Transcript epidemiology - howMed Lectures

INFECTIOUS DISEASE
EPIDEMIOLOGY
Maj Fawad
Topics to be covered in this
lecture
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Definitions
Dynamics of Infectious Disease
Transmission
Control of Infection
Terms
1. Disease
• Manifest / Overt
• Inapparent / Asymptomatic/
Subclinical / Covert
• Latent
• Colonization
2.
Communicable disease /
Contagious disease
3. Contamination
4. Pollution
Parasite
Virus
Infection
Fungi
Bacteria
Infection
The entry and development
or multiplication of an
infection agent in body of
man or animal is called
infection. The infectious
agent can be virus, bacteria,
fungi, parasite etc.
1. Disease (infection)
This results from the infectious agent when it
produces signs and symptoms.
1.1.Manifest
Synonymous with disease.
1.2 .Inapparent, asymptomatic or sub- clinical
infection
Where infectious agent fails to produce signs and
symptoms but infectious agent survives and
multiplies in the body of man or animal and is
capable of being transmitted to another susceptible
individual or animal.
.
1.3
.Latent
When infection is present in the body but does not
Produce any sign and symptoms and at the same time
cannot be transmitted to a susceptible host during
the period of latency i.e. Herpes simplex, Syphilis.
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Latent period:
This period is the interval of subclinical infection
during which the previously active infectious agent
becomes dormant in the host.
Example:
Subsequent to the appearance of genital lesions
induced by a herpes simplex type II infection,
patients often experience period of remission, after
which reactivation of the virus elicits the
reappearance of lesions. This interval of remission
is referred to as the latent period of the virus.
1.4
Colonization
In the case of Commensal or low grade
pathogen, the multiplication may cause
little or no harm to the host and may best
be described as colonization.
2. Communicable disease /
Contagious Disease:
An illness due to a specific infectious agent or its
toxic products. This arises through transmission
of that agent or its products from an infected
person, animal, or inanimate reservoir to a
susceptible host, either directly or indirectly
through an intermediate plant or animal host,
vector, or the inanimate environment.
Spread of Communicable disease
Direct
Indirect
Reservoir
(Man, animal,
Inanimate)
Airborne
Vehicle
Susceptible
Host
(Man, animal)
Vector
Inanimate
Intermediate host
Direct:______
Indirect:………
Mode of transmission
Direct:
1. Close physical contact e.g. sexually
transmitted disease (AIDS) ,skin to skin
contact (leprosy):
2. Droplet infections
3. Contact with the soil
4. Vertical transmission
Indirect :
1.
Vehicle-borne :
2.
Vector –borne :
These infection are transmitted through the
agency of water, food, ice, blood, serum, plasma,
and other biological products e.g. tissues and
organs.
These infection are transmitted by an arthropod or
a living invertebrate carrier such as snails or Cyclops.
Indirect (contd.)
3.
Air –borne: (Droplet Nuclei & Dust )
4. Fomite – borne:
fomites refer to inanimate objects such as
handkerchiefs, bed linen, towels, books,
spoons, forks, etc., which have been soiled
with infective material. Freshly soiled
fomites are likely to spread the agents of
infection.
3.
Contamination:
This refers to the presence of organic material or
micro – organisms on the body surfaces, articles
or inanimate object.
4.
Pollution:
Pollution refers to the presence of
both inorganic and organic matter
such as offensive material like trade
effluvia or industrial pollutants.
Reservoirs of infection
Living
Non living
Human
beings
Animals
patients
carriers
Birds
Arthropods
soil
substances
Reservoir
Any person, animal, arthropod, plant, soil or
substance (or combination of these) in
which an infectious agent normally lives and
multiplies, on which it depends primarily for
a prolonged survival and where it
reproduces itself in such a manner that it
can be transmitted to a susceptible host.
Human reservoirs
Patients
Carriers
Carrier
A person or animal that harbours a specific
infectious agent in the absence of discernable
clinical disease and serves as a potential source
of infection.
Classification of carriers
Based on the stage of disease
Based on the portal of exit of infection
Based on the duration
1.Based on the stage of disease
a. Incubatory carriers e.g. Hepatitis
b. Convalescent carrier e.g. typhoid
c. Healthy carrier e.g. Typhoid
a. Incubatory Carrier
It is that type of the carrier which transmits
micro-organism during incubation period of the
disease.
b. Healthy Carrier.
An individual with an infection that is inapparent throughout its course but can transmit
infection to others, is commonly known as a
healthy carrier.
c. Convalescent carrier
This carrier transmits micro-organisms during
the period of convalescence or recovery.
2. Based on the portal of exit of
infection
a.
b.
c.
d.
e.
Nasal e.g. Streptococcal
Oral e.g. Meningococcal
Urinary tract e.g. Typhoid
Intestinal e.g. Cholera
Genital e.g. HIV
3. Based on the duration
a. Temporary (duration<12 months) carrier
b. Chronic (duration> 12 months) carrier
Types
of host
Susceptible
Host
Immunity
Resistance
Host
A person or another living animal including birds
and arthropods that affords subsistence and
lodgment to an infectious agent under natural
conditions.
A.
Types of Host
1.
Definitive Host.
Hosts in which parasite attains maturity or passes
its sexual stage, are primary or definitive hosts.
2. Intermediate Host:
Hosts in which the parasite is in larval or
asexual stage, are secondary or intermediate
Hosts.
3. Propagative Host:
A transport or propagative host, is the one in
which, the organism remains but does not
undergo development.
4. Obligate Host:
This term refers to the only host of infections
such as man in measles
Host parasite relationship
(Symbiosis)
Parasitism
Harmful
Mutualism
Beneficial
Commensalisms
Neither beneficial
Nor harmful
Mutualism:
It is a type of symbiosis in which there is reciprocal
benefit to both the host and parasite.
Commensalisms:
It is a relationship where the host gives the food and
shelter to the parasite. It can either move to the
mutualism or parasitism. It is neither good nor bad.
Parasitism:
It is of unilateral benefit to the parasite only and
harmful for the host.
Phases in infections
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Infective period
Communicability period
Incubation period
External Incubation period
Prodromal Period
Convalescence period
Generation time
Serial interval
Communicable period/Infective period:
The period during which an infectious agent may
be transferred directly or indirectly from an
infected person to another person, from an
infected person to an animal, including arthropods.
Incubation Period
The time interval between initial contact with an
infectious agent and the appearance of first
symptom of the disease in question, or in a vector,
if The first time transmission is possible
(Extrinsic incubation period).
Example:
After exposure to the measles virus and
before the onset of symptoms, a child is
considered to be highly contagious due to
continued attendance at school and social
encounters with other children.
Importance of Incubation Period:
1. Quarantine regulations
2. Tracing the source of infection
3. Determining the type of epidemic
4. For immunization, active as well as passive
5. Predicting prognosis
Prodromal period:
It is the interval between the onset of symptoms &
appearance of characteristic clinical manifestations,
for example as in measles, the interval between the
onset of symptoms such as fever and coryza and
appearance of rash.
Convalescent Period
This is the period of recovery when the clinical
symptoms have subsided.
Generation time
The interval of time between receipt of infection
by a host and maximum communicability / infectivity
of that host.
Serial Interval
The gap in time between the onset of primary case
and the secondary case is called the serial interval.
Disease spread
Source
Contact
Suspect
Reservoir
Fomites
Source:
The person, animal, object or
substance from which an
infectious agent passes to a
host.
Contact:
A person or animal that has
been in an association with an
infected person or animal or a
contaminated environment
and might provide an
opportunity to acquire the
infective agent.
Suspect:
A person whose medical history and
symptoms suggest that he or she may have or
be developing some communicable disease.
Fomites:
It refers to inanimate object
such as handkerchiefs, bed lines
towels, books, spoons, forks, etc.
Which have been soiled with
infective material. Freshly soiled
fomites are likely to spread the
agents of infection.
Infections
Nosocomial
Opportunistic
Crossinfection
Exotic
Infestation
Epizootic
Zoonosis
Enzootic
Nosocomial infection:
An infection occurring in a patient in a hospital or
other health-care facility and in whom it was not
present or incubating at the time of admission or
residual of an infection during the period of
previous admission. It includes infection (unrelated
to the primary condition) acquired in the hospital
but appearing either before or after discharge and
also such infection among the staff of the facility.
Various types of the nosocomial infections may be.
1.Exogenous:
Which are acquired from contaminated hospital
environment air, water, food, equipment, infected
staff, infectious patients, etc.)
2.Endogenous:
Which are acquired from patient’s normal flora……
Nosocomial infection:
Before admission
1
Case#1
Not nosocomial
infection
Hospital stay
2
Case #2 &3
Nosocomial
infection
After Discharge
3
Infection
symptoms
Cross infection:
This is the infection
acquired by an admitted
patient from the other
patients during the
period of hospitalization.
Infestation:
For persons or animal, the lodgment,
development and reproduction of arthropods on
the surface of the body or in the clothing. Infested
articles or premises are those which harbor or give
shelter to animal forms, especially arthropods
and rodents.
Zoonosis:
It is an infection or
infectious disease
transmissible under
natural conditions from
vertebrate animal to
man. It may be:
1.Enzootic:
It is identical to endemic in man.
2.Epizootic:
It is identical to epidemic. Outbreaks that affect
large number of animals are referred to as epizootic.
Exotic:
It is an infection that is not usually present in
the given country but introduced from abroad.
Iatrogenic:
It is an adverse
consequence of a preventive,
diagnostic or therapeutic
regimen or procedure.
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Opportunistic:
An infection caused by the micro-organism which
are not normally pathogenic but can assume
pathogenecity, when the immune status is compromised.
Pandemic
Epidemic
1
2
Distribution.
3
Endemic
4
Sporadic
1.Pandemic:
Global occurrence of a disease.
Pandemics are widespread
epidemics that achieve large
geographic proportions.
Example :during the influenza
pandemic of the 19th century,
millions of people across the
continents were affected.
Today, AIDS is considered to be
the most alarming pandemic of
the century.
2.Endemic:
The constant presence of a disease or infectious
agent within a given geographic area, may also refer
to the usual prevalence of a given disease within such
area.
3.Sporadic :
it refers to the scattered presence of the disease in the
given population (a case here and a case there). This
indicates that the number of the susceptible is less than
the number of resistant in the population
4.Epidemic :
This is occurrence, in a community or region, of cases of
an illness (an outbreak) clearly in excess of expected
occurrence. This indicates that the number of
susceptible is more than the resistant in the
community.
Epidemic are classified as follows:
1. Common source epidemics
2. Point source epidemics
3. Propagative (progressive) Epidemics
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Disease control
1.
2.
3.
4.
5.
6.
7.
8.
Surveillance
Eradication
Control
Quarantine
Segregation
Isolation
Disinfections
sterilization
Surveillance:
Surveillance of disease is the continuous scrutiny
of all aspects of occurrence and spread of disease
/ other condition of ill health that are pertinent to
effective control.
Isolation:
As applied to patients, it represents separation for
the period of communicability to prevent or limit
the direct or indirect transmission of the infectious
agent from those infected to those who are
susceptible or who may spread the disease to
others.
Segregation:
This refers to the separation of a healthy person
from an infected person.
Quarantine:
Restriction of the activities of well person or
animals who have been exposed to a case of
communicable disease during its period of
communicability (i.e. contacts) for maximum
incubation period to prevent disease transmission
during the incubation period, if the infection should
occur.
Eradication:
It means ending the transmission of the disease and
elimination of reservoirs of infection, in a campaign
limited in time and carried out to such a degree of
perfection that when it comes to an end, there is no
resumption of transmission.
Control :
It means to reduce the prevalence of the disease to
such a low level that it no longer remains a big public
health problem.
Disinfection:
It refers to the destruction of the pathogenic
micro- organisms.
Sterilization:
It refers to the destruction of pathogenic as well
as non-pathogenic microorganisms and it includes
destruction of spores also.
Summary of disease transmission and control
a. Chain of infection
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Infectious agent
Reservoir / source of infection
Exit of infectious agent
Transmission (mode) of infection
Entry into a new host
Host susceptibility
Infectious Agent:
An organism (viruses, rickettsia, bacteria, fungus,
protozoa or helminthes) that is capable of
producing infection or infectious disease.
Exit of infectious agent
Avenues of escape are:
1.
2.
3.
4.
5.
6.
Respiratory tract-droplet nuclei (continuous emission)
Intestinal tract-discharge of faeces (discontinuous emission)
Urinary tract.
Open wounds (escape from the lesions on to clothes)
Mechanical escape (biting or sucking insects e.g. malaria
spread by mosquitoes)
Contact (close physical contact)
Transfer of infection to new host:
Direct
transmission (person to person)
Indirect transmission (requires a vehicle)
Classification of vehicle:
Animate
refers to as vectors e.g. housefly,
flea. Mosquito.
Inanimate-water, milk, food, air soil and fomites
(e.g. clothes, door knobs, money etc.), blood
transfusion.
Entry into new susceptible host
Portals of entry are:
•
•
•
•
Respiratory tract- contaminated food or drink.
Gastro-intestinal tract- contaminated food or
drink.
Direct infection of membranes e.g. diphtheria,
venereal diseases.
Percutaneous infection passage through skin via
bite, for rabies and malaria, direct penetration
by infectious agent e.g. schistosomiasis,
hookworm.
Host susceptibility:
A person or animal presumably not
possessing sufficient resistance against
a particular pathogenic agent to prevent
contracting infection or disease if or
when exposed to the agent.
b. Chain of control
1. Notification
2. Early diagnosis
3. Isolation
4. Destruction of infecting agent
5. Investigation of an attack of illness
6. Immunization
7. Health education
Chain of control
Notification:
It means the immediate intimation of the occurrence
of every case of infectious disease to the healthy
officers.
Early diagnosis:
The first step in the control of a communicable disease
is its rapid identification for..
a)
The treatment of patients
b)
For epidemiological investigations
c)
To study the time, place and person distribution.
Isolation:
Separation for the period of communicability of
infected person or animals from others in such
places and in such conditions as to prevent or
limit the direct or indirect transmission of
infectious agent from those infected to those
who are susceptible or who may spread the
agent to others.
Destruction of the infecting agent
(disinfection)
Destruction of pathogenic microorganism is
called disinfection.
Investigation of an attack of an illness
Broadly the investigation covers the identification
of the source of infection and of the factors
influencing its spread in the community.
Immunization:
Increasing the resistance of the susceptible host.
Health education:
Health education is the process by which
individuals and groups of people learn to behave in
a manner conducive to the promotion,
maintenance or restoration of health