Infectious disease epidemiology

Infectious Disease Epidemiology
Dr. Jayaramachandran S
Assistant Professor
Department of Community Medicine

Importance of Infectious Epidemiology
Epidemiological knowledge
Epidemiological activities (Disease Reporting,
measurement of morbidity, mortality)
Prevention & control of disease
23/04/18 Infectious Disease Epidemiology - Dr. JRC 2

Agent
“A substance, living or non living, or a force, the excessive
presence or relative lack of which may initiate a disease process.”
Eg:
• Biological agent - Bacteria, virus, fungi etc.
• Nutrient – Carbohydrate, protein, fat etc
• Physical agent – Heat, cold, radiation etc
• Chemical agent – Uric acid, bilirubin etc

Infection
Infection is defined as entry and development or multiplication
of an infectious agent in the body of man or other animals.
Transmission of infection is defined as spread of infectious agent
through the environment or to another person from the reservoir and
source.
423/04/18 Infectious Disease Epidemiology - Dr. JRC

Contamination
“Presence of an infectious agent on a body surface or substances
like clothes, beddings, toys, surgical instruments or dressings including
water, milk and food.”
Eg: Water contamination

Infestation
“The lodgment, development and reproduction of arthropods on
the surface of the body of person or animal.”
Eg. Scabies, Pediculosis

Host
“Person or animal, including birds and arthropods that affords
lodgment to an infectious agent under natural conditions.”
• Obligatory host: Man in measles
• Definitive host: malaria cycle in mosquito
• Intermediate host: malaria cycle in man

Contagious disease
Disease that is transmitted through contact.
Eg: Scabies, Pediculosis, Leprosy, STD.

Communicable disease
“An illness due to a specific infectious agent or its toxic product
capable of being directly or indirectly transmitted from man to man, or
animal to animal or animal to man or from environment to man or
animal.”
Eg: Rabies, TB, Leprosy, Diarrhoeal diseases.

Epidemic
“The unusual occurrence in a community or region of cases of an
illness or other health related events clearly in excess of expected
occurrence.”
Eg: Gastro Enteritis, Hepatitis A.

Endemic
“It refers to the constant presence of a disease or infectious
agent within a given geographical area or population group without
importation from outside.” Eg: Hepatitis - A, Typhoid fever.

Endemic
Hyper endemic:
The disease is constantly present in the region and affects all the
age groups. Eg: Diarrheal diseases
Holo endemic:
The rate of infection is high in children and less in adults. Eg:
Malaria

Sporadic
“Cases occur irregularly, haphazardly from time to time and
generally infrequently.” Sporadic means scattered about.
Eg: Rabies, Tetanus, Herpes zoster, Meningococcal meningitis etc.

Pandemic
“An epidemic affecting a large proportion of the population,
occurring over a wide geographical area such as nation, a continent or
world.”
Eg: Influenza, Cholera, HIV.

Zoonosis
“An infection or infectious disease transmitted under natural
conditions from vertebrate animals to man.”
Eg: Rabies, Plague, Anthrax, Brucellosis, Salmonellosis.

Zoonosis
• Anthropozonosis:
Infections transmitted from vertebrate animal to man.
Eg: Rabies, Plague, Anthrax, KFD.
• Zooanthroponoses:
Infections transmitted from man to vertebrate animals
Eg. Bovine tuberculosis

Nosocomial infection
“An infection originating in a patient while in a hospital or other
health care facility.”
Eg: Wound infection, UTI

Opportunistic infection
“Infection by an organism that takes the opportunity provided by
a defect in host defense and cause disease.”
Eg: Candidiasis, Cryptococcal meningitis in AIDS patients.

Iatrogenic disease
“Any adverse consequences resulting from a physician’s
professional activity or from the professional activity of other health
personnel.”
Eg:
• Anaphylactic reaction to any antibiotics or immunoglobulin.
• Acquiring HIV infection through injection or blood transfusion.

Notifiable diseases
A disease that by statutory requirements, must be reported to
the public health authority in the pertinent jurisdiction when the
diagnosis is made.
Eg: Recent addition to the list of notifiable disease is Tuberculosis

Elimination
“Interruption of transmission of disease in a given geographical
area or region.”
Eg. Elimination of Poliomyelitis.

Eradication
“Termination of all transmission of infection by extermination of
the infectious agent.”
Termination of an infection from the whole world, so that
disease will no longer occur in the world.
Eg: Small pox is the only disease that has been eradicated.
Current candidates for eradication are polio, measles

Summary
• Definitions in Epidemiology is important for reporting, planning and
implementation of disease prevention and control programme.
• Every medical students should know these definitions for the future
use.

Dynamics of disease transmission
“Our relationship with infectious pathogen is a part of an evolutionary drama”
- Joshua Lederberg

Source or reservoir

Introduction
Dynamics of disease transmission is the term mainly applicable
for communicable diseases.
• Communicable disease :
Communicable disease is defined as an illness due to a specific
infectious agent or its toxic products capable of being directly or
indirectly transmitted from man to man, animal to animal, or from
environment ( through air, dust, soil, water, food, etc) to man or animal.

Transmission dynamics
27
Source or
reservoir
Modes of
transmission
Susceptible
host

Source of infection
Source of infection is defined as the person, animal, substance or
object from which infectious agent passes or disseminates.
Eg : Soil ---- Source of infection for Hook worm infestation
Sputum -- Source of infection for Tuberculosis

Reservoir of infection
Is defined as any person, animal, arthropod, plant, soil, or
substance (or combination of these) in which the infectious agent lives
& multiplies on which it depends primarily for survival & where it
reproduces itself in such a manner that it can be transmitted to the
susceptible host
E g : Man – Tuberculosis, HIV
Dogs – Rabies
Rodents – Plague

Reservoirs & Sources
Disease Reservoir Source
Tuberculosis Man Sputum
HIV / AIDS Man Body secretions, fluids
Malaria Man / Mosquito Infected blood
Rabies Dog / other animals Saliva
Measles Man Droplets
JE Pig & Birds Infected mosquitoes
Cholera Man Unsafe water & food
Typhoid Man Unsafe water and food

Difference between source and reservoir
Source of infection is an immediate source which may or may not
be a part of reservoir
E g : Hook worm infestation
Source ---- soil
Reservoir ---- Man

Classification of reservoirs
Reservoir
Human Animal
Non – living
things

Homologous reservoir
Is one where another member of the same species acts as a
victim
Eg : Man is a principal reservoir for some enteric pathogens like V.
cholera, Tuberculosis, Measles

Heterologous reservoir
Is applied when infection is derived from reservoir other than
man
Eg : Rabies

Human reservoir
Case
Clinical illness
Subclinical cases
Latent infection
Carrier
Type
Duration
Portal of Exit

Human reservoir – Case
Case is defined as a person in a population or study group,
identified to be having a particular disease, health disorder or condition
under investigation.

Human reservoir – Case
• Criteria to identify a case are
1. Clinical features
2. Biochemical tests
3. Laboratory investigations
• Broadly presence of infection may be
1. Clinical
2. Sub clinical
3. Latent

Human reservoir – Case – Clinical illness
• Clinical cases may be
Mild / Moderate / Severe
Typical / Atypical
• Epidemiologically mild cases are more important than moderate or
severe cases
• As mild cases are ambulating so they spread infection more than
severe cases which have reduced mobility and usually confined to bed

Human reservoir – Case – Subclinical cases
• They are also referred to as in-apparent, covert, missed, abortive
cases
• The infectious agent multiplies in the host but does not manifest itself
by signs and symptoms
• Thus the persons who are not sick [ unbeknown to themselves &
others] contribute more to the infectious disease transmission
• They represent the submerged portion of iceberg they do not come
under any statistics

These cases can only be detected in laboratories by
1. Recovery of organism : Blood culture ---- Staphylococci
2. Antibody response : Anti Hbs Ag antibodies -- Hepatitis B
3. Biochemical test : Acid phosphatase Tuberculosis
4. Skin sensitivity test : Monteux test ---- Tuberculosis
Casoni’s test ---- Hydatid cyst

Proportion of individuals with asymptomatic / sub clinical cases
determines the pathogenicity of the infectious agent.
• Examples of sub clinical cases are Rubella, Hepatitis A / B,
Poliomyelitis, Influenza, Diphtheria

Human reservoir – Case – Latent Infection
In latent infection the host does not shed the infectious agent,
which lies dormant within the host without symptoms (and often
without demonstrable presence in blood, tissues or bodily secretions of
the host).
Eg: Varicella zoster disease
• Primary case: The first case of a communicable disease introduced
into the population unit being studied. Eg : 1st case of measles in a
village.

Human reservoir – Case – Latent Infection
• Index case:
First case of a communicable disease which comes to attention
of an investigator. Eg : 1st case of measles coming to the PHC
• Secondary case:
These are the cases developing from contact with primary case.
Eg : Classmate of first child developing the measles

Human reservoir – Carriers
Carrier is defined any infected person or animal that harbors a
specific infectious agent in absence of any discernible clinical disease
and serves as a potential source of infection for others.
• Factors responsible for production of carriers are Inadequate
treatment & Immune response elicited by host
• Epidemiological importance: They escape recognition, They live
normal life among population or community, They readily infect the
susceptible individuals for a long period of time under favorable
conditions.

Carriers – Classification
Type
Incubatory
Convalescent
Healthy
Duration
Temporary
Permanent
Portal of exit
Urinary
Intestinal
Respiratory
Others

Human reservoir – Carriers – By type
• Incubatory carriers :
Incubatory carriers those who shed the infectious disease during
the incubation period of the disease. That is they are capable of
infecting others before the onset of illness. Eg : Measles, mumps,
pertussis, diphtheria
• Convalescent carriers:
That is those who continue to shed the disease agent during the
period of convalescence. Here the clinical recovery doesn’t co inside
with bacteriological recovery. Eg : Typhoid – 6 to 8 weeks after recovery

Human reservoir – Carriers – By type
• Healthy carriers:
They are the victims of sub clinical infection who developed
carrier state without suffering from overt disease, but are nevertheless
shedding the disease agent. Eg : Typhoid, cholera, meningococcal
meningitis
“A person who’s infection remains subclinical may or may not act
as a carrier” Eg : Polio, Tuberculosis.

Human reservoir – Carriers – Duration
• Temporary carrier: Eg: Mumps, cholera, Polio
They spread infectious agent for a limited period of time, they
include
• Incubatory carriers
• Convalescent carriers
• Healthy carriers
• Permanent carrier:
Is one who excretes the infectious agent over a longer period of
time. Eg : Typhoid, Malaria, Hepatitis B & A.

Human reservoir – Carriers
• Longer the carrier state greater the risk to the community
• Carriers may excrete infectious agent intermittently or continuously
• Duration of carrier state varies with disease
Eg : Typhoid, Hepatitis --- several years
Pseudo carriers (Carriers of avirulent organism) --- Carriers of
nonpathogenic Mycobacteria

Human reservoir – Carriers – Portal of exit
Portal of exit Example
Urinary Typhoid
Intestinal Amoebiasis
Respiratory Tuberculosis
Nasal Diphtheria

Animal Reservoir
• Zoonosis:
An infection or infectious disease transmissible under natural
conditions from vertebrate hosts to man.
There are about 100 zoonosis which are transmissible to man
Eg : Rabies --- Dogs
Anthrax --- Cattle
Influenza --- Pigs, chicks

Animal Reservoir – Amplifying host
Is one in which enormous multiplication of the infectious agent
takes place, without the overt clinical disease.
Eg : Pigs JE
Pigeons Chlamydia
Wild birds Mosquito born febrile illness
There is evidence that genetic recombination between human &
animal viruses might produce new strain of viruses
Eg : Influenza --- Antigenic shift , Antigenic drift

Soil
Soil can harbor infection that can cause
• Tetanus
• Coccidioidomycosis
• Mycetoma

Modes of Transmission

Modes of transmission
Direct
• Direct contact
• Droplet infection
• Contact with soil
• Inoculation – skin , mucosa
• Trans placental / vertical
Indirect
• Vehicle born
• Vector born
• Air born
• Fomites
• Unclean hands and fingers

Direct transmission – Direct contact
This implies an immediate transfer of infectious agent from
reservoir or source to the susceptible host. Eg: Touch – Leprosy,
Scabies, Kissing – Infectious mononucleosis. Sexual intercourse – HIV,
Syphilis.
• The infection may be transmitted from skin to skin, skin to mucosa,
mucosa to mucosa of a same or another person
• Direct contact ensures reduced time required for the infectious agent
to survive in the external environment
• Larger infective dose

Direct transmission – Droplet infection
Direct projection of droplets or saliva , nasopharyngeal
secretions during coughing, talking, singing, sneezing, spitting, talking
to the surrounding atmosphere
They impinge largely over conjunctiva, respiratory mucosa or skin
of close contacts. These droplets which contain millions of organisms
are inhaled by the surrounding susceptible hosts and acquire infection
Eg : Diphtheria, measles, whooshing cough, TB

Direct transmission – Soil
Infectious agent in the soil
Direct exposure of susceptible host
Organisms penetrate the skin
Infection is acquired
Soil acts as a source of infection for hookworm, tetanus mycosis.

Direct transmission – Inoculation into skin / mucosa
Eg : Rabies the organisms in the saliva of dog and are directly
inoculated into the skin/mucosa. Other examples are Hepatitis B, HIV

Direct transmission – Transplacental / Vertical
From mother to fetus.
Eg : Toxoplasmosis, Rubella, Cytomegalovirus, Herpes simplex,
Varicella. Some nonliving things like alcohol, thalidomide. This can
occur through placenta, or during labour

Indirect transmission
Transfer of infectious agent from source or reservoir to
susceptible host in agency of other factors.
5F’s : Flies, fingers, fomites, food, fluid
• Essential requirements for indirect transmission - Capability to survive
in external environment and retain pathogenicity and virulence
• It depends on Characteristics of agent, The inanimate object,
Environmental factors, Resistance to drugs

Indirect transmission – Vehicle borne
Vehicle borne disease transmission implies transmission of
infectious agent through the agency of water, food, blood, serum.
Water: Dead, decaying matter with faeces in soil mixes with rain water.
The infectious agent reaches river and water resources agent continues
to multiply in water reaching susceptible host.
• Man is the commonest contributor for water pollution

Indirect transmission – Vehicle borne
• Water borne diseases:
• Eg : Diarrheal diseases, Cholera, Hepatitis A,E, Bacillary dysentery,
Polio, typhoid
• Food born diseases:
• Include, food poisoning, intestinal parasites
• Staphylococcus : External contamination
• Salmonellosis : Pre existing poison
• Intestinal parasites – Ascaris lumbricoides

Epidemiological features – Vehicle borne diseases
• If the dose of contamination is heavy, outbreak may be explosive
• Cases are confined to those who are exposed to contaminated vehicle in
some infections
• When secondary cases occur primary cases are obscured
• The distance traveled by the infectious agent may be great Eg : Food
poisoning
• It is not always possible to isolate the infectious agent in the incriminated
vehicle. Eg : Typhoid
• When the vehicle is controlled or withdrawn the epidemic subsides
Eg : Cholera
• Common source of infection is often traceable

Indirect transmission – Vector borne
Vector is defined as an arthropod or any living carrier that
transports the infectious agent to the susceptible individual
• Classification : By vector :
•

Classification of Vector Borne : by Vector
Vertebrate type
• Mice
• Rodents
• Bats
Invertebrate type: Arthropod
• Diptera. : Flies / mosquitoes
• Siphonaptera: Fleas
• Orthoptera. : Cockroaches
• Anopleura. : Suckling lice
• Hemiptera. : Bugs
• Acarina. : Ticks
• Copepoda. : Cyclops

Classification of Vector Borne : by vector transmits disease
1. Biting – Malaria
2. Regurgitation – Plague
3. Scratching in of infective faeces – Enterobius
4. Contamination of host with body fluids by vectors

Classification of Vector Borne : by transmission chain
Man & non-
vertebrate Host
• Man – Arthropod
– Man : Malaria
• Man – snail –
man :
Schistosomiasis
Man – Another
vertebrate & Non-
vertebrate host
• Mammal –
Arthropod – Man
: Plague
• Bird – arthropod
– man
(encephalitis)
Man & two
intermediate hosts
• Man – Cyclopes –
Fish – Man : Fish
tapeworm
• Man – Snail –
Crab – Man :
Paragonimus

Classification of Vector Borne : By methods in which
vectors are involved in the transmission
A. Mechanical transmission:
Infectious agent is mechanically transported by crawling, or
flying arthropod with soiling of it’s feet or proboscis or by passing of
organism through it’s GI tract and passively excreted
• There is no development or multiplication of organism in the vector
Eg : Flies transmitting cholera, typhoid

B. Biological transmission:
The infectious agent undergoing development or multiplication
or both in the vector & requires an incubation period before vector can
transmit the disease
• Extrinsic incubation period : This is the interval between entrance of
infectious agent in the vector and vector becoming infective.
i. Propogative type: The infectious agent merely multiplies but no
change in the form. Eg : Plague bacilli in rat flea

ii. Cyclopropogative type:
In this form the agent changes its number and form thus there is
both multiplication and development. Eg : Malaria in Anopheles
mosquito
iii. Cyclodevelopmental type:
In this type the disease agent undergoes only development but
no multiplication. Eg : Wucheraria bancrofti microfilaria

• Transovarial transmission:
When the infectious agent transmitted vertically from female
mosquito to it’s progeny. Eg: Aedes aegypti : In chikungunya
• Trans – stadial transmission:
Transmission of infectious agent from one stage of life cycle to
the next. Eg : Aedes aegypti : Yellow fever
Factors influencing ability of vectors to transmit the disease: Host feeding preference, Infectivity,
Susceptibility & Survival rate of vectors

Air borne transmission – Droplet nuclei
• Droplets ( 0.1 mm) : They evaporate in the environment & dry Droplet
nuclei ( 1-10mico m)
• Droplet nuclei may be formed from evaporation of the particles
coughed and sneezed
• They can also be generated by microbiological labs by variety of
atomizing devises
• These droplets remain suspended in air for long time sometimes
loosing / retaining infectivity
• Particles between 1-5microns liable to easily drawn into alveoli and retain there.
Eg: TB, Influenza

Air borne transmission – Smog & Dust
• Smog :
• Air borne spread of toxic air pollutants resulting in air pollution epidemic
• Dust:
• Larger particles expelled during coughing, sneezing.
• They settle down by their shear weight on the floor, carpets, furniture,
bedding, clothes.
• They become part of dust that is small particles of varying size arising from
soil.
• This dust contains variety of organisms. During act of sweeping, bed making
they are released. Inhalation of this dust leads to infection

Air borne transmission – Dust
• Dust may also be released from floor by wind (Fungal spores)
• Other infections are Pneumonia, TB, Psittacosis
• Dust may remain unsettled in the milk/ uncovered food this type of
infection causes nosocomial infection

Air borne transmission – Fomites borne
They are inanimate articles other than water & milk
contaminated by the infectious discharges from the patient capable of
harboring & transmitting infectious agent to healthy person.
• Eg : Clothes, towels, linen, handkerchiefs, cups
Diseases transmitted are diphtheria, Dysentery

Air borne transmission – Unclean hands and fingers
• Hands are most common medium through which the infectious
agents are transferred to food from skin, nose, bowel
• Transmission takes place directly from hand to mouth
• Eg : Staphylococcal, streptococcal infections, dysentery
• Unclean hands indicate poor personal hygiene
• Parenteral transmission : Occurs by common use of syringes, needles
among IV drug abusers. 2.2% HIV transmission is by parenteral root

Susceptible Host

Susceptible host
• Host: A person or animal including birds or arthropods that affords
subsistence or lodgment of an infectious agent under natural
conditions.
• Obligate host: Means the only host. Eg : Measles
• Definitive host: Hosts in which parasites attains maturation or passes
it’s sexual cycle
• Intermediate host: Host in which parasite in asexual or larval state

Host factors in communicable diseases
• Age : Extremes of age more
chances of infection
• Sex
• Marital status
• Nutritional status
• Immunization status:
• Literacy
• Occupation
• Behavioral factors
• Immune status

Successful parasitism
Four stages:
• Portal of entry
• Site of election
• Portal of exit
• Survival capacity in external environment
Incubation period: Defined as the time interval between the invasion of
infectious agent and appearance of first sign or symptom of disease

Median incubation period
Time required for 50 % of cases to occur following exposure
• Factors determining the incubation period
• Generation time of a particular pathogen
• Infective dose
• Portal of entry
• Individual susceptibility
• Infectious diseases communicable during incubation period are
Measles, chickenpox, whooping cough

Characteristics of incubation period
• Length of incubation period is characteristic of a particular disease
• There is a minimum period below which there is no illness can occur
• Varies from person to person for a same disease
• Different for different diseases
• In some diseases incubation period is very small ranging from few
hours to 2-3 days
• Medium ( 10days – 3weeks ) – Typhoid, chickenpox
• Long : Hepatitis B, leprosy sometimes unpredictable

Why to know about incubation period
• Tracing the source of infection
• Period of surveillance or Quarantine
• Immunization
• Identification of point source or propagated epidemic
• Prognosis
• Serial interval: The gap in time between primary and secondary cases is
known as serial interval
• By getting the series of information about primary & secondary cases we
can calculate incubation period

Generation time
• It is the interval of time between receipt of infection by the host and
maximum infectivity
• Generation time is roughly equal to incubation period
• Maximum communicability may precede or follow the incubation
period
• Incubation period can be calculated only when the infection
manifests with clinical features
• Generation time refers to transmission of infection whether clinical or
subclinical

Communicable period
Defined as time during which an infectious agent may be
transferred directly or indirectly from an infected person to another
person, from an infected animal to man or infected person to animals
including arthropods
• Communicability varies among different diseases
• Some diseases are more communicable during incubation period than
actual illness
• Communicability of some diseases can be reduced by early diagnosis
and treatment

Secondary attack rate
Number of exposed persons developing the disease within the
range of the incubation period following exposure to the primary case.
• The denominator may be restricted to only susceptible contacts.
• The primary case is excluded from both numerator and denominator.
87
SAR = × 100
No. of exposed persons developing the disease with in the range of the I.P
Total No. of exposed/susceptible contacts

Limitations:
1. It is limited to infections with primary case infective only for a short
period of time
2. If the primary case is infective for a long period of time duration of
exposure is an important factor in determining extent of spread

Limitations:
3. Another limitation is to identify susceptible. It is feasible only in
diseases like measles, chickenpox where history can be used as a
basis of identification but in majority of cases the susceptible
cannot be readily identified
4. SAR has limited role in diseases which have numerous subclinical
cases. Further spread can’t be measured without lab investigations

Secondary attack rate – Advantages
• Vaccinees and nonvaccinees from several families can be added to
determine the over all attack rates in the vaccinated and
unvaccinated populations provided the same definitions for cases and
immunization status are used
• SAR was initially developed to measure the spread of infection within
family, household or any close aggregate of persons who have had
contact with case
• It is useful to determine diseases of unknown etiology is
communicable or not
• Evaluation of control measures – Isolation, Immunization

Summary
• Communicable diseases are major public health problem since
ancient days but still remained as problems
• The source & reservoir of infection
• Carriers, sub clinical cases are keys for the spreading the diseases
• Pathogenesity, virulence, resistance are weapons for infectious agent
• Direct, indirect ways of transmission
• Vehicles, vectors , fomites ,Food, dust, droplets are routes of spread

Summary
• Host is the recipient of infectious agent and hence the victim
• Incubation period, generation time, serial interval, secondary attack
rate are tools for measurement and control

Disease prevention & control

Disease Prevention & Control
• Controlling the reservoir
• Early diagnosis & treatment
• Notification
• Epidemiological investigation
• Isolation
• Treatment
• Quarantine

Disease Prevention & Control
• Interruption of transmission
• Susceptible host

Controlling the Reservoir – Early Diagnosis &
Treatment
• Detection of cases and carriers helps in prompt treatment. Early
diagnosis is useful for
• Treatment of patients
• Epidemiological investigation – Eg.. To trace the source of infection from
known or index case to unknown or primary source of infection
• To study the time, place and person distribution
• For the institution of preventive & control measures

Controlling the Reservoir – Notification
• Notification to the local health authority to take up control measure
including the provision of medical care to the patients.
• The serious diseases which are serious menace to public health are included
in the list of notifiable diseases.
• Notification is done by the head of the family including lay people.
• Diagnosis is verified by the local health authority.

Controlling the Reservoir – Notification
• As per international health regulations notifiable diseases are cholera,
plague and typhoid fever. Diseases under surveillance by WHO –
louse borne typhus fever, relapsing fever, paralytic polio, malaria, viral
influenza A, SARS etc…

Controlling the Reservoir – Investigation
• It covers the identification of source of infection and of the factors
influencing its spread in the community which include
• Geographical situation,
• Climatic condition,
• Social, cultural and behavioral pattern
• Character of reservoir, vectors, vehicles and the susceptible host population

Controlling the Reservoir – Isolation
• Separation for the period of communicability of infected persons, or
animals from others in such places and under such conditions as to
prevent or limit the direct or indirect transmission of the infectious
agent from those infected to susceptible or who may spread the
agent to others.
Disease Duration of isolation
Chicken pox Until all lesions crusted – 6 days after the onset of rashes
Measles From the onset of catarrhal stage to the 3rd day of rash
Mumps Until parotid swelling subsides
Hepatitis A 3 weeks

Controlling the Reservoir – Treatment
• Reduces the period of communicability,
• Cuts short the duration of illness and
• Prevents the development of secondary cases

Controlling the Reservoir – Quarantine
The limitation of freedom of movement of such well person or
domestic animals exposed to communicable disease for a period of
time not longer than the longest usual I.P. of the disease in such
manner as to prevent effective contact with those not so exposed.
Eg. All travelers from yellow fever endemic zone is placed on
quarantine for 6 days from date of leaving that area if vaccination
certificate is not available
• Quarantinable diseases are – Plague, Yellow fever, SARS

Interruption of transmission
• It means the breaking the chain of transmission for eg..
• Water borne diseases like Typhoid, Dysentery, Cholera Hepatitis A can be
prevented by provision of safe drinking water supply.
• Food borne diseases can be prevented by improving food sanitation like clean
practices such as hand washing, adequate cooking, prompt refridegeration of
prepared foods and withdrawal of contaminated foods
• Prevention of vector borne diseases by adequate vector control measures.

Susceptible host
• Susceptible host is protected by
• Active immunization
• Passive immunization
• Combined (active & passive) immunization
• Chemoprophylaxis
• Non specific measures

Susceptible host – Active immunization
• UIP vaccination for all children as per national immunization schedule

Susceptible host – Passive immunization
• Eg..
• Anti Rabies Serum for rabies
• Anti Tetanus Serum for Tetanus

Susceptible host – Combined Immunization
• ARS + ARV for class 3 dog bite cases
• ATS + TT in the treatment of Tetanus

Susceptible host – Chemoprophylaxis
• Administration of specific drugs for contacts for the prevention of the
disease before the occurrence of the disease, Eg..
• Diphtheria – Erythromycin and first dose of vaccine
• Cholera – Tetracycline for all household contacts
• Malaria – 300mg of Chloroquine base once a week, same day of each
week should be started one week before entering a malaria endemic
area, continue during the stay there and continued for one week after
returning back.

Susceptible host – Nonspecific Measures
• Improvement in the quality of life
• Better housing
• Water supply
• Sanitation
• Nutrition
• Education - has brought down most of the communicable disease like Tb,
Cholera, child mortality.

Any Questions?

Infectious disease epidemiology

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