Epidemiology The study of the distribution and determinants of disease in populations and its application to the control of health problems

DEFINITIONS OF HEALTH AND DISEASE REQUIRE

DEFINITIONS OF NORMALITY:

• Common (frequent)

• Statistical (within a rangę)

• Pragmatic (related to risks)

• Practical (related to benefits)

DEFINITIONS ARE BASED ON

• Signs

• Symptoms

• Results of tests

MEASUREMENT OF HEALTH AND DISEASE IS

REGUIRED FOR:

• Preventing disease

• Promoting health

• Planning health services

THE CENTRAL TOOL OF

EPIDEMIOLOGY IS THE COMPARISON

OF RATES:

Rate = Numerator

Denominator

Defining health

• The absence of disease

• A feeling of well-being

• Freedom from the risk of

disease and untimely death

• ‘A state of complete

physical, mental and

social well being and not

merely the absence of

disease or infirmity’

(WHO)

– The Definition has not

been amended since

1948

• Many definitions

Population Health

• Conceptual framework

for thinking why some

people are healthier than

others

• Policy development for

resource allocation

– Maintenance of health

– Health research

• Social structures and

processes that contribute

to health

• Epidemiology addresses

health at the population

level

Uses of Epidemiology

• Identifying the cause of disease

– Legionnaires’ disease, Bird Flu

• Determining the effectiveness of

clinical and preventive measures

– Clinical trials, mammograms

• Identifying new symptoms

– Varieties of hepatitis, new

salmonella strains,

• Monitoring the health of a

community, region, or nation

– Surveillance, accident reports

• Identifying risks in terms of

probability statements

– Down Syndrome at increasing

age

• Studying trends over time to make

predictions for the future

– Estimating health services

needs

Epidemiologist’s

checklist

• Define (verify) disease –

what?

– Accuracy of diagnosis

• Describe historical trend of

disease – when?

• Define extent of the problem

– how much?

• Identify determinants of

disease distribution – why?

• Formulate hypotheses about

causation

– how?

• Determine modes of

transmission

• Develop and evaluate

interventions for control and

prevention

Define disease Case definition is the set

of criteria that

differentiate normal (non-

diseased) from abnormal

(diseased).

Case definition may be

based on symptoms

– sometimes the absences

of symptoms

Case definition may be

complicated because there:

– Are no tests;

– Tests are expensive; or

– Test are impractical.

It is important that case

definitions are frequently

reviewed.

Case definition Case definition should

consider how cases will be

detected.

Things to consider:

– Is there a registry of

case?

– Are all cases likely to be

recorded on the

registry?

– Are cases seen by a

medical practitioner?

– Are all cases likely to be

seen by a medical

practitioner?

Defining the

population

Populations may

be defined by:

–Location

–Age

–Sex

–School

– Study

population

the population of

individuals

selected to

participate in the

study (regardless

of whether they

participate)

– Target

population

Target population

= population to

which the results

will apply

Prevalence

Proportion of

population at a defined

point in time that are

suffering from the disease.

Case becomes an

prevalent case until

recovery, death or it leaves

the population

Calculated as the:

P = Number of individuals with attribute at a specified point in time x 10n

Total population at the given point in time

Incidence rate

Where

I = number of cases

PT = Person time a risk

Person time at risk

An individual contributes time at

risk until

– Loss to follow-up

– Death from another cause

– Change in risk status (e.g.

hysterectomy eliminates risk of

uterine cancer

What is person time at risk?

Person-time at risk is the

denominator for rates of disease

1000 person-years at risk =

PT

IIR

– 1000 people followed for 1 year

– 500 people followed for 2 years

– 10,000 people followed for 1/10

of a year

– 100 people followed for 10

years

Calculating

person time at risk

Exact method

– Sum up the total person time

at risk

Cumulative incidence

Where:

– I = Number of cases during

a time period

– N0 = Disease free people at

time zero

Probability that an individual will

become diseased over specified

period

0N

ICI

Case fatality rate:

The proportion of individuals contracting a

disease that die of that

disease.

ROUTINE INFORMATION

MORTALITY (DEATH) DATA: (Coded according to the ICD)

CRUDE MORTALITY RATE (CMR)

Total # people dying CMR = ____________

Total # people

*AGE-SPECIFIC RATES *SEX-SPECIFIC RATES *CAUSE-SPECIFIC RATES *AGE-STANDARDIZED RATES (Also called age-adjusted rates)

LIFE EXPECTANCY

Average number of years of life remaining at

specified ages if current mortality trends

continue

Standardization:

The process by which you derive a

summary figure to compare health

outcomes of groups

The process can be used for mortality,

natality, or morbidity data

Infectious disease

– diseases involving an agent

(bacterial, viral, parasitic) which is

transmitted from infected to non-

infected hosts

Infectious diseases

– bacterial

• salmonellosis, campylobacteriosis

– viral

• polio, HIV, influenza

– parasitic

• malaria, lymphatic filariasis

Non-infectious diseases

– cancers, heart disease, diabetes

mellitus

Unlike non-infectious diseases, the

occurrence of infectious disease events

in a given host depends on

– occurrence of the disease in other

members of the host population

– length of time that infected hosts

remain infectious

Infectious disease epidemiology

– centres around understanding the

relationship between the host and

infectious agent (or parasite) and the

transmission of the infectious agent

between hosts

Components of the infectious process

1. Agent

2. Reservoir

3. Portals of entry and exit

4. Transmission

5. Immunity

Agent

1. infectious = biologic

organism living and

replicating within a host

2. infectious disease =

infection + illness

3. contamination = agent

on exterior surface near

host

Reservoirs the whole species or other

environment where the agent

multiplies

without the reservoir, the agent

can’t perpetuate

types of reservoirs

symptomatic cases

carriers

animals

inanimate objects

Source of infection

Any living organism in which

infectious agent lives, multiplies &

from which the host acquires the

agent.

Transmission

– contact

– direct (host host)

– indirect (host exudate or

secretion host)

– droplet (airborne short distance)

– nuclei (airborne suspended)

– intermediary

– vector (animate)

– vehicles (inanimate)

Terminology used to describe the various

phases an individual goes through during

the time course of an infectious disease

process …

• Latent period

– the time interval from infection to

development of infectiousness

• Incubation period

– the time interval from infection to

development of signs of disease

– not always a fixed period of time,

shows some variation due to

• route of incubation

• virulence of agent

• host factors (age, immune status)

– follows a log normal distribution

• Infectivity

– the ability of an agent to cause

infection in a susceptible host

– in theory, depends on the minimum

number of infectious particles

required to establish infection

– in diseases spread from person to

person, the proportion of susceptible

individuals who develop infection

after exposure

– the secondary attack rate is a

measure of infectivity

• Pathogenicity

the ability of the infectious agent to

induce disease

agents with high pathogenicity

• viruses causing rabies, smallpox,

measles, chicken pox

agents with low pathogenicity

• polio virus, arboviruses

(mosquito borne)

•

• Virulence

describes the severity of disease,

after infection has occurred

measured using the case fatality rate

• Carrier = contagious person without

discernable signs of disease

types of carriers

inapparent throughout (e.g.

polio)

incubatory (e.g. Hepatitis B,

HIV)

convalescent (e.g.

Salmonella typhi)

The carrier state

– asymptomatic carriers of infection

– play an important role in

spreading disease

– ‘Typhoid Mary’

• Irish cook in New York in the

early 1900s

• worked as a cook in many

homes where residents

developed typhoid fever after

she was hired

• 53 cases of typhoid fever traced

to her

Infectious period

– the time during which time the

host can infect another susceptible

host

–

Non-infectious period

– the time interval between infection

to development of clinical disease

• Terms used to describe the temporal

pattern of disease in a population:

– endemic

disease occurs at expected frequency

– epidemic

disease occurs at greater than expected

frequency

– pandemic

huge epidemic (international)

– sporadic

single case or a cluster of cases

• Endemic

– disease occurs at expected

frequency

– disease present in population or

region at all times

– level of disease usually low and

predictable

• Epidemic

– disease occurs at greater than

expected frequency

– incidence exceeds expected

– usually infectious disease or

poisoning

– may be point source or

propagated

• Sporadic

– single case or cluster of cases

– infrequent disease occurrence

– irregular and unpredictable

– examples:

• Legionnaire’s disease

• food poisoning

• Seasonal variation

– vector-transmitted diseases

(malaria, dengue fever, St Louis

encephalitis) depend on exposure

to infected mosquito vectors

– disease transmission only occurs

during the warmer months of the

year

• Annual variation

– many infectious diseases exhibit

marked and repetitive cyclical

trends

– due to infection exhausting the

susceptible population and then

infecting the birth cohort

replenishing it

– examples

• measles, pertussis, polio

• Temporal patterns of onset can

provide insight into the nature of the

disease we’re dealing with (even

when we don’t know the cause)

Type of epidemics

1.Common source

2.Propagated

1. Common source epidemics

– subjects are exposed to a common

noxious influence

– common point source epidemics

• group is exposed over a relatively

short period then disease cases

will emerge over one incubation

period

• curve rises rapidly and contains a

definite peak at the top, followed

by a gradual decline

– common continuous source epidemics

• group is exposed continuously

and cases emerge over more than

one incubation period curve rises

rapidly, no definite peak

2. Propagated epidemics

– occur when the agent is transmitted

through the population from host to

host (typically infectious conditions)

– propagated epidemics, in theory,

show a series of progressively

taller peaks one incubation period

apart

Nature of epidemic depends on

• characteristics of an agent

(virulence) and host

(susceptibility)

• contact rate

• population density

CHAIN OF INFECTION

AGENT

↓

TRANSMISSION

↓

HOST

FACTORS REQUIRED TO PRODUCE INFECTIOUS

DISEASE

ETIOLOGIC AGENT

RESERVOIR

PORTAL OF EXIT

TRANSMISSION

PORTAL OF ENTRY

SUCCEPTIBLE HOST

Methods of transmission of an infectious agent

Direct transmission

Touching

Kissing

Sexual intercourse

Other contact (e.g. childbirth, medical

procedures, injection of drugs, breast-feeding)

Airborne, short-distance (via droplets,

coughing, sneezing) Transfusion (blood)

Transplacental

Indirect transmission

Vehicle-borne (contaminated food, water,

towels, farm tools, etc.)

Vector-borne (insects, animals)

Airborne, long-distance (dust, droplets)

Parenteral

(injections with contaminated syringes)

The routes of infection vary for different viruses and bacteria. Generally, there are one or

two main methods of transmission; no infectious agent can transmit via all the methods in

the table. A detailed knowledge of these transmission routes is required for effective

primary prevention.

Investigation of an epidemic

1. Verification of diagnosis

2. Confirmation of the existence of an epidemic

3. Identification of cases and their characteristics

4. Definition and investigation of the population at risk

5. Formulation of a hypothesis as to the source and spread of the epidemic

•

Types of epidemiological study

Type of study Alternative name Unit of study

Observational studies

Descriptive studies

Analytical studies

Ecological Correlational Populations Cross-sectional Prevalence Individuals Case-control Case-reference Individuals Cohort Follow-up Individuals

Experimental studies Intervention studies Randomized controlled trials Clinical trials Patients Field trials Healthy people Community trials Community intervention Communities

studies

This classification of study types includes the most commonly used terms for different study

designs. Unfortunately, not all epidemiologicaj study reports use the terms in the same way,

and in addition, terms other than those listed here may occasionally appear in the literature.

The terms in the left column are those recommended for use in WHO-sponsored research

and teaching programmes.

Design of a case-control study

Start with: Exposed ‹——————— ,

___ cases ________(people with

Not exposed‹—————— disease) POPULATION Exposed‹———————

controls ———————— (people without

Not exposed‹—————— disease)

TIME

————————›

direction of inquiry

<—————————

In a case-control study the starting point is cases with the disease in a population. The previous

exposure of the cases is analysed and a representative sample of controls (without the disease

being studied) selected from the same population. The direction of enquiry is always backwards in

time, but the actual data collection can be carried out in either a retrospective manner (backwards

in time from the time the study started) or a prospective manner (forwards in time from the start of

the study).

Design of a cohort study

direction of inquiry

TIME ———————————› ———————————›

———→Exposed —→ disease —→ no disease

People without

the disease ————→Not —→ disease

exposed —→ no disease

Population———→

In a cohort study a group of people free of the disease which is being investigated is selected.

Exposure to the factor which is being studied is measured during "follow up", as is the subsequent

development of the disease. The direction of enquiry is always in the same direction as time, but data

can be recorded both retrospectively and prospectively, as for case-control studies.

———————↓

Participants ↓ Randomization ————————↓ Control ↓ Treatment

Design of a randomized controlled trial

Study population

↓

↓——————Selection by defined criteria ——————↓

Non

participants

Nonparticiparits (do

not meet selection

criteria)

Potential

participants

↓

Invitation to

participate

↓

This scheme does not indicate that examination of the impacts of interventions on the health of

the treatment and control groups will be similar to the study of disease or absence of disease in

the cohort design. The random allocation of participants to either a group receiving some

treatment or subject to an intervention, or to a group not subject to any intervention (the control),

provides a very powerful scientific basis for drawing conclusions about causation of the health

impact of the interventions. In the ideal cohort study the occurrence of exposure is a random

event, as in the randomized controlled trial.

40

Applications of different observational study designs

Ecological Cross- Case- Cohort sectional control Investigation of rare + + + + — + + + + + — Disease Investigation of rare + + — — + + + + + cause Testing multiple effects + + + — + + + + + of cause Study of multiple + + + + + + + + + + + exposures and determinants Measurements of time + + — +b +++++ Relationship Direct measurement of — — +c +++++ Incidence Investigation of long — — + + + — latent periods

This table makes it possible to assess the most appropriate study

design for different situations. For example, if a rare disease is being

studied, an ecological or case-control design would be the most

suitable. If the cause is rare, a cohort design would be more

appropriate. The indications of applicability of different designs

should be used only for initial guidance. The final choice of design will

depend on availability of crucial data and other operational issues,

and deviation from the table's proposals may be required.

a Key: + ... + + + + + indicates the degree of suitability - not suitable

b If prospective. c If population-based.