natural habitat of the infectious agent.”
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:
•Temporar
y
•Chronic
carriers
Portal of exit:
•Urinary
•Intestinal
•Respiratory
•others
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”
Carriers

It occurs either due to inadequate treatment or immune
response, the disease agent is not completely eliminated,
leading to a carrier state.

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:
1.
2.
3.
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.
Animal reservoirs
 Zoonosis is an infection that is
transmissible under natural conditions
from vertebrate animals to man, e.g.
rabies, plague, bovine tuberculosis…..
 There are over a 100 zoonotic
diseases that can be conveyed from
animal to man.
Reservoir in non-living things
 Soil and inanimate matter can also
act as reservoir of infection.
 For example, soil may harbor agents
that causes tetanus, anthrax and
coccidiodomycosis.
(II): Modes of transmission
Mode of transmission
Direct
transmission
Direct contact
Droplet infection
Contact with soil
Inoculation into skin or mucosa
Trans-placental (vertical)
Indirect
transmission
Vehicle-borne
Vector-borne:•
•Mechanical
•biological
Air-borne
propagative
Cyclo-prop.
Cyclo-develop.
Fomite-born
Unclean hands
and fingers
(III): Susceptible host
 An infectious agent seeks a susceptible
host aiming “successful parasitism”.
 Four stages are required for successful
parasitism:
1.
2.
3.
4.
Portal of entry
Site of election ‫ انتخاب‬inside the body
Portal of exit
Survival in external environment
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.
There are 4 types of transmission
probabilities.
TPR (cont.)
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.
Immunity: Protection against a disease.
There are two types of immunity, passive
and active.
Immunity is indicated by the presence of
antibodies in the blood and can usually be
determined with a laboratory test.
Community Immunity ("Herd Immunity")
Vaccines can prevent outbreaks of disease and
save lives.
When a critical portion of a community is
immunized against a contagious disease, most
members of the community are protected
against that disease because there is little
opportunity for an outbreak. Even those who
are not eligible for certain vaccines—such as
infants, pregnant women, or
immunocompromised individuals—get some
protection because the spread of contagious
disease is contained. "
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